% Scott's thesis: Chapter 2 

\chapter{The Flow Mow Experiment}
\label{ch:flowmow}

The Flow Mow field program (August 3--21, 2000) was an effort to measure the
heat flux of a hydrothermal vent field, specifically the Main Endeavour vent
field (MEF) on the Endeavour segment of the Juan de Fuca ridge
(Figure~\ref{location-map}).  While previous studies measured heat flux in
plumes near their sources or their equilibration depths, the Flow Mow strategy
was to intercept plumes at an intermediate, optimal distance above their
sources.  By establishing a control volume that enclosed the best-mapped
hydrothermal field in the world, the stage was set to ``mow'' the ``flow'' ---
to survey the top control surface repetitively, in a back and forth pattern
akin to mowing a lawn, as plumes rose through it.  The side surfaces were also
monitored to assess possible lateral fluxes associated with fluid entrained by
the rising plumes or plumes bent over and advected by ambient currents.  The
methodology was economical because it avoided the expense of acquiring
measurements at vents on the sea floor.  It was also effective, leading to heat
flux measurements that could be accurately associated with specific areas of
hydrothermal activity, unlike some past measurements made in advected plumes.  

\begin{figure}[htbp] \begin{center}
\includegraphics*[width=0.75\textwidth]{figs/maps/field_location_map.eps}
\end{center} \caption[Endeavour segment bathymetry and hydrothermal
sources]{Endeavour segment bathymetric map showing known hydrothermal sources:
open boxes are major fields (MF=Mothra, MEF=Main, HRF=High Rise, SDF=Salty
Dawg, SF=Sasquatch) and small solid squares are diffuse or isolated focused
vents (Q=Quebec, R=Raven/ReddFox, CB=Clam Bed, D=Dune, C=Cirque).  Contours are
every 100\,m and at the sill depth, $\sim$2170\,m.  
%Mean current vectors are superimposed on mooring locations ($+$) labeled with
%year/depth/elevation; 5\,cm/s vector scale at lower right.  
Open black box aligned with ridge axis is perimeter of MEF --- the boundary of
the Flow Mow control volume.  }
%Solid circles ($\bullet$) are CTD monitoring stations (NoMEF and SoMEF).  }
%May want to remove means from this figure and create new fig with means of all
%FM meters, and perhaps those from other studies (at a larger scale?)...
\label{location-map} \end{figure}

The key to the success of the Flow Mow study was a careful combination of
theory for guiding the methodology and technology for acquiring the
observations.  A numerical model implemented by R. McDuff predicted the
elevation at which rising plumes were large enough to be intercepted
frequently, but still intense enough to yield a strong velocity and temperature
signal relative to noise.  An autonomous underwater vehicle called the
Autonomous Benthic Explorer (ABE) provided an efficient, economical, and stable
platform for repetitively sampling the control surfaces.  More traditional
instruments --- a lowered CTD and moored current meters --- served to monitor
the ambient hydrography and flow, critically supplementing the ABE surveys of
the side control surfaces.

While the primary use of the Flow Mow data is heat flux estimation, the data
also represent an unprecedented characterization of hydrothermal plume
hydrography.  The majority of plume observations to date are of fluid that is
just exiting a vent or is equilibrated; observations in and near buoyant plumes
are relatively rare.  In contrast, the Flow Mow study focused almost
exclusively on intercepting buoyant or equilibrating plumes, and characterizing
the environment through which they rose.  

This thesis presents the majority of the Flow Mow field data, with an emphasis
on synthesizing CTD surveys and current measurements.  This chapter presents an
overview of the hydrographic and current meter data, while the next 2~chapters
present CTD observations made on specific surfaces of the Flow Mow control
volume and combine them with data from the current meter moorings to make heat
flux estimates.  This thesis does not describe in detail the Flow Mow data
acquired on the top control surface, as those observations are presented in
\citeN{stahr+03}.  It does, however, present the magnitude of the vertical
heat flux through that top surface, combines it with magnitudes of other heat
flux estimates in a heat flux budget for the MEF, and discusses the
implications of the budget.

This chapter provides a synopsis of some of the Flow Mow observations, places
them in the context of regional hydrography, and sets the stage for further
analysis of plume distributions and heat flux in subsequent chapters.  First, I
present the details of the study: setting, methodology, and instrumentation.
Second, I describe patterns in currents with an emphasis on how the flow
differs within and above the axial valley.  Third, I scrutinize how the
hydrography changes from beyond the ridge flanks, distant from known
hydrothermal sources, to the axial environment where both equilibrated and
buoyant hydrothermal plumes become increasingly intense and prevalent.
Overall, this chapter will introduce some new perspectives on how plumes and
currents transport hydrothermal energy and products into the ocean, and will
also establish the context for calculating and interpreting heat fluxes in the
subsequent chapters.


\section{Setting}

The Endeavour segment presents an unusual opportunity to measure heat fluxes
for 2 main reasons.  First, the hydrothermal sources and geology within the MEF
are well-mapped.  This guarantees that a measurement of net heat flux through
control surfaces bounding the field can be related accurately to a specific
suite of sea floor hydrothermal phenomena.  Second, the topography of the ridge
and the hydrography of hydrothermal plumes have similar scales.  Unlike at
faster- or slower-spreading centers, the vertical relief of the axial valley
($\sim$100\,m) is greater than the rise height of diffuse plumes, but less than
the rise height of focused plumes (Figure \ref{nsf_cross_sxn}).  The top of the
axial valley also marks a change in currents --- from strong, multidirectional flow
above the ridge crests to relatively slow, rectilinear flow within the axial
valley.  

The combination of topographic relief, plume rise height, and currents motivate
different methodologies for measuring heat flux within the axial valley versus
above it.  Above the ridge crests, flux measurements depend on interception of
advected plumes (\emph{e.g.}~\citeNP{baker+massoth87}).  This approach is also
required at fast spreading centers where the axial valley, or caldera, is
typically too shallow to constrain hydrothermal plumes from either focused or
diffuse sources.  In contrast, a typical slow spreading center has a deep axial
valley that can confine all hydrothermal plumes
(\emph{e.g.}~\citeNP{rudnicki+94}) and motivates a budgetary approach to
quantifying fluxes \cite{murton+99} and diapycnal diffusivities
\cite{thurnherr+02}.  

The Flow Mow methodology took advantage of the Endeavour setting by
intercepting plumes within the axial valley, accounting both for equilibrated
plumes advected laterally by the rectilinear flow and for buoyant plumes rising
out of the valley, but not yet exposed to the overlying flow (Chapter
\ref{ch:lower}).  In order to compare the Flow Mow methods and results with
those of previous heat flux measurements made above the ridge in advected,
equilibrated plumes, a secondary investigation was conducted near the end of
the cruise.  At the same time the vertical heat flux in buoyant plumes was
being measured, the equilibration depths of the same plumes were surveyed,
providing estimates of net horizontal heat flux in the multidirectional flow
above the ridge (Chapter \ref{ch:upper}).

\begin{figure} \begin{center}
\includegraphics*[width=.75\textwidth]{figs/setting/depth_bin_dth_hist_nsffig.eps2}
\caption[Axial valley relief and current shear relative to plume rise heights]{
Axial valley relief and current shear relative to plume rise heights.  The left
panel shows an axis-perpendicular cross-section of the Endeavour segment
Hydrosweep bathymetry (D.~Kelley, 1996), with the location of the MEF indicated
by a black triangle.  The mean flow within and above the axial valley is
indicated, along with the range of rise heights expected in cross flows of
1--10\,cm/s.  The right panel shows stacked histograms of temperature anomaly
data that were acquired during a 14\,hr CTD station $\sim$500\,m south the MEF
and subsequently grouped into 25\,m depth bins.} \label{nsf_cross_sxn}
\end{center} \end{figure}

\subsection{Bathymetry}

From a regional perspective, the Endeavour segment is a series of linear ridges
striking $\sim$20$^\circ$ east of north.  Along the central ridge, the actively
spreading axis, the segment is about 46\,km, long when defined by the 2400\,m
contour (Figure~\ref{segment_bathy}).  While the surrounding basins have depths
of 2500--2700\,m, the ends of the segment ends descend as deep as 2800\,m.
Prominent bathymetric features in the vicinity are the Heck and Endeavour sea
mounts about 35\,km to the north and Split sea mount about the same distance to
the south-southeast (Figure~\ref{segment_bathy}).  There are other sea mounts
within $\sim$100\,km west of the ridge, but none to the east, an asymmetry that
has been noted (\emph{e.g.}~\citeNP{delaney+92}), but not related to the
regional flow or hydrography near the Endeavour segment.
% Could sea mounts be important to consider when contemplating regional
% currents?

\begin{figure} \begin{center}
\includegraphics[width=.75\textwidth]{figs/maps/fullsegmentmap.eps}
\caption[Bathymetry of the Endeavour segment]{Sea Beam bathymetric map of
prominent topography in the vicinity of the Endeavour segment.  Contour
interval is 100\,m.  Solid lines are inferred tectonic boundaries.  Dotted lines
are hypothetical boundaries and fracture zones (FZ), suggested by
microearthquake distributions.  Figure courtesy S. Archer and V. Bhat.  }
\label{segment_bathy} \end{center} \end{figure}

The Endeavour segment is essentially a short linear ridge with an elongate
summit depression, or axial valley, that is open at both the north and south
ends, but shoals to a sill near the north end, at $47^\circ59.75$'N
(Figures~\ref{location-map}~and~\ref{parallel_bathy}).  When defined by the
2170\,m isobath (the approximate depth of the sill), the east and west crests
of the segment are $\sim$10 and $\sim$12\,km long, respectively.  At the
latitude of the MEF, the segment is $\sim$4\,km wide when measured between the
points where the outer flanks rise abruptly from adjacent depressions that are
2350--2400\,m deep.  The rounded outer flanks rise about 250\,m to crests near
2100\,m depth.  The shallowest point on the ridge is $\sim$2050\,m atop a
rounded summit at the north end of the west crest, adjacent to the axial sill
(Figure~\ref{parallel_bathy}).


\begin{figure} \begin{center}
%\includegraphics*[width=.45\textwidth]{figs/setting/parallel_bathy_es_map.eps}
\includegraphics*[width=.95\textwidth]{figs/setting/alongaxisbathy.eps}
\caption[Axis-parallel bathymetric cross-sections and current meter depths]{
Axis-parallel bathymetric sections along the valley floor and ridge crests.
Short vertical lines are 3 moorings (FM-S, MZ, and FM-N) with individual
current meters (solid circles) at specific depths.  Vertical lines with labels
at top margin are boundaries of major vent fields.  Solid horizontal line at
2170\,m marks the depth of the sill near 48$^\circ$N.  Cross-sections are based
on Hydrosweep bathymetric data (D. Kelley, 1996).  Axis depths are generally
deeper than the depths of the mooring bases or hydrothermal fields because the
moorings and fields are often located on the shallower sides of the valley,
rather than directly on the axis.} \label{parallel_bathy} \end{center}
\end{figure}

\subsection{Geology}
\label{sec:geo}

Tectonic spreading at a half rate of 29\,mm/yr \cite{elvers+73} has created an
axial valley that has a uniform width of $\sim$1\,km between the rims
\cite{kappel+ryan86}.  Near the center of the segment, the valley floor is
$\sim$500\,m wide and is bounded by basaltic talus slopes and/or fault scarps.
Analysis of segment bathymetry (Figures
\ref{nsf_cross_sxn}~and~\ref{parallel_bathy}) shows that these valley walls
have an average relief of $\sim$100\,m and slopes of 10-30$^\circ$.  Along-axis,
the valley floor shoals to the north at an average rate of $\sim$18~m/km.  

%\begin{figure} \begin{center}
%\includegraphics*[width=.75\textwidth]{figs/setting/w_ecrest_axis.depth_hists.eps2}
%\caption[Histograms of depths from axis-parallel sections]{ Histograms of
%depths from axis-parallel sections.  Bathymetry data source ref? }
%\label{axial_sxn_hists} \end{center} \end{figure}

%Kappel and Ryan on surface roughness (flanks, crests, valley) in preparation
%for estimation of boundary layer thicknesses in flow section?

%Touch on earthquakes, seismic reflector (integration figure?)

%Conductive heat flow measurements are lower than expected near fields (Ref?  Paul?)

% Add delaney thoughts on geology of MEF here
Within the MEF, it is striking that all known hydrothermal sources rest on a
nearly-level, 100$\times$600\,m, terrace between 2190--2200\,m that narrows
dramatically to the north and south (Figure~\ref{abebathy}).  
% Add delaney thoughts on geology of terrace MEF here.  Is it a more recent
% lava flow, elevated relative to the adjacent valley?  Why not expect dike
% injections to follow major faults and lead to eruptions on the edge of the
% valley, rather than the axis?  Take a look at the sm2000 map and old Delaney
% hi-res imagery to consider this hypothesis further!
The terrace ends abruptly on its southwest side where the western scarp attains
slopes up to $45^\circ$.  The western wall of the axial valley is less well
defined at the north end of the terrace; while the basalt--talus contact at the
base of the scarp trends $\sim$20$^\circ$N, the western rim of the valley wall
diverges to $\sim$10$^\circ$N.  Like the other known fields to the north and
south (Figure~\ref{location-map}), the MEF rests on the relatively flat valley
floor.  The surface of the terrace consists of low-relief basaltic pillow and
sheet flows that are nearly continuous, except for being broken intermittently by
10--100\,cm wide fissures and 1-10\,m throw normal faults with strikes
parallel or oblique to the 20$^\circ$N.

\begin{figure} \begin{center}
\includegraphics*[width=.75\textwidth]{figs/maps/abebathy_ctdmap.eps}
\caption[Bathymetric map of the MEF]{Bathymetric map of the MEF vicinity
annotated with locations of CTD stations within the NoMEF, SoMEF, and Quebec
survey areas.  Bathymetric data were acquired with a Mesotech downward-looking
sonar mounted on ABE during dive 43, processed by D.~Yoerger and C.~Parker
Sarason.  Large rectangle aligned with the ridge axis delineates the boundaries
of the Flow Mow control volume.  Outlines of actively venting sulfide
structures are overlain in red, based on the geologic observations made from
the Alvin submersible (within rectilinear polygon) and cataloged by V.~Robigou
in the map of \citeN{delaney+92} (see Figure~\ref{mefmap}).  Discrepancies of
$\lesssim$30\,m between the outlines and local bathymetric highs are likely due
to inaccuracies in the absolute location of the Alvin submersible in 1988.
Relative locations of structure outlines are accurate to within a few meters.
UTM $X$ and $Y$ coordinates are based on zone~9 and the WGS84 ellipse, with the
origin of the Alvin coordinate system at $X=$492638.34, $Y=$5310510.09.}
\label{abebathy} \end{center} \end{figure}

The MEF is the most thoroughly mapped hydrothermal area on the Endeavour
segment \citeN{delaney+92}, having been the subject of myriad Alvin dives (more
than any other site in the world) and numerous geophysical surveys.  It was
first located in 1982 when a dredge brought up unexpected sulfide blocks
\cite{tivey+delaney86} and was explored via submersible in 1984
\cite{hammond+84,merge84,tivey+delaney86}.  The field consists of 2 clusters of
sulfide structures separated by $\sim$150\,m (Figure~\ref{mefmap}).  The
structures are unusually large, having volumes of 10$^5$--10$^6$\,m$^3$
\cite{wilcock+delaney96} and heights of 3--21\,m.  Most structures are
10--15\,m across \cite{tivey+delaney86} and are based on basalt near 2195\,m
depth.  While some structures in the north straddle the northeast edge of the
terrace and have bases as deep as 2205\,m, and a few in the south have bases as
shallow as 2190\,m, source depths are similar throughout the field because the
7 major northern structures are distinctly larger than the $\sim$11 southern
structures.  Based on dimensions noted in the map of \citeN{delaney+92}, the
active sulfide structures in the north have larger diameters (20--40\,m versus
10--20\,m in the south) and are taller on average ($\sim$12$\pm$4\,m versus
$\sim$7$\pm$2\,m).  Consequently, the average depth of MEF high $B$ sources is
about the same in the north and south clusters (2185$\pm$6\,m versus
2188$\pm$5\,m).

\begin{figure} \begin{center}
\includegraphics*[width=.75\textwidth]{figs/maps/mefnamemap.eps}
\caption[Geologic map of the MEF]{Digitized version of the geologic map of the
MEF created by V.  Robigou \cite{delaney+92}.  Arrows connect names to sulfide
structures that are actively venting from high $B$ sources (black dots).
Inactive sulfide structures are yellow.  Areas of diffuse venting are light
blue.  Major fissures and faults are delineated in grey.  Approximate location
of contact between talus scarp and valley floor is shown by jagged black line
near western edge of field.
} \label{mefmap} \end{center} \end{figure}

\subsection{Hydrothermal activity}
\label{sec:mefhta}

%Major vent fields and known diffuse flow sites.  Field distribution details (number 
%of hiB vs loB sources in each; w vs e side)?

As shown in Figure~\ref{location-map}, the major vent fields are separated
along-strike with spacing of $\sim$2\,km \cite{delaney+96}.  A growing number of
relatively isolated high and low $B$ sources have been located or are suspected
to exist in between \cite{veirs+99,johnson+02,kelley+02}., some of which are
mapped Figure~\ref{location-map}.

The location and intensity of hydrothermal sources close to the MEF, but
outside the Flow Mow control volume, are particularly important to this study
because of their potential influence on the MEF hydrography.  One such source
is Quebec, a low $B$ area $\sim$500\,m south of the center of the MEF.  Quebec
is roughly 40\,m square, rich in tube worm beds venting $\sim$12$^\circ$C fluid
of unknown salinity (see dive summary at
\href{http://www2.ocean.washington.edu/zephyr/divesum/2998.sum.html}{www2.ocean.washington.edu/zephyr/divesum/2998.sum.html}),
and potentially close enough to the MEF to exert an influence on measurements
of horizontal heat flux through the south side of the MEF control volume.
Another known source near the MEF perimeter is a lone high $B$ source
$\sim$500\,m north-northeast of the MEF recently dubbed Raven (H. P.  Johnson,
pers.\ comm.) and previously noted as Redd Fox (V.  Bhat, pers.\ comm.).  A
buoyant plume was detected in the Raven/Redd~Fox vicinity by M.  Landsteiner
during VOT~t06a of the Mixing Zephyrs hydrographic program (1995). 

%Get brief descriptions + coordinates from Deb and Paul.

Within the MEF, $\sim$110 individual focused sources are distributed atop the
actively-venting sulfide structures.  The hydrothermal fluid typically exits
near the tops of sulfide structures through mineralized chimneys that are a few
meters high.  While there is a gradient of temperature and salinity in the MEF,
with temperatures increasing and salinities decreasing to the south
\cite{butterfield+94}, the typical vent fluid has $\theta\sim$350$^\circ$C and
$S$ between 35 and 15\,psu.  Analysis of the digitized and updated geologic
map of \citeN{delaney+92} indicates that there are 96~``active smokers''
(\href{http://www2.ocean.washington.edu/gis}{Endeavour Segment GIS web site: 
\\http://www2.ocean.washington.edu/gis}).  Cathedral, a cluster of
10--20 high $B$ vents founded on the talus scarp $\sim$50\,m SW of Peanut and
Bastille (Figure~\ref{mefmap}) was discovered in 2000 (D. Kelley, pers.\ comm.), 
and brings the total number of high $B$ sources within the MEF to $\sim$110. 

Also within the MEF, the top and side surfaces of the active sulfide structures may
act as low $B$ vents with areas up to 20--30\,m in diameter.  Tube worm patches,
typically 2--10\,m in extent, constitute low $B$ fluid sources on the sea floor,
either abutting sulfide structures or distributed along faults near sulfide
structures (Figure~\ref{mefmap}).  The maximum vertical separation of MEF
sources is 38\,m --- between a high $B$ source, the 21\,m high chimney at 2174\,m
on Dante, and a low $B$ source, a tube worm bed at 2212\,m just southwest of
Crypto.  More typical separations are $\lesssim$1\,m between sulfide top
surfaces and high $B$ vents, and 10--15\,m between tube worm beds and nearby
black smokers.


\section{Methodology}
\label{sec:cv}

Quantification of the heat flux through the MEF requires the specification of a
control volume that encloses all sources in the field and intercepts all
hydrothermal plumes.  Detailed, accurate information about the spatial
distribution of distinct hydrothermal sources within the MEF defines the extent
of the field with a certainty not possible at less well-mapped fields.  With a
width of $\sim$300\,m and a length of $\sim$700\,m, the base of the Flow Mow
control volume is a rectangle aligned with the strike of the valley
(Figure~\ref{location-map}) that encompasses all the known MEF hydrothermal
sources with a buffer of $\gtrsim$100\,m.  

The main Flow Mow field program focused on assessing the net heat flux through
the top and sides of a lower control volume, while a secondary experiment
monitored horizontal fluxes through an upper control volume
(Figure~\ref{fmcv}).  The top surface of the lower control volume is a
horizontal (constant pressure) plane $\sim$80\,mas, an altitude near which the
signal strength of rising plumes and the likelihood of intercepting them are
simultaneously optimized \cite{stahr+03}.  

The Flow Mow methodology was to survey the control surfaces as often as
possible, measuring temperature and velocity in order to calculate heat flux.
A high priority was repetitive surveys of the top surface with which to obtain
a mean vertical flux with low variance.  A total of 9 ABE dives resulted in 12
complete surveys of the top surface that successfully acquired both $T$, $S$,
and $w$ data.  On a few occasions when the modular acoustic velocity sensor
(MAVS) mounted on ABE failed to record useful data, the vertical velocity of
the ambient fluid was derived using a hydrodynamic model of ABE
\cite{yoerger+00}.  In addition, ABE surveyed the north and south surfaces 3
separate times each, and the west and east surfaces once.

During the substantial periods when ABE was on deck, the ship was kept occupied
with a second high priority: assessing the side surfaces and nearby hydrography
with the lowered CTD.  Especially for the hydrographic transects conducted
within the axial valley, efforts were made to acquire data at all phases of the
tide, from both north and south of the MEF.  Since the bottom time of ABE was
limited to 6--30 hours, primarily by battery endurance, and because of
troubleshooting early in the cruise and frequent rough weather, enough time was
available to complete 40 CTD stations during 17~days spent over the Endeavour
segment (for details, see
\href{http://www2.ocean.washington.edu/flowmow/ecr/ctd}{Flow Mow electronic
cruise report: \\http://www2.ocean.washington.edu/flowmow/ecr/ctd}).  The side
surfaces of the lower control volume were surveyed from the top surface
($\sim$2110\,m) to within 5--10\,m of the local sea floor (as deep as
$\sim$2210\,m).  

%Need good schematic here (convert the poster powerpoint fig?).  Possibly
%useful to show data coverage relative to tidal phase??  Another opportunity to
%include the ABE-acquired bathymetry...  Worth discussing the actual area
%surveyed on the N and S MEFCV walls?  Did the shoaling topography in the SW
%corner of the CV cause the surveyed area at the S end to be significantly
%smaller than at the N end?

Overall, the Flow Mow study consisted of intermittent monitoring of the
control surfaces with ABE and the lowered CTD.  When opportunity arose, the
lowered CTD was also used to assess the hydrography of the axial region, the
MEF vicinity, and areas suspected to be hydrothermally active based on previous
reconnaissance \cite{veirs+99}.

%Refer forward to CTD/ABE maps and temporal distribution?

\section{Instrumentation}
\label{sec:instrumentation}

Hydrographic data were collected with pumped and ducted Sea Bird
Electronics~911\emph{plus} with conductivity and temperature (CT) sensors
mounted either within a lowered cage or in free flow on the front of ABE
(Figure \ref{ctd_abe_pix}).  On the lowered CTD, the 2 pairs of CT sensors were
separated vertically by 142\,cm in order to detect vertical gradients.  In
addition to deriving potential temperature ($\theta$) and $S$ from the CT
sensor pairs, the lowered CTD recorded pressure, transmissivity, optical
backscatter, fluorescence, oxygen concentration, redox potential, and cage
altitude.  An acoustic altimeter enabled ABE to monitor its elevation and avoid
contact with the sea floor.

\begin{figure} \begin{center}
\includegraphics*[width=.45\textwidth]{figs/ctd_on_deck.eps}\hfill
\includegraphics*[width=.45\textwidth]{figs/abe_front.eps} \caption[Photographs
of CTD and ABE]{The lowered CTD package (left) and the Autonomous Benthic
Explorer (right) that were utilized during the Flow Mow cruise.  Note the
144\,cm vertical separation of the CT sensor pairs (arrows) on the CTD package, and the
relative location of CTD and MAVS on ABE.} \label{ctd_abe_pix} \end{center}
\end{figure}

For the lowered CTD sensors, pre- and post-cruise calibrations did not differ
significantly.  The SBE3\emph{plus} temperature sensor resolution was
$0.0003^\circ$C at 24\,samples/second and had an accuracy of $\pm0.001^\circ$C.
The SBE4 conductivity sensor has a resolution of 0.00004\,Siemens/m at 24\,Hz and an
accuracy of $\pm0.0003$\,Siemens/m.  Raw 24\,Hz data were bin averaged to 2\,Hz.  
%The 2\,Hz series were hand-edited for non-hydrothermal outliers and the
%resulting gaps were filled through linear interpolation.  
Processing of data from the ABE-mounted CT sensors is detailed in
\citeN{stahr+03}.
% from Baker and Massoth, p61 SeaBird CTD: Sensor resolutions were
% $\pm$0.003$^\circ$C for $T$, 
%$\pm$0.00004\,Siemens/m for conductivity, $\pm$0.00?? for $P$...

The hydrographic observations were located through long-baseline acoustic
navigation whenever only one platform (CTD cage or ABE) was submerged.  Only at
stations $>$5\,km from the MEF did the long baseline navigation degrade.  When
both platforms were submerged simultaneously, only ABE was navigated to
preclude  multiple interrogations of the transponders.  In these instances, the
CTD cage locations were estimated from the R/V Thompson GPS fixes, the ship
heading, and the geometric relation between the block on the CTD boom and the
ship GPS antenna.  Comparison of CTD long-baseline and ship GPS fixes during
similarly slow tows ($\lesssim$10\,cm/s) indicates that the CTD rarely deviated
laterally more than 10\,m from the tip of the boom that supported it.  All fixes
were edited for outliers and interpolated onto the 0.5\,Hz hydrographic time
base.

%Map of CTD and ABE coverage.  Make sure that VOT and VOC have been defined and
%abbreviated... Also temporal coverage relative to tidal phase (and/or
%predicted flux... or maybe save the flux juxtaposition for later) Table of ABE
%sensors?  

% consider adding ctd_table.tex here...


The primary sources of horizontal current measurements during the field program
were Aanderaa RCM5 instruments moored $\sim$1.1\,km north and south of the MEF
(Figure~\ref{escm_map}).  Both moorings were deployed and recovered by Rick
Thomson of the Institute for Ocean Sciences, Canada, and his associates.  He
also calibrated and processed the data according to standard IOS procedures.
%The meters generated hourly mean speed and direction continuously throughout
%deployments that are summarized in Table~\ref{cm_table}.  
The north mooring (FM-N: 47$^\circ$57.521'N, 129$^\circ$5.542'W) consisted of
2 RCM5s, mounted one above the other on the same mooring, 14 and 15 meters
above bottom (mab).  In calculating heat fluxes in Chapter~\ref{ch:lower},
velocity data are exclusively from the shallower meter (2161\,m depth) on the
northern mooring because the deeper meter failed prior to the beginning of the
Flow Mow field program.  The southern mooring (FM-S: 47$^\circ$56.281'N,
129$^\circ$5.886'W) supported 5 current meters at 50\,m intervals (from
50--250\,mab).  In Chapter \ref{ch:lower}, the focus is on the deepest meter
from the southern mooring because its depth (2142\,m) is comparable to that of
the northern meter (2161\,m), and both are approximately centered in the Flow
Mow control volume depth range (2100--2200\,m).  The upper meters on the
southern mooring are utilized in Chapter \ref{ch:upper} to interpret
hydrographic variability and compute flux estimates above the ridge crests.

\begin{figure} \begin{center}
\includegraphics*[width=.75\textwidth]{figs/maps/all_cm_map2.eps}
\caption[Current meter mooring locations and mean vectors]{Location of current
meter moorings, both historic and utilized in the planning or conduction of the
Flow Mow field program.  The northern and southern Flow Mow moorings are FM-N,
and FM-S; the 1995 Mixing Zephyrs mooring is MZ95.  Historic moorings are
located with large open circles to emphasize the imprecision of Loran-C
coordinates; adjacent or central labels are consistent with past studies listed
in the key.  For each meter on the FM and MZ moorings, the average flow
observed over the full record is indicated with a vector.  White vectors
indicate flow within the axial valley (meter located deeper than 2100\,m);
black vectors show flow at or above the ridge crests (shallower than 2100\,m).
A 5\,cm/s reference vector is provided in the lower right hand corner.  To
minimize clutter only major hydrothermal fields are delineated.}
\label{escm_map} \end{center} \end{figure}

Additional current observations were made with Nobska MAVS.  One MAVS was
mounted on ABE and sensed relative vertical and horizontal flow components;
these were corrected for vehicle motion using the record of ABE depth and
navigated location, respectively.  Other MAVS were placed near-bottom
($\lesssim$1\,mab) within the MEF by collaborating scientific programs and
recorded velocity for short periods during the Flow Mow field program.  Data
from the latter instruments have provided useful comparisons (courtesy of
M.K.~Tivey, S.~Hautala, I.~Garcia-Berdeal, and their associates), particularly
the MAVS deployed by M.K.~Tivey $\sim$0.5\,mab near the center of the MEF, about
40\,m north of the S\&M sulfide structure (Table~\ref{cm_table}).

%Include more specifics on Tivey MAVS deployment and setting, ideally in
%Table~\ref{cm_table} and Figure~\ref{escm_map}, or perhaps the MEF geo map.

A final source of information about Endeavour currents that is utilized in this
dissertation is the hourly mean record from the MZ95 mooring deployed over the
summer of 1995, $\sim$200\,m east of the northeast corner of the MEF perimeter
(Figure~\ref{escm_map}).  The mooring was approximately centered in the axial
valley and supported 2 Aanderaa RCM5 meters, one within 25\,m of the axial
valley floor, the other 300\,mab at a depth of 1900\,m (see
Table~\ref{cm_table}).  The 1995 data were used by H.~Mofjeld to predict the
phase of the tidal currents within the axial valley in order to guide
hydrographic deployments during the Flow Mow field program.  They are also
utilized in Chapter~\ref{ch:lower} to characterize the typical near-bottom
oscillations in a model of plume dispersion.

\begin{sidewaystable}

\caption[Current meter deployments]{Current meter and pressure sensor
deployments that collected data used in this thesis.  Elevation in meters above
bottom (mab) is based on mooring engineering specifications and the assumption
that each mooring was deployed on level terrain.  MZ = 1995 Mixing Zephyrs
cruise; FM=2000 Flow Mow cruise.  MJD=modified Julian day.  $T_e$=Expanded
temperature sensor (small range, high resolution).  Y=sensor present, N=no
sensor, M=sensor malfunctioned, and P=partial record.  MEF~MAVS data provided
courtesy of M.~Tivey.  
%MEF~$P$ data provided courtesy of M.~Lilley.  
For reference, the Flow Mow cruise period is 8/04/00--8/21/00, or MJD
51760--51777.  All $T$ values recorded on the moorings are in $^\circ$C on the
International Temperature Scale 1968 (ITS-68).} 

\label{cm_table}

\begin{center}
\begin{tabular}{lrrccrrrcccc}
Name 	& Depth & Elevation & Deploy date & Recover date & Data start & Duration & Sample & Sensors &         &     &    \\
	& m 	& mab       &             &              & MJD        & days     & period 
                                                                     &$T$ &$T_{e}$ &$S$ &$P$ \\
\hline
MZ25	& 2175	& 25  & 05/24/95 & 07/23/95 & 49866.076 &  52.56 & 10 min & Y & Y & Y & N\\
MZ300	& 1900	& 300 & 05/24/95 & 07/23/95 & 49866.076 &  52.56 & 10 min & Y & Y & Y & N \\
FM-N15  & 2161  & 15  & 05/21/00 & 10/09/00 & 51685.000 & 141.75 & 1 hr   & Y & Y & Y & N \\
FM-N14  & 2162  & 14  & 05/21/00 & 10/09/00 & 51685.042 &  58.75 & 1 hr   & Y & Y & Y & N \\
FM-S50  & 2142	& 50  & 07/27/00 & 10/09/00 & 51752.833 &  73.96 & 1 hr   & Y & Y & N & N \\
FM-S100 & 2092  & 100 & 07/27/00 & 10/09/00 & 51752.833 &  73.96 & 1 hr   & Y & Y & N & N \\
FM-S150 & 2042  & 150 & 07/27/00 & 10/09/00 & 51752.833 &  73.96 & 1 hr   & Y & Y & M & M \\
FM-S200 & 1992  & 200 & 07/27/00 & 10/09/00 & 51752.833 &  73.96 & 1 hr   & Y & Y & N & N \\
FM-S250 & 1942  & 250 & 07/27/00 & 10/09/00 & 51752.833 &  73.96 & 1 hr   & Y & N & M & P \\
MEF MAVS& $\sim$2200  & 0.5 & 07/25/00 & 09/25/00 & 51710.000 &  87.74 & 2 min  & Y & N & N & N\\
%MEF $P$ & 22??  & 0   & 06/13/00 & 06/23/00 & 51708.889 &   9.00 & 3 min  & Y & N & N & Y \\
%MEF $P$ & 22??  & 0   & 06/23/00 & 09/13/00 & 51718.687 &  81.20 & 10 min & Y & N & N & Y \\
\end{tabular} 
\end{center}

\end{sidewaystable}

During the summer of 2000, pressure sensors were deployed atop the southern
mooring, on the sea floor within the MEF (by M. Lilley and associates), within
ABE, and on the lowered CTD.  The sea floor and moored $P$ records provide
information about the ambient pressure field, as well the effect of currents on
the tall southern mooring, but neither are analyzed in this thesis.  
%Only the moored record is discussed in this thesis. 

%The moored pressure record is from a sensor associated with the uppermost
%meter on the southern mooring (FM S250).  Although about $2/3$ of the record
%is suspect, showing essentially no change except  occasional ($\sim1/$month),
%discontinuous jumps indicating downward displacement of 1--2\,m, there is a
%short central section that shows plausible oscillations
%(Figure~\ref{mooring_p}).  The oscillations have an amplitude of 2.3\,m,
%indicating that the mooring remained nearly vertical; a downward displacement
%of 2.3\,m on a 250\,m mooring line would imply a lateral displacement of about
%34\,m, or a deflection of about 8$^\circ$ from vertical.  However, the
%pressure variations do not have a clear phase relationship with the current
%magnitude measured by the current meter at the same position.  An alternative
%interpretation of the pressure oscillations is that they are due to changes in
%surface tidal height, and that the mooring was not significantly deflected by
%the currents.  
%
% Bill asks: but are they at tidal freq and do the have mag predicted for
% surface tides in the area? 

%The sea floor pressure sensor was deployed within the MEF
%(Figure~\ref{escm_map}, Table~\ref{cm_table}).  Is the amplitude of the
%pressure variation consistent with both the expected surface tide amplitude
%(and Marv's P sensor?)...

%\begin{figure} \begin{center}
%\includegraphics*[width=.75\textwidth]{figs/cms/mab250.pres_dep_mag.70hr_tseries.eps2}
%\caption[Pressure oscillations on the southern current meter mooring]{
%Oscillatory pressure (upper) and depth (middle) variations recorded at the
%uppermost meter on the southern mooring (FM S250) for a short period during
%the middle of the deployment.  The lower panel shows the magnitude of the
%hourly mean current observed at the same meter. } \label{mooring_p}
%\end{center} \end{figure} 


\section{Flow}
\label{sec:flow}

A broad spectrum of plume-related research projects have involved deployment of
current meters at the Endeavour segment during the last 20\,years
\cite{dymond+roth88,kadko+90,franks92,thomson+90,thomson+92,allen+thomson93,veirs+99}.
The data from moored meters have been supplemented with ADCP observations
(\emph{e.g.}~\citeNP{thomson+89}).  These measurements have been acquired near
and above equilibration depths over the ridge crests and axis, within the axial
valley, and occasionally on the southwest flank of the segment
(Figure~\ref{escm_map}).  The patterns of flow observed during these projects
and the Flow Mow study are discussed in this section.  

In distilling basic features of the Endeavour flow, it must be noted that past
mooring arrays have been primarily axial, and that the moorings deployed during
the summer of 2000 were exclusively axial (Figure~\ref{escm_map}).  Mooring
deployments on the ridge crests or flanks are comparatively rare, especially on
the historically neglected east side.  

This section presents the current observations made during the Flow Mow field
program and compares them with past observations of Endeavour currents.  After
an initial discussion of regional flow patterns --- from the surface currents
of the northeast Pacific down through the flows observed over the entire Juan
de Fuca ridge ---  the focus narrows to the Endeavour segment.  Current
observations made at the Endeavour during the Flow Mow and past studies are
juxtaposed in 3 subsections that describe mean flows, characteristic
oscillations, and spatial coherence.  In each of these subsections, the
differences between flow within versus above the axial valley are highlighted.  


\subsection{Regional overview}

The Endeavour segment of the Juan de Fuca ridge is centered latitudinally
beneath the divergence of the Subarctic current which shifts seasonally between
$\sim$45$^\circ$N (winter) and $\sim$50$^\circ$N (summer) \cite{thomson81}.
The bifurcation of the Subarctic current occurs between 150--130$^\circ$W
longitude and generates a complicated field of meanders and 10--100\,km
diameter eddies.  By the time the flow has reached the Endeavour segment
($\sim129^\circ$W), it typically has split into 2 distinct currents: the
California current going southeast and the Alaska current headed
north-northeast to feed the Alaska gyre.  Consequently, the area over the
Endeavour, between the diverging currents, tends to experience confused and
variable near-surface currents.  While the mean eastward velocity is typically
$<$10\,cm/s, absolute velocities in eddies commonly attain speeds of 25\,cm/s,
at least for short periods \cite{thomson81}.  In the region over the ridge and
as far east as Station Papa (50$^\circ$N, 145$^\circ$W), oscillations within
the wind-mixed layer (15\,m drogue) and at the base of the pycnocline (120\,m)
are dominated by inertial currents with RMS speeds of $\sim$50\,cm/s, but also
involve diurnal currents of similar magnitude ($\sim$10\,cm/s)
\cite{thomson+98}.  \citeN{thomson81} notes that ship drift has identified a
Subarctic counter-current that flows westward in mid-winter at
$\lesssim10$\,cm/s between 48--51$^\circ$N and most frequently east of
160$^\circ$W (Figure~\ref{regional_map}).

%Read refs in Thomson drifter paper, or ask Mihaly for recent reviews/articles
%regarding nature of surface currents directly over ES (only one of their
%drifter tracks crosses 130W near the ES...).

\begin{figure} \begin{center}
\includegraphics*[width=.75\textwidth]{figs/maps/bigannotmap.eps}
\caption[Regional bathymetry and currents]{Bathymetric map of the northeast
Pacific ocean off the coast of Washington \cite{smith+sandwell97} with regional
mean currents overlain.  Dashed arrows indicate bifurcating surface currents.
Solid arrow shows westward Subarctic counter-current on surface.  Solid arrow
indicates long-term mean flow over the Endeavour segment near 2000\,m, a
typical plume equilibration depth (denoted by white diamond).  }
\label{regional_map} \end{center} \end{figure}

Regional patterns of flow from below the surface currents to within a few
hundred meters of the segment topography ($\sim$1800-2400\,m) are relatively
less well understood, but some basic characteristics have been discerned.  Mean
values from the uppermost meters presented in \citeN{thomson+90} indicate that
flow decreases from $\sim$1.19\,cm/s to the east-southeast at 60\,m depth to
only 0.15\,cm/s to the northwest at 486\,m depth.  At mid-depth (near 1500\,m,)
the mean flow is generally faster, $\sim$1\,cm/s, but in directions that are
inconsistent; mid-depth flow recorded by \citeN{franks92} varied between east,
south, and southwest directions, while \citeN{thomson+90} observed flow to the
southwest to southeast.  \citeN{franks92} discerned a vertical shear between
mid-water depths ($\sim$1600\,m) and plume equilibration depths (near 1900\,m).
Examination of her progressive vector diagrams (PVDs), based on moorings with
3--4 meters distributed between 1600--1900\,m, suggests that with every 100\,m
descent, the mean current direction shifted counter-clockwise ($-5$ to
$-20^\circ$) while the magnitude increased by about a factor of 2.  

%Summarize with mean vectors and/or PVD over bathy map and suggest that there
%aren't really clear patterns, though Cannon suggests patterns in large-scale
%flow...  Mention POM modeling effort by Thomson group, plus others.  And cite
%the Nature paper regarding the 2001 deployments.

% Possibly reiterate theoretical expectations re flow over ridges and even the
% nearby seamounts \cite{lavelle+02, JGR?}, both to the north and the local
% 1950m summit?

%Include Marv's P record, perhaps juxtaposed with P record from mooring (is
%there a horizontal pressure gradient (phase difference?).  Also, Bill wants a
%PSD of the P record.  And maybe a pressure prediction (and currents?) from the
%OSU model?   Talk about forcing (ocean tide vs tidal stream vs Bernoulli vs
%storms and other inertial)?

Models provide one perspective on the flow expected over the Juan de Fuca ridge
and the Endeavour segment, with the assumption of an ideal forcing function and
simplified topography \cite{allen+thomson93,lavelle+cannon01}.  For diurnal and
weather band forcing, motions are amplified over the ridge and the flow is
rectified on the ridge flanks, generating a mean northward flow on the western
flank and a mean southward flow on the eastern flank \cite{lavelle+cannon01}
(\href{http://www.pmel.noaa.gov} {animation by J.W.~Lavelle}: \\
pmel.noaa.gov/$\sim$lavelle/Review/Workstation/Anim/ridge.fli).  While the
models indicate that oscillatory flows can displace isotherms and isopycnals
and perturb the velocity field in a broad region over the ridge flanks and
crests, the resolution of the \citeN{lavelle+cannon01} model is too coarse to
elucidate the details of the shear immediately above the axis or within the
axial valley.  This is problematic when attempting to interpret the current
meter records from the Endeavour segment because, for the most part, they
derive from moorings placed close to the bottom and almost exactly on axis,
rather than atop the ridge crests or on the outer ridge flanks.

Some model predictions have been confirmed by observations of the flow over the
Juan de Fuca ridge.  Most studies have focused on segments south of the
Endeavour and depths of the ridge crest and equilibrated hydrothermal plumes
($\sim$1700--2300\,m).  Early studies \cite{cannon+91} detected opposite mean
flows along the flanks in the directions later predicted by models.  Denser
arrays of meters stretched across the northern Cleft segment on the southern
Juan de Fuca ridge discerned similar flank flows \cite{helfrich+98} and
elucidated details of the anticyclonic shear near an axial hydrothermal source
\cite{cannon+pashinski97,joyce+98}.  Near 2100\,m, the mean flows parallel to
the Cleft segment had magnitudes that peaked at 2--3\,cm/s about 8\,km
off-axis, and decreased to 0.8--1.0\,cm/s at the axis
\cite{cannon+pashinski97}.  The broadest geographic perspectives on flow near
2100\,m depth suggest that the circulation in the interior of the northeast
Pacific may be relatively simple, mimicking the gyre above, but the flow over
and around the Juan de Fuca ridge and associated sea mount chains is strongly
steered by topography \cite{cannon+91,cannon+93,cannon+pashinski97}.


\subsection{Mean currents}
\label{subsec:meanflow}

\subsubsection{Mean flow above the ridge crests}

The general consensus among Endeavour researchers is that the mean flow
\emph{above} the ridge is usually to the southwest.  While it is true that the
mean flow typically has a southward component, the extant observations contain
many exceptions to the rule of mean flow to the southwest.  A review of past
studies in this section suggests that the upper level mean flow can be
consistent for many days but can also change direction dramatically for periods
of days to weeks.  There is evidence of horizontal coherence during some
periods, while at other times there is also some evidence of horizontal shear
along and across axis on spatial scales of a few km \cite{franks92}.  Some of
the confusion that often occurs at sea when tracking and mapping hydrothermal
plumes may stem from assuming that the long-term mean flow is characteristic
when the flow at any given moment is likely to be much more variable.  This
section also highlights how the flow patterns over the ridge differ from those
\emph{within} the axial valley.

The idea that mean flow over the Endeavour segment is generally to the
southwest, in rough alignment with the ridge axis, is an oversimplification
that appears to have originated in measurements made by S.~Roth~Franks
\cite{thomson+90,franks92} in support of the plume study of
\citeN{baker+massoth87} (\emph{c.f.}~Section~\ref{sec:advectflux}).  During the
field program of \citeN{baker+massoth87} the mean flow above the ridge was at
205$^\circ$ true, almost directly along-axis.  Just prior to the hydrographic
survey, however, the mean flow was to the southeast.  Examination of the PVDs
from all meters above the ridge crests over the non-synoptic and primarily
axial array deployed by \citeN{franks92} indicates that the mean flow almost
always has a southward component.  In some PVDs, the flow is aligned with the
axis consistently for periods of up to 259\,days; in others, the mean flow
ranges through the southeast and southwest quadrants.  

In a subsequent study by \citeN{kadko+90} that did not overlap temporally with
the deployments of \citeN{franks92}, periods of mean flow to the southwest were
recorded, but only at 1 of the 3 meters located near plume equilibration depths
above the ridge crests.  The 3 moorings were deployed by \citeN{kadko+90} for
17--19\,days in September, 1988.  Their mooring~F was located on or near the
outer southwest flank, mooring E was at the same latitude but on axis south of
Mothra, and mooring~C was on axis near the High Rise field
(Figure~\ref{escm_map}).  Comparing simultaneous periods for the meters near
1950\,m depth, mooring~E showed consistent flow to the southwest at
$\sim$2.25\,cm/s, mooring~F experienced flow to the west at
$\sim$0.6--1.2\,cm/s interrupted by a few days of northward flow, and mooring~C
was exposed to a brief southward flow and subsequent, consistent flow to the
east-northeast at $\sim$1.4\,cm/s.  This suggests that the currents above the
ridge can exhibit substantial horizontal shear over spatial scales as small as
2\,km across axis and 5\,km along axis.  

Southwest flow was only observed during a few days of a 45\,day deployment
(ER02) by \citeN{thomson+92} on the western crest near 2075\,m
(Figure~\ref{escm_map}) that also did not overlap temporally with previous
records.  The low-pass filtered data (40\,hr cutoff) reveal currents of
$\sim$2--3\,cm/s in almost all directions, but primarily to the northwest to
northeast.  Such extended periods of northward mean flow are unusual enough
above the ridge to raise questions about whether this variable record
represents clear flow above the crest or is substantially influenced by local
topography, being only 25\,mab.  While the text of \citeN{thomson+92} indicates
the mooring was based near the 2100\,m bathymetric contour about 4\,km
southwest of the MEF, the accompanying map suggests it was closer to the
2200\,m contour, which would put the meter near 2175\,m depth.  Near the
latitude of the deployment, the closest 2100\,m bathymetric contour is about
2000\,m to the north atop a prominence at the south end of the western crest.
The uncertainty of the mooring depth and location, given a Loran-C location
error of $\sim$1\,km, make it difficult to know whether this record should be
associated with flow over the western ridge crest or on the western flank.

The 1995 deployment of a meter 300\,mab on the MZ95 mooring was definitively on
axis, just a few 100\,m northeast of the MEF \cite{veirs+99}, and recorded mean
flow to the southwest at a depth of 1900\,m over $\sim$2\,months
(Figure~\ref{escm_map}).  During periods of a few days, however, the average
flow ranged from southeast to west-southwest (Figure~\ref{mz_pvd}).  This
variability may explain why efforts to intercept the MEF hydrothermal plume
during the same Mixing Zephyr field program were successful on some days, but
found no plume in the expected downstream direction on other days.

\begin{figure} \begin{center}
\includegraphics*[width=.65\textwidth]{figs/cms/95/pvdMZ300.eps2}
\caption[Progressive vector diagram for 1995 current meter at 1900\,m]{
Progressive vector diagram for the 1995 current meter (300\,mab, 1900\,m) near
the northeast corner of the MEF field.  The mean flow for this 52.5\,day record
is 4.10\,cm/s at 206$^\circ$.  Black asterisks symbolize 10\,day intervals.  }
\label{mz_pvd} \end{center} \end{figure}
 
Overall, past measurements of the mean flow above the ridge suggest that a
southward component is typical.  At depths shallower than 2100\,m, northward
mean flow is rarely observed over periods longer than a few days.  In the 3
exceptional current meter records \cite{kadko+90,thomson+92} it is most likely
that topographic effects are the cause of the northward mean flows.  Both
mooring ER02 \cite{thomson+92} and f \cite{kadko+90} were unique in
being located on the outer edge of the western flank of the Endeavour segment
(Figure~\ref{escm_map}) where they may have been subject to northward
ridge-parallel mean circulation.  Such flows have been observed on the western
flank of the southern Juan de Fuca ridge \cite{cannon+pashinski97} and are
generated by nonlinear rectification of oscillatory flow over model ridges
\cite{lavelle+cannon01}.  The third exceptional record, from mooring c of
\citeN{kadko+90} was located on-axis and is therefore less likely to have been
affected by an episode of northward flow along the western flank. 

% Possible explanations.  Seasonal variations?  Events?  Significant
% cross-ridge shear that is not well resolved by mostly axial deployments?
% Should I not expect the mean flows observed further south on the JdFR?

An alternative explanation for the distinct sequences of mean flows observed by
nearby meters deployed simultaneously above the ridge is that rotational
dynamics of hydrothermal plumes affect the flow field above the ridge, at least
intermittently overpowering the ambient mean flows
(\emph{e.g.}~\citeNP{speer89}).  According to this theory, a meter placed west
of a strong hydrothermal source and within the equilibration depths where an
anticyclonic flow is expected could record a northward mean flow.  This may
have been the case for mooring c which was located very near the High Rise
vent field (Figure~\ref{escm_map}), but the uncertainty in the location of that
mooring means it is unclear whether the mooring was west or east of the field.
Rotation associated with hydrothermal plumes is also a tenable explanation for
the northward flow observed at moorings ER02 and f, both of which were
located $\sim1$\,km west of the Mothra vent field.  
%If rotation were affecting meters so distant from large vent fields, then the
%record from mooring R1 of \cite{thomson+90}, located $\sim$1\,km due west
%of High Rise would also be expected to show a northward mean flow; in fact, it
%recorded a mean flow to the south-southeast over more than 100\,days.  Ah, but
%it sits pretty high for geostrophic flow, at 1700\,m.

Analysis of the Flow Mow current meter data, presented next, cannot resolve
questions about what processes govern the patterns of mean flow over the
Endeavour segment, primarily because the moorings were all axial.  Discerning
the relative influence of topography and plume dynamics on the flow field will
probably be contingent on a \emph{synoptic} observational program that
encompasses not only the Endeavour axis, but also the crests and flanks. 

% Here or at end of Ch2 or in Ch 5: Say something about the historical bias
% towards axial current measurements!  There are interesting studies to be done
% at larger scales, with deployments on flanks, in adjacent basins, on outer
% rigdges, at segment ends, near topographic highs, etc.  Mention POM/modeling
% efforts afoot?  Do patterns Cannon detected further south hold at ES?  Do sea
% mounts play a role as they do at Axial?  

\subsubsection{Vertical shear in the mean flow}

Data from the meters at 5~different depths on the southern mooring FM-S
(Figure~\ref{parallel_bathy}) show with greater vertical resolution than ever
before how the mean flow above the ridge differs from that within the axial
valley.  The progressive vector diagrams from FM-S (Figure~\ref{5level_pvd})
provide a rare perspective on the vertical structure of the horizontal flow.

The 3 uppermost meters generate remarkably similar progressions: southwest for
the first 30\,days, northwest for the next 30\,days, and west-southwest for the
remainding 13\,days of the record.  The sequence of flow resulted in average
flow to the west-southwest over the full record of each meter
(Figure~\ref{escm_map}).  These meters, at depths of 1942, 1992, and 2042\,m,
are each well above the east and west ridge crests, which at the latitude of
FM-S are at $\sim$2110 and 2150\,m, respectively (Figure \ref{parallel_bathy}).
Of the 3 upper meters, the central one at 1992\,m exhibits the greatest PVD
displacements, which implies relative enhancement of the mean flow at that
depth.  This enhancement may be related to the equilibration depth of MEF
plumes being 2000\,m (\emph{c.f.}~Figure~\ref{4meanprofiles}).  Is it possible
that the mean flow is accelerated, perhaps by divergence of the equilibrating
plumes, in the same 200\,m thick zone centered at 2000\,m in which
near-inertial motions are amplified by a factor of 1.2--1.7 \cite{thomson+90}?
ADCP profiles taken in the same vicinity also show a maximum relative velocity
near 2000\,m \cite{thomson+89}.  

The next deepest meter, FMS100 (Figure~\ref{escm_map}) at a depth of 2092\,m
--- just above the adjacent valley walls --- shows a similar sequence of
direction changes.  However, the direction is shifted $\sim30^\circ$ relative
to the overlying flow: counterclockwise during the first 30~days and clockwise
during the next 30.  During the last 13\,days the flow direction is identical
to the overlying meter.  Additionally, the individual displacements are
smaller, resulting in a reduced net displacement.  The deepest meter,
relatively protected within the axial valley at a depth of 2142\,m, shows
shifts in direction that are comparatively slight and not clearly correlated
with the overlying changes.  

%\begin{figure} \includegraphics*[width=.95\textwidth]{figs/cms/5level.pvd.eps}
%\caption[Progressive vector diagrams for 5 meters on southern mooring]{
%Progressive vector diagrams for 5~meters on southern mooring.  Left hand
%panels show entire record, while right hand panel shows only the Flow Mow
%cruise period.  Black dots are separated by 10~days in the left panels and by
%5~days in the right panels.  Red sections indicate the 17~day Flow Mow cruise
%period (MJD~51760--51777).} \label{5level_pvd} \end{figure} 

\begin{figure} \begin{center}
\includegraphics*[width=.75\textwidth]{figs/cms/5cmmultipvd.eps}
\caption[Progressive vector diagrams for 5 meters on southern mooring]{
Progressive vector diagrams for 5~current meters on mooring FM-S
(Table~\ref{cm_table}).  Entire 74~day record is used with asterisks indicating
10~day intervals.  Relative to these intervals, the 17~day Flow Mow cruise
period (Julian day 217--234) was from day~7 to day~24.  } \label{5level_pvd}
\end{center} \end{figure}

Figure~\ref{5level_pvd} also shows that \emph{during the Flow Mow study}, the
mean flow at each depth was in a consistent direction.  Above the ridge
(150--250\,mab), the flow was to the southwest at $\sim$4\,cm/s, resulting in a
moderate cross-axis component to the west.  Near the depth of the ridge crests
(100\,mab) flow due south (180$^\circ$ true) at $\sim$3\,cm/s resulted in a
cross-axis component to the east.  Just within the axial valley, the flow was
to the northwest at 1--2\,cm/s --- almost directly opposed to flow of the same
magnitude only 50\,m above.  Thus, the greatest shear observed between the
deepest pair of meters in the 74~day record happened to occur during most of
the 17~day Flow Mow cruise period.  During other periods the flow measured at
the 2 deepest meters was either aligned or orthogonal.

While most of the vertical current meter arrays deployed by \citeN{franks92}
did not extend deep enough to characterize the flow within the axial valley,
one mooring did (ER3 in Figure~\ref{escm_map}) and generated PVDs that show a
similar pattern of shear.  For the 4~meters at depths of 1686, 1936, 2036, and
2136\,m, the respective mean current speeds and directions were 1.1\,cm/s at
210$^\circ$, 2.0\,cm/s at 190$^\circ$, 1.4\,cm/s at 200$^\circ$, and 0.5\,cm/s
at 30$^\circ$.  The 3~records from above adjacent ridge crests ($\sim$2100\,m)
indicate southward mean flow, while the meter located below the crests shows
northward flow.  
%I am not aware of any other published evidence of this relative flow reversal.

% Are there similar shifts in Frank's mean flow earlier in the spring/summer?
% How bout in the MZ record, or in longer through-winter deployments of
% Thomson/Franks?  Is this the 4-10 oscillation, or something else?

Similar patterns of vertical shear persists at shorter averaging periods.
Figure~\ref{daily_mean_stack} displays a daily resampled time series of the
low-pass filtered (35\,hr) currents from all of the FM-S meters.  The upper
3~records are remarkably similar with little speed or direction shear, despite
the variable, multidirectional currents.  The flow at 100\,mab is directionally
similar to the overlying flow, but is consistently diminished in speed.  Near
Julian day 235 and 260, the low frequency flow is essentially zero, despite
substantial flow at the other depths.  Finally, the flow at 50\,mab seems
independent of the overlying flow; it is directly opposite to the flow 50\,m
above near Julian day 217, but perfectly aligned near Julian day 254. 

\begin{figure} \begin{center}
\includegraphics*[width=.75\textwidth]{figs/cms/thomson/5level_dailymean.eps}
\caption[Daily mean currents from 5 meters on southern mooring]{ Daily mean
currents from 5 meters on southern mooring based on the 35\,hr low-pass
filtered record.  Flow Mow cruise was from Julian day 217--234.  Used with
permission, courtesy R.~Thomson, from \cite{thomson+03}.}
\label{daily_mean_stack} \end{center} \end{figure}


\subsubsection{Mean flow within the axial valley}

Prior to $\sim$1995, the mean flow \emph{within} the axial valley received
little attention, because most current meters were deployed to facilitate
interpretation of the hydrography of plumes equilibrated \emph{above} the ridge
crests.  While mean flows to the north had been observed previously in
near-bottom axial records (R. Thomson, pers.\ comm.; \citeNP{franks92}),
deployment of the MZ mooring in 1995 (Table~\ref{cm_table}) also revealed a
consistent northward mean flow at 25\,mab, or 2175\,m
(Figure~\ref{valley_pvds}).  This observation helped guide the design of the
Flow Mow study because rectilinear flow within the Flow Mow control volume
simplifies the task of measuring heat flux by reducing the number of side
surfaces that need to be monitored.  Whether the flow within the axial valley
is uniform as the valley is crossed, or if lateral shear is common is a topic
of ongoing investigation; initial analysis of Jason-mounted ADCP transects of
the axial valley collected in 2000 and 2001 suggests that substantial
cross-valley shear may occur, albeit intermittently (S.~Hautala, pers.\ comm.).  
%Mention or list other recent sources of information about currents near MEF:
%the Hautala/Johnson MAVS arrays and ADCPs, plus others (?).

\begin{figure} \begin{center}
\includegraphics*[width=.45\textwidth]{figs/cms/95/pvdMZ25.eps2} \hfill
\includegraphics*[width=.45\textwidth]{figs/cms/pvdFM-N15.eps2}
\caption[Progressive vector diagrams: northern (FM-N) and central (MZ)
moorings]{Progressive vector diagrams for axial valley currents measured at the
central mooring in 1995 (MZ95, 25\,mab, 2175\,m) and the northern mooring in
2000 (FM-N, 15\,mab, 2162\,m).  Mean flow is 4.74\,cm/s at 31$^\circ$ for MZ95,
and 2.29,cm/s at 0$^\circ$ for FM-N15.  Relative to the multidirectional flow
above the ridge, the mean flow in the valley at these meters is rectilinear and
consistently northward.  Black asterisks are 10\,day intervals.  Note that
domain and range are different between plots.
%Black dots are separated by 6 hours, red stars by 10 days.  
} \label{valley_pvds} \end{center} \end{figure} 

Northward mean flow within the axial valley is also evident in the records from
both Flow Mow moorings (Table~\ref{cm_table}).  The mean flow at FM-N is as
consistently northeastward ($\sim$35$^\circ$) as the flow at MZ95 is due
northward (Figure~\ref{valley_pvds}).  In contrast, the northward mean flow at
FM-S varies from northeast to northwest
(Figures~\ref{5level_pvd}~and~\ref{daily_mean_stack}).   While the flow within
the axial valley is primarily rectilinear, it is not necessarily aligned with
the overall strike of the ridge axis (20$^\circ$).  This may be due to a
combination of the along-axis variations in the strike of the valley walls and
the proximity of each mooring to the topographic variations along each wall and
ridge crest.  Using the 2200\,m isobath in Figure~\ref{escm_map} as an
indicator of the local valley width and axial strike, FM-S is close to the
eastern wall and a gap in the east ridge crest, while FM-N is closer to the
western wall; MZ95 is close to the eastern wall, but is not far from the
$\sim$20\,m sulfide structures and complex topography within the MEF
(Figure~\ref{abebathy}).  In all three cases, the mean flow vector is roughly
aligned with the strike of the nearest 2200\,m isobath (Figure~\ref{escm_map}).
Intermittent flow through the gap in the east crest may account for the
westward component that is occasionally evident in the FM-S record
(Figure~\ref{5level_pvd}).

The magnitude of the northward mean flow increases from FM-S to FM-N (1.6, 
%2.3, 
4.7\,cm/s), while the meter elevation decreases (50, 
%25, 
15\,mab).  This suggests that the flow within the axial valley is intensified
near the bottom.  This possibility is discounted, however, by a similar 
% plume entrainment may be most intense in the 10--20\,mab range?
set of observations made in 2001.  With moorings deployed during the same 4
summer months, approximately from July through October, and in the same
locations as FM-S and FM-N, \citeN{thomson+03} measured a similar increase in
mean flow within the axial valley, but the meters were both $\sim$15\,mab.  An
alternative explanation for the north-to-south increase in the mean flow is the
decrease in cross-sectional area of the axial valley.  Based on cross-sections
taken orthogonal to the ridge axis and through the mooring locations, the area
beneath the 2170\,m isobath decreases from $\sim$56,000\,m$^2$ at FM-S to
$\sim$23,000\,m$^2$ at FM-N.  This could account for a factor of 2 increase in
the along-axis flow within the valley.

An additional, important aspect of the \citeN{thomson+03} deployment in 2001
was the  placement of a meter $\sim$15\,mab on a mooring located further north
in the axial valley, just south of the 2170\,m saddle.  The mean flow recorded
there was southward at 1--2\,cm/s \cite{thomson+03}.  Thus, the 3 meters at
$\sim$15\,mab in 2001 a most striking pattern: convergence of the near-bottom
mean flow within the axial valley. 

One possible exception to the pattern of near-bottom convergence within the
axial valley comes from the southernmost axial current meter mooring described
in \citeN{franks92}.  The mooring A3, was deployed $\sim$1\,km south of the
MEF near the 2250\,m bathymetric contour in 1985--1986 (Figure~\ref{escm_map})
and supported near-bottom meters at 2155 and 2156\,m depth, or $\sim$100\,mab,
and upper level meters at 2055 and 1955\,m.  Since the adjacent axial valley
walls are well-defined up to $\sim$2110--50\,m at this latitude
(Figure~\ref{parallel_bathy}), the deepest 2 meters are just within the axial
valley.  While the uppermost meter recorded mean flow to the southwest, the
three deeper meters have PVDs that indicate extremely consistent mean flow
($\sim$2.7\,cm/s) directly along-axis to the south.  The persistent southward
flow at the pair of deepest meters, roughly 50--100\,m above the bottom, does
not match the northward flow observed in 2000 by a meter at roughly the same
location, elevation, and depth (2142\,m on the FM-S mooring).  While a latter
portion of the $v$ record at 2155\,m is erroneous and the mean value of the $T$
record at 2156\,m is suspect, the good $v$ data that overlap temporally
generate completely consistent PVDs that suggest the southward flow persisted
from October, 1985 to June, 1986.  

The differences between the winter 1985 and summer 2000 data are puzzling.  One
explanation is that the prevailing flow within the axial valley shifts
seasonally from north in summer to south in winter.  This is consistent with
the observations made \emph{south} of the MEF at depths near the top of the
axial valley, but is not satisfactory for similar depths north of the MEF
because in the winter of 1986--1987, during a deployment of similar duration
but $\sim$2\,km \emph{north} of the MEF (mooring ER-3 in
Figure~\ref{escm_map}), \citeANP{franks92} recorded northeastward mean flow at
$\sim$0.5\,cm/s within the axial valley near 2136\,m depth.  Another
explanation is that during winter 1985, unusual conditions -- either
hydrothermal or in the ambient flow -- led to southward mean flow within the
axial valley.  While it is true that all of the meters deployed by
\citeANP{franks92} that winter \emph{above} the ridge crests did exhibit
southward components of flow, the PVD for the 1\,month record from the deepest
meter on mooring A2 at 2106\,m (north of the MEF and \emph{below} the ridge
crests) shows mean flow to the north-northwest at $\sim$1\,cm/s.  An
alternative explanation is that flow within the axial valley is northward near
the eastern valley wall and southward near the western valley wall
(Figure~\ref{escm_map}).  More observations close to the western valley wall
would help test this explanation.  Another explanation is that the mean flows
within the valley are generally convergent, but also influenced by proximity to
topography.  The uncertainty in the Loran-C coordinates of the moorings
deployed by \citeANP{franks92} precludes assessment of this possibility, but it
remains plausible that the ER-3 mooring fell close to the western valley wall
adjacent to topographic features that generated a southward flow at the meters
near 2155/6\,m depth.  The possible importance of valley topography on
10--100\,m scales, including the $\sim$20\,m sulfide structures, is reinforced
by observations made by an array of MAVS distributed within the MEF
$\sim$1\,mab during summer 2000; despite tidal coherence across the array, the
mean flows were random with each MAVS apparently affected differently by the
local topography (I.~Berdeal, pers.\ comm.). 

%Make plot of daily mean t-series for WASP 2000... It's not in Nature fig for
%some reason.
%
%(Didn't Thomson see occasional reversals in the WASP 2000 data?)  [According
%to his 2001 figure, daily mean currents were 3\,cm/s south-southwest for only
%one day; there was no reversal at the south meter at that or any other time in
%the 2001 data.

Anticipating consideration of current oscillations in the next section, it is
helpful to recall that the magnitudes of instantaneous flows within axial
valleys may be much higher than the mean flows.  This is illustrated by 2
interesting accounts from submersibles operating in axial valleys.  Within the
Mid-Atlantic ridge FAMOUS axial valley, near a meter that recorded a mean flow
of 8.2\,cm/s and a maximum speed of 24.2\,cm/s over a 50-s averaging period,
French submersible divers claimed to have struggled against a 39\,cm/s current
\cite{keller+75}.  Similarly, the Alvin submersible was overpowered by a strong
current during a dive within the MEF in 2000, causing it to be rammed into a
sulfide structure (M. Lilley, pers.\ comm.).
%  (Dive time or location?).  
The differences between these first-hand observations of near-bottom velocities
and associated, simultaneous current meter measurements made nearby that
topographic constrictions may significantly accelerate mean flows within the
axial valley.  It is also possible that entrainment by hydrothermal plumes may
accelerate ambient flows, but significant effects are likely to occur only
close to the vent where the entrainment velocities are highest ($\sim$10\,m
laterally and $\sim$25\,m vertically from a high $B$ source). 


\subsection{Oscillatory currents}

Current oscillations are ubiquitous in the open northeast Pacific, and are
prominent both above the Endeavour topography and within the confines of the
axial valley.  Both observations and models show that oscillatory currents are
nearly rectilinear away from the ridge, but are amplified and predominantly
clock-wise rotary immediately over the ridge
\cite{franks92,allen+thomson93,lavelle+cannon01}.  Relative to mid-water
conditions, the semi-diurnal, inertial (16.1~hr), diurnal, and low-frequency
oscillations are amplified just above the ridge crests, especially in a depth
range of 1800--2100\,m, and attenuated within the axial valley
\cite{franks92,allen+thomson93}.
%,cannon+thomson96}.

Analysis of data from FM-S by \citeN{thomson+03} characterizes this depth
progression with unusually fine vertical resolution for component inertial and
tidal oscillations (Figure~\ref{5level_ellipses}).  Relative to the highest
meter, the diurnal, inertial, and M$_2$ semi-diurnal bands are amplified with
proximity to the ridge crest (near 2100\,m).  The diurnal and inertial bands
are also become more circular at ridge crest depth, and are attenuated within
the axial valley.  Relative to their orientation above the ridge, both the
M$_2$ and the S$_2$ ellipses are more aligned with the topography within the
axial valley.  

\begin{figure} \begin{center}
\includegraphics*[width=.75\textwidth]{figs/cms/thomson/ellipses.eps}
\caption[Current ellipses and vectors at 5 depths on southern mooring]{Current
ellipses and vectors at 5 depths on the southern mooring for the main tidal
frequency bands (O$_1$, K$_1$, M$_2$, S$_2$) and the inertial frequency band
($f$).  Meter elevation is noted on the left in meters above bottom.  Current
vectors indicate a reference time to illustrate relative phase.  Small circles
denote hourly mean values for current vectors.  Outer arrows show direction of
vector rotation.  Along-axis (20$^\circ$ True) direction is upward.  Figure
used with permission from \citeN{thomson+03}, courtesy R. Thomson.}
\label{5level_ellipses} \end{center} \end{figure} 

The most prominent oscillation within the axial valley is at the M$_2$
semidiurnal frequency.  At all depths on all moorings deployed by
\citeN{thomson+03} in 2000 and 2001 the semidiurnal frequency band, dominated
by M$_2$ but also including S$_2$ energy, accounts for a greater proportion of
the variance than any other constituent (low- or high-, inertial, or diurnal
frequencies).  Within 100\,m of the bottom the M$_2$ consistently accounts for
more than $50\%$ of the total variance \cite{thomson+03}.  Most of the rotary
spectra from the Juan de Fuca ridge showcased in \citeN{franks92} and
\citeN{cannon+91} have spectral energy density maxima in the semi-diurnal band.
The M$_2$ tidal ellipses in Figure \ref{5level_ellipses} show amplitudes that
diminish from $\sim$3.5\,cm/s at and above the ridge crest to 3\,cm/s within
the valley \cite{thomson+03}.  The $\pm$3\,cm/s flow within the valley, aligned
with local topography, competes with the mean flow to produce occasional
reversals at the FM-S50; in contrast, the oscillatory amplitude rarely exceeds
the mean flow at FM-N15, so such reversals are rare (Figure~\ref{quiver}).  At
the 1995 meter MZ25, the semi-diurnal and mean flow also combine to generate
reversals, less frequently than at FM-S50, but more frequently than at FM-N15. 

\begin{figure}[htbp] \begin{center}
\includegraphics*[width=0.9\textwidth]{figs/cms/fm1_quiver.eps2} \end{center}
\caption[Hourly mean current within the axial valley]{A characteristic 65~hr
time series of hourly-mean currents within the axial valley north (top) and
south (bottom) of the MEF.  True north is up.} \label{quiver} \end{figure}

The influence of the topography on distinct oscillations is also illustrated by
power spectra of available near-bottom $v$ time series from different parts of
the axial valley (Figure \ref{psd_v_ns}).  Relative to FM-S50, FM-N15 has
greatly reduced inertial energy, equivalent semi-diurnal energy, and enhanced
diurnal energy (see also upper panel in Figure \ref{cohphase}).  The reduction
in inertial energy is consistent with the inhibition of near-inertial motions
by the topography of the axial valley (\emph{e.g.}~\citeNP{thomson+90}), as
FMS-50 is less constrained by the valley walls than FM-N15.  The relative
increase in diurnal energy is probably also a consequence of the greater
topographic constriction of flow at FM-N15.

\begin{figure} \begin{center}
\includegraphics*[width=.45\textwidth]{figs/cms/power/CM50V.power.eps2}\hfill
\includegraphics*[width=.45\textwidth]{figs/cms/power/Wasp2161V.power.eps2}
\caption[Power spectra of near-bottom $v$ at north and south moorings]{Power
spectra of near-bottom $v$ at FM-S50 (left) and FM-N15 (right).  The diurnal,
inertial, and semi-diurnal frequency bands are labeled D, f, and SD.  }
\label{psd_v_ns} \end{center} \end{figure}

%PSD of $v$ from cm95 2175?  Does it show inertial damping (perhaps relative to cm95 1900)?

At the Endeavour, the combined oscillatory components of flow are capable of
transporting fluid about the same distance as the mean component during any
given half-period.  This is true both above the ridge and in the valley.  The
displacement by a pure oscillation with amplitude $v_o$ and period $\tau$ that
occurs during $0 <= t <= \tau/2$ is 
\begin{equation} 
\Delta x = \int\limits_{0}^{\tau/2} v_o \sin{(\frac{2\pi}{\tau} t)} dt = v_o\tau/\pi.
\label{displace}
\end{equation}

%For typical oscillations over the Endeavour ridge, this distance indicates
%that it is unlikely that hydrothermal plumes could be displaced far enough to
%affect the Flow Mow hydrographic background stations.  
Values of $v_o$ typical of the flow above the ridge and within the axial valley
can be taken from the maximum amplitude of the current ellipses in
Figure~\ref{5level_ellipses}.  Using these values and the associated period,
Equation~\ref{displace} yields the maximum displacement that can be affected by
each oscillatory component of the characteristic flow.
Table~\ref{displace_table} presents these displacements and compares them to
the distance traveled in typical mean flow (1--5\,cm/s) during the
corresponding half-periods.

Above the ridge, oscillatory flow can displace fluid 0.48--1.37\,km, depending
on the relative duration and amplitude of a particular component.  These
component displacements are comparable to the $\sim$1\,km semi-diurnal and
diurnal ``excursions'' noted by \citeN{baker+massoth87} as being much smaller
than the horizontal scale of the Endeavour hydrothermal plumes.  They are also
similar to the ``maximum daily movements'' of $\sim$0.7\,km east-west and
$\sim$1.0\,km north-south estimated from currents measured at Endeavour
\cite{roth+dymond86,dymond+roth88}.

Larger displacements are possible, however, if the component oscillations act
together and have the same direction.  A function that describes a
\emph{composite displacement} $\Delta x_c$ caused by combined oscillatory
components (in phase) is the sum of 5 terms, one for each of the 5 main
component frequencies and amplitudes:
\begin{equation}
 \Delta x_c= \sum_{i=1}^5\frac{\tau_i}{2\pi}{v_o}_i \cos{\frac{2\pi}{\tau_i}}t.
\label{composite}
\end{equation}
The maximum of this function, obtained graphically in this case, is an estimate
of the maximum composite displacement.  The maximum for the characteristic
oscillations above the ridge is 2.2\,km; for oscillations within the axial
valley it is 1.3\,km (Table~\ref{displace_table}).  

\begin{center}
\begin{table}

\caption[Typical displacements by oscillatory and mean currents] {Typical
displacements ($\Delta x$) by oscillatory and mean components of Endeavour
currents over $1/2$ tidal or inertial periods ($\tau$).  Values for main tidal
frequency bands (O$_1$, K$_1$, M$_2$, S$_2$) and the inertial frequency band
($f$) are calculated with Equation~\ref{displace}, using amplitudes ($v_o$)
taken from major axes of ellipses in Figure~\ref{5level_ellipses}.  This is
done first for typical flow above the valley, then for characteristic flow
within the valley.  Last 2 columns show displacements due to mean flows of
1~or~5\,cm/s acting over $1/2$ of each period.  Last row gives maximum
displacement by composite oscillations (calculation described in text). }

\label{displace_table}
\begin{tabular}{r||c|cc|cc|cc}
\multicolumn{2}{c}{}& \multicolumn{2}{|c|}{\emph{Above valley}}& \multicolumn{2}{|c|}{\emph{In valley}} & \multicolumn{2}{|c}{\emph{Mean flow}} \\
\hline
\emph{Constituent} & $\tau$ & $v_o$ & $\Delta x$ & $v_o$ & $\Delta x$ & $\Delta x$ & $\Delta x$    \\
\emph{frequency}   
          & hr     & cm/s  & km        & cm/s  & km        & km @1\,cm/s& km @5\,cm/s \\
\hline\hline
O$_1$     &25.82   &3.0    &0.89       &1.5    &0.44       &0.46        &2.30\\
K$_1$     &23.93   &5.0    &1.37       &2.0    &0.55       &0.43        &2.15\\
$f$       &16.10   &3.5    &0.65       &2.0    &0.37       &0.29        &1.45\\
M$_2$     &12.42   &7.0    &1.00       &6.0    &0.85       &0.22        &1.10\\
S$_2$     &12.00   &3.5    &0.48       &3.0    &0.41       &0.22        &1.10\\
\hline
Composite & \multicolumn{3}{|c|}{Max $\Delta x$: 2.2\,km} & \multicolumn{2}{|c|}{Max $\Delta x$: 1.3\,km} & & \\ 
\end{tabular}
\end{table}
\end{center}

Displacements by typical mean flow above the ridge ($\sim$5\,cm/s) range from
1.1--2.3\,km, depending on the duration of $1/2$ period.  This means typical
component oscillations are unlikely to overcome the mean flow above the ridge.
Only maximum composite oscillations are likely to reverse the mean flow and
displace fluid upstream.  Transport of fluid over distances greater than
2.2\,km, however, are rarely accomplished through composite oscillatory flow
alone; such large displacements require an extended period of mean flow in a
steady direction.  This insight will also be realized through an
advection/diffusion model in Chapters~\ref{ch:lower}~and~\ref{ch:upper}.

The range of displacements caused by mean flows ($\sim$1--5\,cm/s) within the
axial valley are also rarely overcome by the oscillatory components.  Relative
to the 1.1--2.3\,km displacements caused by the $\sim$5\,cm/s mean flow at
FM-N15, the displacements caused by individual oscillations in the valley are
small: 0.37--0.85\,km.  But the 1.6\,cm/s mean flow at FM-S50 is close to
1\,cm/s, generating displacements of only $\sim$0.22--0.46 over the respective
$1/2$ periods.  This suggests that a typical diurnal, inertial, or semi-diurnal
oscillation can displace fluid upstream in the slower mean flows in the axial
valley, but is unlikely to do so where the mean flow is faster.  

The maximum composite displacement in the axial valley, 1.3\,km, indicates that
combined oscillatory flow will commonly overpower a slower mean flow and may
even counter the faster flow on occasion.  The maximum displacement is also
similar to the 2\,km spacing of known hydrothermal vent fields along-axis.
This has interesting ramications for larval dispersal and other exchange
processes within the axial valley.  In the absense of a mean flow, fluid
parcels may be moved $\pm1.3$\,km by the composite oscillatory flow; the M$_2$
component alone could transfer parcels back and forth over $\pm$0.85\,km.  With
the addition of only a slight mean flow ($\sim$5\,cm/s), larvae might move
between major vent fields in a single semi-diurnal period, and will gain
multiple exposures to an individual point on the sea floor.

% Mention scale of MZ distribution?  Include animation of $\Delta_S\theta$
% distribution by depth bin!
%
%Compare with baker+massoth87 "advective flushing time" $\Delta t =$ field
%diameter/$\overline{v}\simeq$ 500\,m/2\,cm/s$\sim$0.3\,days.  
%
% Lupton+85, p.621--2: "striking" profiles that led to MEF discovery were from
% $\sim$4\,km E of the 
%axis (stable and $\Delta_?\theta\sim0.05^\circ$C).  "This feature is present
%to some degree in all CTD profiles collected within 5--10\,km of the vent area
%both in 1983 and 1984, and we conclude, therefore, that this hydrothermal
%layer is a long-lived feature of the water column in this region."


\subsection{Coherence and phase}

%Build on uniformity (modest vertical shear) of Figure \ref{5level_pvd}...

When comparing current records from similar depths above the Endeavour ridge
crest, both along and across-axis flow are in phase and have high horizontal
coherence at semi-diurnal, inertial, and diurnal frequencies within several
kilometers of the MEF \cite{thomson+90,franks92}.  This is also true over
larger scales, albeit to a lesser extent; for tidal and inertial oscillations
recorded near 2000\,m at both the A2 mooring (Figure~\ref{escm_map}) and a
mooring 15\,km to the SE, coherence is near 1 and phase is $\sim$0$^\circ$ for
the along-axis component and $\lesssim45^\circ$ for the across-axis component
\cite{franks92}.  At near-inertial frequencies, horizontal coherence scales
exceed 50\,km in this depth range \cite{thomson+90}.  Taken together, these
observations imply that the \emph{oscillatory components} of horizontal flow
are uniform near 2000\,m, the depth at which many MEF hydrothermal plumes
equilibrate and are dispersed, over horizontal scales of at least a few km. 

% Should say something here about scale of vertical coherence of tidal and
% near-inertial oscillations (~50--100m? thomson ref?)... as opposed to
% low-frequency mean flow which was essentially addressed in previous section
% on shear

The majority of historical current meter arrays positioned above the Endeavour
axis, crests, or flanks, suggest that the \emph{low-frequency component} of the
horizontal flow is relatively uniform near the plume equilibration depths and
within $\sim$1\,km of the axis.  For example, PVDs from an array of meters
deployed by \citeN{franks92} simultaneously near 2000\,m indicate that the
low-frequency motions induce net southward transport in all cases; at the same
time, the presence of horizontal (or possibly vertical) shear caused meters
separated by only $\sim$1\,km (A2 at 2005\,m and R2 at 1985/86\,m) to differ in
mean flow direction by up to 45$^\circ$.  Similarly, the mean flow recorded at
1942--2042\,m by the 3 highest meters on the FM-S mooring
(Table~\ref{cm_table}) is vertically uniform, showing only slight differences
in patterns of flow (Figures~\ref{daily_mean_stack}~and~\ref{5level_pvd}).
\citeN{franks92} describes a counterclockwise or ``helical'' rotation in mean
flow recorded by vertical pairs of meters near plume equilibration depths,
specifying that on average the mean flow backs $-25^\circ$ for every 100\,m of
descent, while the flow magnitude decreases.  Studies at other ridges indicate
that this degree of uniformity may be typical of the flow over spreading center
topography.  Within the MAR/FAMOUS axial valley, for example, \citeN{keller+75}
report high coherence between 2 meters (A and C) offset 150\,m vertically and
$\sim$3\,km horizontally.

Within the axial valley, there are limited opportunities to assess horizontal
flow coherence.  During the Flow Mow study, the only meters occupying similar
depths simultaneously were within the axial valley, near 2150\,m, the central
depth of the Flow Mow control volume.  The meters, FM-N15 at 2161\,m and FM-S50
at 2142\,m, were separated along-axis by 2.2\,km.  Before calculating and
modeling horizontal heat fluxes through the MEF control surfaces using
particular measured current velocities, it is important to examine similarities
and differences the FM-N15 and FM-S50 records by means of horizontal coherence
calculations (Figure~\ref{cohphase}).  Auto- and cross-spectral analysis of
these near-bottom meters reveals statistically significant coherence and a
0--20{$^\circ$} phase difference at the dominant (semidiurnal) frequency; the
south meter (50\,mab) leads the north meter (15\,mab) by $\lesssim$45~min.
This phase lag is unexpected and may be insignificant, given its similarity to
the sampling period of the current meters (30\,min).
%Error bars on the phase estimates?!  Bill asks why would you expect a phase
%difference or lack of coherence?
Unfortunately, the horizontal coherence of near-bottom currents cannot be
assessed using the data of \citeN{franks92} because only 1~meter was deployed
within the axial valley at any given time.

\begin{figure}[htbp] \begin{center}
\includegraphics*[width=0.9\textwidth]{figs/cms/power_coh_phase_NS.eps2}
\end{center} \caption[Power spectrum, coherence, and phase for 2 axial valley
current meters] {Power spectral density of the along-axis component of flow
measured at the N and S moorings (top), and coherence (solid line) and phase
(asterisks) between the 2 moorings (bottom).  The level of 95\% confidence is
indicated by the horizontal line.  The three significant peaks (excluding the
low frequency shoulder) correspond with the diurnal, inertial, and semidiurnal
periods.  Positive phase means that S leads N.} \label{cohphase} \end{figure}

Figure~\ref{quiver} provides an additional demonstration of the similarity of
the oscillations at FM-N15 and FM-S50.  In these 65\,hr records, the magnitude
of the mean flow relative to the amplitude of the tidal oscillations determines
whether flow reversals occur.  These short time series also characterize how
rarely fluid is advected southward in the axial valley.

\begin{figure}[htbp] \begin{center}
\includegraphics*[width=0.9\textwidth]{figs/cms/fm1_v-tseries-all.eps}
\end{center} \caption[Along-axis flow in and near the Main Endeavour field]
{The zero-mean, along-axis component of flow measured at A) FM-N15 north of MEF
(dashed blue) and FM-S50 south of MEF (solid green), and B) by ABE on the Flow
Mow control surfaces (dashed blue) and by a near-bottom MAVs in the middle of
the MEF (solid green).  Each series covers the same period as in Figure
\ref{quiver} and is overplotted with a solid black curve: flow extrapolated to
the year 2000 through harmonic analysis (H. Mofjeld, pers.\ comm.) using 60
days of 10~min mean data from MZ25 (Table~\ref{cm_table}).  Time series in B)
derive from: a MAVS deployed by M.~Tivey $\sim$0.5\,mab and 40~m north of the
S\&M sulfide structure (Figure~\ref{mefmap}); a MAVS mounted on ABE during the
2 longest dives (ABE50 and ABE51).  All MAVS data were smoothed with
Butterworth low pass filters (\emph{e.g.}~\citeNP{emery+thomson01}).}
\label{v-series} \end{figure}

Further evidence that currents within the axial valley were strongly tidal and
approximately in phase is given by a few short records acquired \emph{within}
the MEF during the Flow Mow field program.  Figure~\ref{v-series} juxtaposes
the zero-mean, along-axis component of near-bottom flow from FM-N15 and FM-S50
with 3 additional time series: the flow predicted through harmonic analysis of
MZ25 (Table~\ref{cm_table}, H.~Mofjeld, pers.\ comm.); the record from a MAVS
deployed by M.~Tivey 1\,mab in the center of MEF; and ambient current inferred
from a MAVS mounted on ABE during 2 of the longest dives.  The latter series
was calculated by subtracting ABE velocity, derived from navigational fixes of
$\sim$1\,m accuracy, from the ABE-mounted MAVS readings.  That this method
resolved the ambient current at all is a tribute to both the MAVS, the
stability of the ABE platform, and the long baseline navigation system.  During
this characteristic 65\,hr period (the same as in Figure~\ref{quiver}), the
flow is dominated by semidiurnal oscillations and all records are approximately
in phase.  Most zero-crossings of the de-meaned records fall within $\pm$1\,hr
of the predicted flow reversals.  

% Might want to mention what the means of ABE50/51 and Tivey MAVS were... or
% put in a table of means somewhere??

\subsection{Synopsis}

As the resolution of the flow field above the Endeavour segment and within its
axial valley has increased, some consistent characteristics have become
evident.  First, the mean horizontal flow is dramatically sheared near 2100\,m
depth over the axis.  Above the ridge axis (shallower than $\sim$2100\,m), mean
currents are typically directed southwestward in rough alignment with the ridge
axis.  Within the valley, the mean flow is often in an opposite direction and
can also be reduced in magnitude.  South of the sill (near 48$^\circ$N), it is
typically directed up-valley at 1--5\,cm/s to the north or northeast.  A recent
synoptic deployment indicates convergent flow within the axial valley, with
near-bottom ($\sim$15\,mab) mean flow to the south at the sill and to the north
adjacent to the MEF \cite{thomson+03}.  A second characteristic is that the
mean flows in both depth ranges are remarkably consistent.  Only intermittently
do the axial valley currents appear to be overpowered by the overlying mean
flow \cite{thomson+03}.  Finally, oscillations are prominent at all depths, but
rectilinear and dominated by the M$_2$ semi-diurnal frequency only within the
confines of the axial valley.

The methodologies for quantifying heat flux in the Endeavour flow field will be
applicable in other parts of the oceans only to the extent that the current
characteristics are similar there.  
%While the typical flow over abyssal plains in deep sea is , 
Combinations of oscillatory and mean flows have been observed near hydrothermal
sites within the axial valleys of other oceanic ridges
\cite{keller+75,thurnherr00}.  In the vicinity of the Rainbow vent field on the
Mid-Atlantic ridge, for example, mechanical mixing at sills driven by tidal
oscillations appears to maintain a northward mean flow along an axial valley
that is open only at its south end \cite{thurnherr+02}. 
% Ask Andreas about similarities in the EPR data?
Because the problem of measuring horizontal heat flux is significantly simpler
in slower, rectilinear, nearly unidirectional flow than in faster,
multidirectional, oscillatory flow, Chapter \ref{ch:lower} presents the problem
within the axial valley, while Chapter \ref{ch:upper} examines the more
complicated setting above the ridge crests.  



\section{Hydrography}
\label{hydrography}

% Information about Endeavour hydrography has been gathered by many
% plume-related research projects over the last few decades (refs?), many of
% which were reviewed in Chapter~\ref{intro}.  The increase in resolution has
% been dramatic... 

% This section...

% Mention that most studies only look at plumes from the density anomaly
% perspective, but here the plumes are also examined from the perspective of
% isohaline and level anomalies, as well as $theta$--$S$ space

\subsection{Regional overview}

%General NE Pacific $\theta-S$ plot and interpretation with respect to water
%masses.  Pretty good overview in Pickard+Emery.  Juxtapose station Papa with
%Levitus?  
%
%Near-linear profiles characterize the deep ocean environment.  (Stommel on why?)
%
%Cannon93 overview: ``Common water'' ($\sigma_{\theta}=27.76$, ) and ``Intermediate
%water'' ($\sigma_{\theta}=27.28, z=$800-900\,m).  Ref to Munk, Abyssal recipes,
%1966.  Also to Roemmich and McCallister (1989) on large-scale flow in the
%North Pacific.  Maybe good to combine his mean vectors with those from Franks
%in 1 figure to give overview?

The hydrography in the northeast Pacific near the Juan de Fuca ridge is
generally consistent with long-term mean flow to the southwest at the depths
hydrothermal plumes equilibrate (2000\,m) and $\sim$500--1500\,m below
(\emph{e.g.}~\citeNP{cannon+93}).  Anomalies of $\theta$, $S$, Si, and He on
the $\sigma_\theta=27.675$ isopycnal surface tend to have maxima atop the Juan
de Fuca ridge and decrease to the west along or near 45$^\circ$N.  In a study
of biochemical transformations in plumes advected from the MEF,
\citeN{lilley+95} detected concentrations of Mn and ATP 25\,km southwest of the
MEF that were elevated relative to undetectable levels at background stations
10\,km east of MEF, but decreased by a factor of $\sim$4 from concentrations
directly over the MEF.  There is some evidence that hydrothermal signatures
remain detectable in $\theta$--$S$ plots as far as 1000\,km to the
west-southwest of the ridge.  The lateral distributions are relatively
heterogeneous at large scale for the mid-depth open ocean, but often show local
maxima that have lateral scales from 10 to a few 100\,km, and are separated by
similar to slightly larger distances \cite{cannon+93}.  An outstanding question
is whether these ``patches'' of anomalous fluid are coherent mesoscale
hydrothermal eddies, rotating in geostrophic equilibrium after separating from
their vent field sources \cite{speer89,helfrich+battisti91} or simply a
manifestation of the way that equilibrated hydrothermal plumes are dispersed by
a combination of rotary oscillations and mean flow.  Given the depths of nearby
volcanically active ridge segments and the $1000$\,m rise height of event
plumes observed above the Juan de Fuca ridge \cite{baker+89}, these patches
appear to be too deep to be remnant megaplumes from local sources.
% Get feely+92!  Tracking the dispersal of hydrothermal plumes from the JdFR
% JGR 97 3457-3468: how far apart are the blobs?

% Upstream of the JdFR are the waters overlying Cascadia basin.  Notes from Thomson+95: fig5 shows 
% increase in slope of dth/dz and dth/ds below about 2250.  Above this depth the th profiles 
% 4 casts around Baby Bear show substantial variability ($\sim$0.05), while below it they 
% are extremely uniform ($\sim\pm$0.01 at most).  Variability returns to the higher levels at 
%depths below about 2550\,m.

%Note the FM background and axial mean $\sigma_{\theta} profiles cross at
%27.662--27.664\,kg/m$^3$ and 1975--2000\m.

Current records from \emph{above} the ridge crests near 1900--2100\,m during
the Flow Mow field program and previous Endeavour segment studies also indicate
that regional mean flow is to the southwest, at $\sim$5\,cm/s.
Although tidal and inertial oscillations are amplified in this depth range
\cite{thomson+03}, none are capable of displacing hydrothermal plumes more than
a few km from their axial sources, even when acting together to affect a
maximum composite displacement (Table~\ref{displace_table}).  Consequently, we
assume that vertical CTD casts acquired 10 and 15\,km due east of the MEF prior
to the axial investigations define the regional hydrographic context, or
``background'' for the Flow Mow field program.  The more distant station was at
47$^\circ$57.01'N, 128$^\circ$54.02'W, while the closer station was at
47$^\circ$56.99'N, 128$^\circ$58.02'W.  

% For those who rest assured that this distance eastward of the ridge is 
% sufficient to guarantee a hydrocast uninfluenced by hydrothermal plumes, 
% there are a few observations that suggest otherwise.  For example, 
%Franks92: First 2 months of A2 2005 was due east, resulting in a PVD
%displacement of 100 km.  This certainly suggests that contamination of the FM
%background stations is possible, though unlikely during the FM S current meter
%record...  UNLESS there is a lot of across-axis shear in the mean flow
%field... 
% Also the MZ encounters in the SE
% 
% Thomson cascadia obs also emphasize some upstream affects and hydrothermal
% influence

I assume that these eastern, upstream stations define the condition of the northeast
Pacific deep water prior to topographic and hydrothermal influences.  Like
other casts taken at similar distances east of the axis, 
% reference Irene's data; MZ? REVEL?
these stations display a near-linear $\theta$--$S$ trend and constant (clear
water) transmissivity or backscatter signals over the depths where Endeavour
hydrothermal plumes typically equilibrate (1700--2300\,m).  
% The negative slope of the background $\theta$--$S$ curve results in a
% temperture anomly that is $\sim$2\times the isopycnal anomaly.  cite Thomson
% Cascadia and Thurnherr and lupton.  Because it tends to magnify the thermal
% intensity of the plume, I use it along with backscatter values to visualize
% the plume distributions against the temperature-stratified background.  In
% calculating fluxes, however, I use unadjusted $T$ and $S$.
I calculate an \emph{isohaline} potential temperature anomaly (hereafter
simply $\Delta\theta$), using as a reference a 2nd-order polynomial fit to the
near-linear $\theta$--$S$ background data (10-m depth-binned average of both
stations).  

%Now compare all available ''background'' casts.  Need a map of the locations:
%MZ stns, REVEL stns?, FM stns, Hautala stns.  Then pressure-binned overlain
%profile plots for full water column and plume depths

\subsection{Transition to geothermal hydrography}
\label{sec:transition}

% Probably should start with the background and nsew most distant stations plot
% of sig-z and th--S
%
% Note that similar upward ``bows'' from the background th--S trends are
% presented in \cite{hautala+riser93} (they note that Reid first recognized in
% bottle data and cite similarity to lupton+85 observations in north Pacific.

When averaged in 10\,m depth bins, the data from the 20 Flow Mow CTD stations
located over the Endeavour axis (722 separate up or down casts) show positive
$\theta$ and $S$ anomalies relative to background trends that are linear with
respect to potential density, $\sigma_\theta$
(Figure~\ref{axial_mean_sig-th+s}).  Deviation from the background trends
typically begins near 1900\,m, the same depth at which light attenuation
decreases to a local minimum from a maximum near 2000\,m \cite{thomson+92}.
From an isopycnal perspective, the overall influence of the hydrothermal system
on the ocean below $\sim$1900\,m is a shift from background fluid to ``plume
fluid'' which is saltier and warmer, on average, at all densities greater than
$\sigma_{\theta}\simeq27.64$.  

\begin{figure}[htbp]
\begin{center} 
\includegraphics*[width=.65\textwidth]{figs/setting/sgth_T2.mean.vs.bkgrd.eps2}

\includegraphics*[width=.65\textwidth]{figs/setting/sgth_S2.mean.vs.bkgrd.eps2}
\caption[Background and axial CTD stations: $\sigma_\theta$ versus $\theta$
and $S$] {$\sigma_\theta$ versus mean $\theta$ and $S$ for Flow Mow
background and axial CTD stations.  The solid line represents the background
data (from both stations and both pairs of sensors), while the open circles
represent 10\,m depth-binned averages of data (from the second pair of C/T
sensors only, Figure~\ref{ctd_abe_pix}) at stations located along the ridge axis.  Specific station
numbers included in these averages are: 3--8, 10, 12--14, 16, 17, 19, 20, 21,
23, 24, 27, 28, 33, and 34 (see
\href{http://www2.ocean.washington.edu/flowmow/ecr/ctd}{Flow Mow CTD web site},
www2.ocean.washington.edu/flowmow/ecr/ctd for details and data).}
\label{axial_mean_sig-th+s}
%\label{axial_mean_sig-S}
\end{center} \end{figure}

% consider adding light attenuation plot for comparison with thomson's

The axial average $\theta$, $S$, and $\sigma_\theta$ data in
Figure~\ref{axial_mean_sig-th+s} show the deviation from the background trends
begins at $\sigma_\theta=$27.64\,kg/m$^3$, or $\sim$1860\,m on the background
$\sigma_\theta$ profile (see also Figure~\ref{profile_comp_sigt-th-s}).  The
axial mean values diverge from the background trend until
$\sigma_\theta=$27.656.  At higher densities, the data parallel the background
slope with remarkable consistency, generating average isopycnal temperature and
salinity anomalies of $\Delta_{\rho}\theta\simeq+$0.04$^\circ$C and
$\Delta_{\rho}S\simeq+$0.004\,psu, respectively.  Then, at densities
higher than $\sigma_\theta=$27.666 there is a subtle convergence back toward
the background data.  The observed mean isopycnal anomalies in
Figure~\ref{axial_mean_sig-th+s} are comparable to the means reported by
\citeN{thomson+92} for the neutral plume: 0.05\,$^\circ$C, 0.005\,psu,
and 0.03\,m$^{-1}$ light attenuation.  They noted maximum isopycnal anomalies
in the plume core (2000--2050\,m) of +0.17\,$^\circ$C, +0.016\,psu, and
+0.20\,m$^{-1}$.  

While the average profiles have positive isopycnal anomalies at all depths,
\emph{individual} axial casts commonly exhibit layers of plume fluid
($\Delta_{\rho}\theta>0$) interspersed with background fluid
($\Delta_{\rho}\theta=0$).   Above the ridge crests, individual casts show that
background and plume layers are both common and occur at variable depths,
evidencing a hydrography that is heterogenous; when averaged together spatially
or temporally, however, the individual casts generate a mean profile with
$\Delta_{\rho}\theta>0$ at all depths (Figure~\ref{axial_mean_sig-th+s}).
Within the axial valley, however, background fluid is rarely seen --- even in
individual casts, and the hydrography is less variable.  Valley fluid almost
always has positive $\Delta_{\rho}\theta$ and $\Delta_{\rho}S$, even within
5\,m of the sea floor.  
%Show an example!
Further from the axis and downstream of the volcano (with respect to the mean
flow above the ridge), casts that extend deeper than the valley floor depths 
%suggest that even the positive isopycnal anomalies near the bottom ultimately
%intrude the linear background; descending through the plume depths, 
show that the positive anomalies typical of axial valley depths on axis
decrease to zero below $\sim$2250\,m.  This downstream return to background
conditions below plume equilibration depths has has often been used to justify
the definition of an anomaly by extrapolating a slope from above the plume
through intermediate depths, rather than by fitting a line to data from
background data above \emph{and} below the equilibration depths.

%It is fascinating to think about the entrainment process and plume rise in
%$\theta$--$S$ space.
Plotting these same $\theta$ and $S$ data against each other (rather than
density) also reveals positive temperature anomalies
(Figure~\ref{axial_mean_th-S}), though in this case they are isohaline
(referenced to salinity).  On average, the observed isohaline temperature
anomaly is $\sim$0.05$^\circ$C roughly 2 times the isopycnal anomaly
(\emph{c.f.}~Section~\ref{sec:anomrel}).  As with the isopycnal anomalies,
individual axial casts show highly variable isohaline anomalies above the ridge
crests --- from intense plumes to background conditions at different depths at
different times --- and relatively uniform conditions within the confines of
the axial valley.  Stations as close as 1\,km from the axis often reveal
positive isohaline anomalies near the plume equilibration depths that are
bounded above and below by background fluid.  

\begin{figure}
\begin{center} 
\includegraphics*[width=.75\textwidth]{figs/setting/T2-S2.mean.vs.bkgrd.eps2}
\caption[$\theta - S$ data from Flow Mow background and axial CTD stations]
{The depth-binned average $\theta$ and $S$ from background (solid line) and axial (open 
circle) stations, over the depth range common to all stations.}
\label{axial_mean_th-S}
\end{center}
\end{figure}

A number of mixing processes could be responsible for the departure in
$\theta$--$S$ conditions at the axis from the background trend
(Figure~\ref{axial_mean_th-S}).  The relatively rough topography of the ridge
may cause enhanced vertical mixing by increasing turbulence and boundary layer
thickness, but in the absence of a warm, salty water mass, this process can
only redistribute the $\theta-S$ data along the background trend.  The elevated
(warmer/saltier) values observed over the Endeavour axis could be due to mixing
of background fluid with a variety of other fluid sources: a water mass on the
west side of the ridge that is warmer and/or saltier than the fluid sampled on
the east side at the Flow Mow background stations; deeper, saltier background
water that has been warmed geothermally; shallower, warmer background water to
which salt has been added; or an end member that is both warmer and saltier
than the background.  
% Need to clarify logic and presentation here...  Maybe interpret depth
% profiles first, then discuss mixing in th-s space and idea that the focused
% plumes have a large-scale effect on the isopleth curvature, and then diffuse
% plumes see the perturbed mean profiles and add a "second bulge" to them, and
% sometimes even an average inversion (due to the prevalence of bottom
% heating??  These alternatives are should be illustrated schematically in
% figure \ref{th-s_diag}.  
%Need a schematic here!!

% YES THIS IS A BIG GAPING HOLE that could connect up a lot of pieces: the anom
% definition, the choice of isohaline anoms, the interpretation of th-S plots
% later in this chapter, and...
%
% THE COOL IDEA THAT A POPULATION OF TH-S SOURCE POINTS MIXES THROUGH A
% VARIABLE RANGE OF DEPTHS TO CREATE A POPULATION OF PLUMES WITH TH-S TREND
% DRAWN OFF, AND OFTEN PARALLEL TO THE BACKGROUND TREND!!

Interleaving of distinct water masses across the Mid-Atlantic ridge has been
observed near ridge crest depths \cite{thurnherr00}.  The northeast Pacific,
however, has a much more uniform hydrography.  Stations from multiple years and
both sides of the Endeavour segment suggest that the regional hydrography is
temporally stable and does not show across-axis gradients.
% (Figure \ref{th-s_regional_bkg_comp.eps2}).  

A final and illuminating way to visualize the density, temperature, and
salinity fields over the Endeavour segment is in space.  A depth profile of the
mean density observed at background and axial stations
(Figure~\ref{profile_comp_sigt-th-s}) immediately highlights the dynamic
interaction of the hydrothermal plumes and the deep ocean.  On average, the
fluid above 2000\,m is denser than the surrounding ocean, while the deeper
axial water is less dense than the background.  This is the mean manifestation
of the equilibration process at the segment scale, and represents a dynamic
equilibrium in which the plume layer, supplied by hydrothermal venting, is
actively intruding into the deep ocean.  
%The difference between background and axial data at a particular depth defines
%a depth-referenced, or level-to-level, anomaly (hereafter denoted $\Delta_z$).  

%Discuss \emph{isobathic} versus \emph{isobaric}?  

Geographic grouping of the vertical profiles provides evidence that
hydrothermal activity causes the transition from background to axial
conditions.  Figure~\ref{profile_comp_sigt-th-s} emphasizes how the level
anomalies increase as one progresses from the background stations, through the
axial stations at the north and south ends of the segment, and into the axial
valley.  Both near the bottom and at upper levels, the absolute values of the
level anomalies (\emph{c.f.}~Section~\ref{sec:anomdef})
$\Delta_z\sigma_{\theta}$, $\Delta_z\theta$, and $\Delta_zS$ increase to maxima
at the SoMEF and NoMEF stations, in the vicinity of the MEF.
%specify station by number and on a map??

\begin{figure} \begin{leftfullpage} \caption[Depth profiles of mean
$\sigma_{\theta}$, $\theta$, and $S$: geographic transition] {Depth profiles of
mean $\sigma_{\theta}$, $\theta$, and $S$ from Flow Mow stations grouped
geographically.  Emphasis is on the transitions between background and axial
stations.  Axial stations are: ``Deep S,'' the southernmost stations (23, 24,
27, and 34), from the southeast flank to the Mothra vicinity; ``SoMEF,''
stations 4, 6, and 33, $\sim$500\,m south of MEF; ``NoMEF,'' stations 3, 5, 14,
and 28, $\sim$500\,m north of MEF; ``Deep N,'' stations 17 and 19, just north
and south of the saddle.  } \label{profile_comp_sigt-th-s} \end{leftfullpage}
\end{figure} \begin{figure} \begin{fullpage} \begin{center}
\includegraphics[width=.58\textwidth]{figs/setting/S-N.mean_sgt2_z.eps2}

\includegraphics[width=.58\textwidth]{figs/setting/S-N.meanT2_z.eps2}

\includegraphics[width=.58\textwidth]{figs/setting/S-N.meanS2_z.eps2}
\end{center}
\end{fullpage}
\end{figure}

Given the positive $\Delta_{\rho}\theta$ and $\Delta_{\rho}S$ in
Figure~\ref{axial_mean_sig-th+s} at all densities, a critical question is how
the fluid below $\sim$1975\,m in Figure~\ref{profile_comp_sigt-th-s} has
negative $\Delta_z\sigma_{\theta}$.  At first glance, this density structure in
Figure~\ref{profile_comp_sigt-th-s} indicates vertical mixing, but the $\theta$
profile is not consistent with vertical mixing because it is not displaced to
values cooler than the background above 1975\,m.  The fluid deeper than
$\sim$2075\,m has positive $\Delta_z\theta$ and negative $\Delta_zS$.

A similar geographic grouping of $\theta$--$S$ profiles
(Figure~\ref{geog_th_s}) cannot be explained by mechanical vertical mixing, for
that process could only redistribute depth-bin averaged points along the
background trend.  Instead, the average profiles are displaced furthest from
the near-linear background trend near the center of the Endeavour segment,
where the hydrothermal activity is most intense.  This suggests that
hydrothermal water is being mixed into the deep ocean over a broad region,
affecting the spatially- and temporally-averaged $\theta$--$S$ profiles over
the length of the segment and within $\sim$ 1\,km of the Endeavour axis.  Below
$\sim$2075\,m, the positive $\Delta_z\theta$ and negative $\Delta_zS$ must be
due to a hydrothermal input of relatively warm, fresh water, not simply a
vertical circulation of relatively warm, fresh water from overlying depths.

\begin{figure} \begin{center}
\includegraphics*[width=.75\textwidth]{figs/setting/S-N.meanT2_S2.eps2}
\caption[Endeavour axis $\theta$--$S$ profiles: geographic
transition]{$\theta$--$S$ profiles based on data, averaged in 10\,m depth bins,
from Flow Mow stations grouped geographically and labeled as in
Figure~\ref{profile_comp_sigt-th-s}.} \label{geog_th_s} \end{center}
\end{figure}

%The increase in the magnitude of $\Delta_z\sigma_{\theta}$ in
%Figure~\ref{profile_comp_sigt-th-s} from the north and south ends of the
%segment to the vicinity of the MEF evidences persistent, average, horizontal
%density gradient within the axial valley depths.  If the observed gradient of
%$\sim$0.005\,kg/5\,km is assumed to drive a flow balanced by simple frictional
%term, a noteworthy implication is that the resultant flow has a magnitude
%close to the observed along-axis mean flow that is observed near the MEF
%within the axial valley.  The simple balance in the along-axis ($y$) direction
%can be expressed as \begin{equation} - \frac{1}{\rho_o} \frac{\dif P}{\dif y}
%= \mu v, \label{friction_flow} \end{equation} where $\rho_o$ is a reference
%density (1030\,kg/m$^{3}$), $\frac{\dif P}{\dif y}$ is the observed horizontal
%pressure gradient at 2150\,m, and \mu is the multiplicative inverse of the
%friction time scale (which is, in turn, the turbulent dissipation rate divided
%by the kinetic energy of the flow).  

The observations in Figure~\ref{profile_comp_sigt-th-s} indicate, overall, that
below the equilibration depth, average isopycnal, isohaline, and isotherm
surfaces all dip downward over the axis.  The density profiles imply that
isopycnal surfaces are level near 1975\,m and bowed upward between 1975\,m and
$\sim$1800\,m.  Such downward and upward ``bowing'' of isopycnals has been
observed in many hydrothermal environments (\emph{e.g.}~\citeNP{helfrich+98})
and inspired ideas about plume-induced motions: radial motion in response to
horizontal pressure gradients and pursuant rotation
(\emph{e.g.}~\citeNP{speer89}).  Isohaline surfaces mimic the isopycnal ones,
although they are level at a greater depth, near 2075\,m, and must slope more
steeply than the isopycnals to generate positive $\Delta_{\rho}S$ at all
depths.  Isotherms dip downward at all depths (also more steeply than do
isopycnals) below 1900\,m, causing positive $\Delta_z\theta$; they may bow
slightly upward at shallower depths, accounting for the slight negative
$\Delta_z\theta$ above 1900\,m.  At the north end of the segment the isotherms
begin to dip only below $\sim$1975\,m.  

% An alternative cause of the isopycnal distortion is periodic flow over 
% the ridge topography...  lavelle+cannon01  

% Why is the axial S and sigma crossover with background at different depths
% 1975 vs 2075\,m?

Positive $\Delta_z\theta$ in near-bottom fluid has been attributed to
hydrothermal venting within the topographically-constrained depths of the
Broken Spur segment of the MAR \cite{murton+99}.  There, the depth profile of
\emph{in situ} temperature $T$ is nearly vertical within the confines of the segment,
but about 6.7$\times10^{-4}$\,$^\circ$C/m both at similar depths in the adjacent
Atlantis fracture zone and at depths above the Broken Spur ridge crests.  

East of the Endeavour segment in the Cascadia basin, \citeN{thomson+95}
identify positive $\Delta_z\theta$ relative to a background profile acquired at
the southern end of the basin, near the deep Blanco fracture zone where the
north Pacific deep water (NPDW) is thought to intrude the basin.  They use the
positive $\Delta_z\theta$ to define a ``geothermal boundary layer'' (GBL) that
extends from the basin sea floor at $\sim$2700\,m up to $\sim$2250\,m.  They
posit that the warming is caused by a combination of conductive heat flux
through the sea floor sediments, convective heat flux from diffuse vents on
basement outcrops near 2600\,m, and long-term turbulent mixing of the
geothermal signal as high as 250\,m above the outcrops
\cite[,~Figure~14]{thomson+95}.  A composite of the $\theta$--$S$ data obtained
near the Baby Bare diffuse flow site shows a departure within the GBL from the
near-linear trend in the overlying water.  Inversions in the $\theta$ profiles
were interpreted as plumes from the diffuse vents, while vertical near-bottom
gradients evidenced benthic boundary layers 10--30\,m thick.
\citeANP{thomson+95} also suggest that a change in the slope of the $\theta$
profile near 2250\,m marks the upper boundary of the NPDW; $\dif\theta/\dif z$
decreases from 5.19$\times10^{-4}$\,$^\circ$C/m between 2000--2200\,m to
1.91$\times10^{-4}$\,$^\circ$C/m between 2250--2525\,m.  In comparison, the Flow
Mow background slope is 5.7$\times10^{-4}$\,$^\circ$C/m between 2000-2200\,m, but is
decreased to 4.4$\times10^{-4}$\,$^\circ$C/m within the axial valley, close to the
MEF.  Taken together, these observations imply that the GBL is established over
the Cascadia basin below 2250\,m, but is thickened by additional geothermal
sources near 2250--2150\,m on the Endeavour segment, ultimately extending up to
near $\sim$1900\,m (Figure~\ref{profile_comp_sigt-th-s}).

%bkg-S2.transition_profiles.mean.eps2
%bkg-T2.transition_profiles.mean.eps2
%depth_bin_dth_hist_stn34.10m.gt2100.eps2
%depth_bin_dth_hist_stn24.10m.gt2100.eps2
%depth_bin_dth_hist_stn20.10m.gt2100.eps2

%ALTERNATIVE PHRASING: Plotted against density, both $\theta$ and $S$ have
%positive anomalies at all depths.  Depth profiles of $\theta$ and $S$,
%however, suggest that the deep layer is isobathically fresh and warm.  This
%means that isopycnals must be sloped differently at different depths (as
%predicted by plume equilibration theory) --- plunging into topography below
%the mean equilibration depth ($z^*$) and bowing upward above.  These profiles
%also imply that isohalines dip downward less than isopycnals below 2100\,m, and
%bend upward more than isopycnals above $\sim$2100\,m.  

The same pattern of level anomalies is evident in an average of data from NoMEF
and SoMEF stations, including 2 long (10--14\,hr) VOCs
(Figure~\ref{4meanprofiles}).  The profile of backscatter evidences that the
particle rich plume layer is centered on average near 1980\,m, the depth where
the background and axial mean $\sigma_\theta$ trends intersect.  Note that the
mean values below 2200\,m are derived only from the SoMEF area because the sea
floor depth is $\sim$2200\,m in the NoMEF area.  The mean
$\Delta_z\sigma_\theta$ is positive up to a depth of $\sim$1800\,m.  The main
plume has positive $\Delta_z\theta$ below 1900\,m, and $\Delta_z S$ that is
positive from 1900--2060\,m, but negative at greater depths.  The negative
$\Delta_z\theta$ above 1900\,m may be the result of small changes in the water
properties relative to the background, but may also be evidence that vertical
circulation induced nearby buoyant plumes causes isotherms to bulge upward on
average.  The negative $\Delta_z\theta$ begins in the 50\,m below the top of
the particle plume, a depth range where $\Delta_z S$ and
$\Delta_z\sigma_\theta$ are positive as expected.

Many aspects of the vertical profiles observed at this distance from high $B$
sources are explained by a numerical model of a hydrothermal plume rising from
a 1\,MW/m line source in a steady cross flow of $\sim$1.5\,cm/s
\cite{lavelle94}.  The model includes ambient stratification (with $T$ and $S$
profiles typical of the Juan de Fuca ridge), a cross flow velocity that is
steady $>$100\,mab and decreases to zero at the sea floor, constant source heat
flux, and variable source salinity.  Although the linear nature of the source
exaggerates the vertical recirculation between the plume cap and stem, and may
therefore perturb nearby isopleths more than a plume from a point source
(J.W.~Lavelle, pers.\ comm.), vertical $S$ and $T$ profiles taken through the
model domain upstream and downstream of the source are remarkably similar to
the mean MEF profiles (Figure~\ref{4profileslavelle}).  Source salinities of
\emph{both} 34.7 and 15\,psu result in positive $\Delta_z\theta$ below the
equilibration depth ($\sim$2100\,m) and a $\Delta_z S$ ``dipole.''  The
upstream $S$ plume profile intersects the background profile at a depth
25--50\,m below the equilibration depth, while the downstream $\Delta_z S$ is
zero even deeper, 75--125\,m below 2100\,m.   

% \citeN{helfrich+speer95} develop a model of a steadily venting plume rotating
% in geostrophic equilibrium, and the radial extent of the recirculating
% streamlines and deflected isopycnals is about 1~Rossby radius ($\simeq N Z_m
% / f \simeq $2\,km, where Z_m is the equilibration elevation in mas).
% Rotation acts to impose virtual walls, forcing recirculation and erosion of
% local stratification.  Perhaps it is the mean cross flow that continually
% restores the stratification, leading to the remarkably consistent maximum
% rise height?

%Examination of the temporal sequence of individual casts (about 1 every half
%hour) suggests that the isopycnals move vertically over time, but not with any
%apparent periodicity. 

\begin{figure} \begin{center}
\includegraphics[width=.95\textwidth]{figs/setting/bkg_nosomef.z_th-s-sgt-bks.eps2}
\caption[Observed profiles of mean $\sigma_\theta$, $S$, $\theta$, and
backscatter at NoMEF and SoMEF]{Observed profiles of mean $\sigma_\theta$, $S$,
$\theta$, and backscatter data combined from the NoMEF and SoMEF areas,
$\sim$500\,m north and south of the MEF.  Each circle is the mean of all data
within a 25\,m depth bin centered at that depth.  Note that the deepest 2 bins
include data from the SoMEF area only.} \label{4meanprofiles} \end{center}
\end{figure}

\begin{figure} \begin{center}
\includegraphics[width=.95\textwidth]{figs/setting/lavelle.50.eps}
\caption[Modeled profiles of $S$ and $T$ up- and downstream of a line plume]{
Modeled profiles of $S$ and $T$ 300\,m downstream (A \& B) and 200\,m upstream
(C \& D) of a line plume source.  The solid lines represent background profiles
from the Juan de Fuca ridge, the dotted lines correspond to a source
$S=34.7$\,psu, and the dotted lines are for a fresher source, with
$S=15$\,psu.  Reproduced with permission from \citeN{lavelle94}.}
\label{4profileslavelle} \end{center} \end{figure}

% An additional feature of the NoSoMEF mean profiles is the secondary bulge which ought to be 
% discussed along with the trends in the near-field backscatter profiles

Figure~\ref{4meanprofiles} also displays typical near-bottom conditions within
the axial valley, close to the MEF.  Mean and individual profiles of $\sigma_2$
are often vertical near the valley floor, evidencing a well-mixed boundary
layer that is $\lesssim$50\,m thick.  On rare occasions the near-bottom fluid
is unstable.  The fluid below $\sim$2150\,m typically has $\sim1/5$ the
backscatter intensity found in overlying equilibrated plumes.  Taken together
with the near-bottom mean flow, these observations suggest that the increase in
$\Delta_z\theta$ and $\Delta_S\theta$ toward the center of the segment is
caused by a heat flux to the fluid as it progresses along the axial valley.
This could occur through multiple processes: sea floor geothermal heating of
relatively fresh, clear background water; injection of warm, fresh, clear vent
fluid; or downward recirculation of overlying plume fluid in which the
backscatter signal is decreased.  Subsequent mechanical mixing may help to
homogenize the added heat, resulting in a reduction of $\Delta\theta$ variance
with increasing time and/or distance from a sea floor source.

%The implication for vertical calculation is that south is consistently cooler
%than north.  Can I add evidence that the near-bottom layer (especially below
%2150m) always is clear and has a T-profile toe at 1.70$^\circ{C}$ , whereas
%the toe on north side (and E side?) has toe at 1.72? 
%
%What is the toe, anyways?  Include idea in $theta$--$S$ space that the toe is
%created by diffuse plumes mixing with background fluid (regional and/or
%local), but through entrainment only in the bottom 50\,m, not from 2190 (MEF
%high $B$ source depths) all the way up through 2000 and beyond.  
%
%Consider discussing stns 28/33 th--S for temporal averages (explain the linear
%tails and ``toes''!)

Approaching Endeavour ridge from the east, hydrothermal influence on the
$\theta$--$S$ trend first becomes evident on the outer ridge flank
\cite{veirs+99}.  Above 2100\,m on the southeast flank (only about 1\,km
southeast of the MEF) water properties are nearly indistinguishable from the
background stations.  Below 2100\,m, the water is slightly above background in
$\theta$--$S$ space, but less so than at any more central axial station.
Similarly, a station taken on axis, but at the extreme north end of the
segment, $\sim$15\,km north of MEF, reveals no departure from the background
trend over a wide range of depths (1700--2250\,m).  

In contrast, axial stations located over or between the vent fields always
sample fluid that has positive $\Delta\theta$ (Figure~\ref{nosomef-th-s-sig})
and high backscatter, indicating hydrothermal plume influence.  Above and near
ridge crest depth, background water is interspersed with layers of this plume
water that vary in depth and thickness between sequential casts that are
typically 0.3\,hr apart or offset by $\sim$100\,m.  In this upper layer
(1850--2100\,m), stations taken $\lesssim$1\,km southwest (downstream with
respect to the regional mean current) of known fields tend to be the most
anomalous.  The intermittent returns to the background in this depth range (as
well as below plume depths in distal downstream casts) assure that the
near-linear background relationship can be extended, rather than extrapolated,
through equilibrated plumes, as well as the axial valley depths.

\begin{figure} \begin{center}
\includegraphics[width=.75\textwidth]{figs/setting/bkg_nosomef.th-s-sgt-z.eps2}
\caption[NoSoMEF mean $\theta$--$S$ and $\sigma_\theta$ profile]{ Mean
$\theta$--$S$ and $\sigma_\theta$ profile based on data combined from NoMEF and
SoMEF areas and averaged in 10\,m depth bins.  The equilibration depth,
1980\,m, is associated with the $\sigma_\theta=$27.66\,kg/m$^3$ isopycnal.
Note that isopycnals in the $\theta$--$S$ plot are only parallel to isobaths
(lines connecting bins of the same depth) at the equilibration density,
27.66\,kg/m$^3$; that is, at all depths greater than $\sim$1800\,m the mean
isopycnal surfaces are sloped above and below 1980\,m.}
\label{nosomef-th-s-sig} \end{center} \end{figure}

Within the axial valley ($\gtrsim$2100\,m) near MEF, water with background
$\theta$--$S$ character is extremely rare.  The $\Delta\theta$ of near-bottom
fluid is almost always positive and generally increases along-axis toward the
center of hydrothermal activity.  In $\theta$--$S$ space, the average profile
within the valley typically plots nearly parallel to, but above the background
trend (Figure~\ref{nosomef-th-s-sig}).  

Other investigators have also noted that the fluid below $\sim$2100\,m within
the axial valley has distinctive characteristics.  Based on samples taken near
the MEF, \citeN{rosenberg+88} note that the concentration of radon relative to
$\mathrm{^3He}$ increases with depth, and does so dramatically below 2100\,m.
The most enriched sample was taken from a depth of $\sim$2180, likely
$\sim$20\,mab, and was almost 3 times greater than the next sample above at
2110\,m.  They explain the trend by suggesting that the near-bottom water is
enriched in radon through low $T$ venting and is subsequently entrained and
transported vertically by high $B$ plumes.  This supposition is strengthened by
the observation that the ratio Rn/$T$ is higher at low $T$ vents than at high $T$ vents
\cite{kadko+moore88}.


%\subsection{Variability observed at moorings}

%What are the potential causes of variability?

%\begin{figure} \begin{center}
%\includegraphics*[width=.75\textwidth]{figs/cms/5cm.T.t-series.eps}
%\caption[Moored temperature time series from 5 depths]{Expanded(?) temperature
%time series from the 5 meters on the southern Flow Mow mooring.  What are the
%deep cold events?  What is expectation in non-hydrothermal ridge topography?}
%\label{} \end{center} \end{figure}

%Worth comparing PSD of these series with the velocity series?  Consider juxtaposing 
%velocity or pressure time series from same mooring...  

%\citeN{franks92} presents time series of low-pass filtered data from $T$
%sensors moored along the Endeavour axis from $\sim$1650--2050\,m depths show
%day-to-day amplitudes of 0.03--0.05~$^\circ$C and peak-to-peak ranges of
%0.1--0.2~$^\circ$C during any given 259--337~day record.  Her T3 mooring,
%located $\sim$15\,km southeast of the MEF, provides an important observation of
%how $T$ variability is reduced off-axis.  The 2 meters on the T3 mooring were
%located at 1955 and 2055\,m and recorded $T$ series with day-to-day amplitudes
%of 0.01--0.025~$^\circ$C and peak-to-peak ranges no greater than
%0.05~$^\circ$C.  This could be because isothermal vertical displacements are
%greatly reduced away from ridge topography, or it could indicate that thermal
%variability on-axis is greater than expected from internal wave displacement
%of isotherms because of the proximity of volcanic heat sources.  

% Probably could quote Thomson 90 or 2003 regarding amplification of inertial
% $v$ over ridge, which presumably means amplitude of isotherm displacement is
% also enhanced...  
%
%For what it is worth, the $v$ records from T3 are comparable in magnitude and
%variability to many of the on-axis $v$ records.

%\begin{figure} \begin{leftfullpage} \caption[Power spectra of expanded $T$
%from near-bottom current meters]{ Power spectra of expanded $T$ from
%near-bottom current meters.  } \end{leftfullpage} \end{figure} \begin{figure}
%\begin{fullpage} \begin{center}
%\includegraphics*[width=.6\textwidth]{figs/cms/power/wasp2000--15mab.expT.power.eps2}
%\includegraphics*[width=.6\textwidth]{figs/cms/power/cm1995--25mab.expT.power.eps2}
%\includegraphics*[width=.6\textwidth]{figs/cms/power/scm2000--50mab.expT.power.eps2}
%\label{psd_Texp} \end{center} \end{fullpage} \end{figure}


\subsection{Observations in and near buoyant hydrothermal plumes}
\label{sec:hts}

This subsection advances what is known about the physical characteristics of
rising hydrothermal plumes and how they evolve as the plumes equilibrate within
the ocean.  The focus is on new insights derived from the Flow Mow hydrographic
data set.  Some of the data analyzed here have also been utilized to
estimate the net vertical heat flux from the MEF \cite{stahr+03}.

%\subsection{Previous observations}
%ES: Bemis, Ginster Lupton et al mention buoyant plume Thomson 92 Kadko 
%Rona and Bemis imaging efforts Jackson and Jones results Chemical evolution (particles?) McLaughlin

%\subsection{Flow Mow observations}

During the Flow Mow hydrographic survey numerous rising plumes were intercepted
by the lowered CTD when it was towed horizontally through the MEF at heights of
$\sim$5--60\,mas.  ABE traversed hundreds of plumes as they rose through the top
of the control volume, $\sim$85\,mas.  The final Flow Mow CTD VOT characterized
the plume tops above the MEF.  And the many CTD casts within $\sim$100--1000\,m
of the MEF inform the equilibration process, helping to explain how differences
between axial and background hydrography arise.  

This section is organized as a tour through rising plumes at increasing
elevations above the source.  Examples are taken from the CTD stations during
which buoyant plumes were observed (Table~\ref{ht_table}).
%beginning closest to the sources and ending high above the sources near the
%equilibration depths.  Subsequent sections in this chapter will dissect the
%differences between plume and ambient fluid.

\begin{table}

\caption[$\Delta\theta$ in rising plumes at various depths and elevations]{
$\Delta\theta$ observed at various depths and elevations in rising plumes
during Flow Mow CTD stations.  The depth ranges result from minor changes in
the CTD or ABE depth during a single station (CTD 10 depths refer to horizontal
tow section only).  Elevation ranges are due to both changes in CTD 
%or ABE
depth and the topography of the underlying sea floor and sulfide structures.
Last 2 columns show the elevation at which the buoyant plume was intercepted
and the associated peak $\Delta\theta$ value.}

\label{ht_table}

\begin{center}
\begin{tabular}{lrrrr}
Station& Depth     & Elevation   & Buoyant plume & Peak $\Delta_S\theta$ \\
       & range (m) & range (mas) & (mas)         & ($^\circ$C)           \\
\hline
CTD 10 & 2177--2198 & 05--20 & 07         & 0.79 \\
CTD 11 & 2145--2155 & 25--35 & 22         & 2.71 \\
CTD 15 & 2150--2160 & 25--45 & $\sim$40   & 0.75 \\
CTD 30 & 2140       & 30--60 & 33         & 1.25 \\
CTD 31 & 2110--2125 & 65--90 & 80         & 0.35\\
CTD 32 & 2105--2125 & 50--70 & 70         & 0.72 \\
%ABE  & 21        & -- & \\
\end{tabular}
\end{center}

% Consider plotting with Bemis values?  Ideally over 1-D model of $\Delta_z
% \theta$ profile?
%
% From Ch1:
%From deepest to shallowest thermister, the maximum $\Delta T$ measured during
%this shorter record are 3.10, 1.24, 1.02, 1.28, and 0.71$^\circ$C; the mean
%values of all ``in plume'' $\Delta T$ data (the $T_{hot}$ of
%\citeANP{bemis+93}) for each thermister are 0.86, 0.50, 0.43, 0.40, and
%0.27$^\circ$C.
\end{table} 

Station~10 began as a VOT along the north MEF control surface between
1800--2200\,m.  Toward the end of the station, after encountering multiple
intense plumes at similar positions but on different passes along the surface,
an excursion was made southward in an attempt to intercept buoyant plumes from 
the supposed source, the sulfide structure called Hulk.  A map of the excursion 
and the associated time series are shown in Figure~\ref{stn10}.
% Mention that the locations of Hulk N/S structures may be off by ~30m?

\begin{sidewaysfigure} \begin{center}
\includegraphics*[width=.45\textwidth]{figs/rising/stn10.hitmap.eps2}\hfill
\includegraphics*[width=.45\textwidth]{figs/rising/stn10.hulkhits.tseries.eps2}
\caption[Horizontal tow through buoyant plumes: CTD~10 location map and time
series]{Location map and time series from horizontal tow through buoyant plumes
during CTD 10 which proceeded from north to south.  Left panel symbols: dashed
line is MEF control volume perimeter; each $+$ is a known individual high $B$
source; colored x and * symbols indicate thresholds of $\Delta\theta$ annotated
in upper left corner.  Right panels shows time series of depth and bottom
traces (upper), $\Delta\theta$ (middle), and backscatter (lower).  Bottom depth
values are sum of CTD pressure depth and altimeter reading.} \label{stn10}
\end{center} \end{sidewaysfigure}

The data from Station~10, acquired in and around plumes 5--400\,m above known
sources, provide an unusual opportunity to compare the physical hydrography of
buoyant plumes with the fluid through which they rise -- from the sea floor to
above the equilibration depth.  Figure~\ref{stn10_th-s-sig-z} shows the raw
$\theta$--$S$ distribution and $\sigma_\theta$ profile for the entire station,
including 3~traverses of the north surface (\emph{c.f.}~Figure~\ref{ns_surfs})
and the horizontal tow.  The left panel reveals the degree of scatter that
underlies the average $\theta$--$S$ distribution near the
MEF(\emph{c.f.}~\ref{nosomef-th-s-sig}) --- equilibrating plumes interspersed
with background fluid above ridge and points more uniformly above the
background trend within the valley --- but superimposed are data points from
within the buoyant plume.   The right panel also shows the majority of points
scattered around the mean density profile typical of the MEF vicinity, but the
buoyant plumes are clearly evident in the bottom 100\,m.  Indeed, the magnitude
of the near-bottom $\Delta_z\sigma_\theta$ appears to decrease rapidly with
elevation above the sea floor.  Above $\sim$2030\,m where
$\Delta_z\sigma_\theta$ goes to zero, there is a population of points with
moderate positive $\Delta_z\sigma_\theta$ comparable to the average anomaly,
but there are also individual layers with greater $\Delta_z\sigma_\theta$ and
sharp upper boundaries.  I interpret these signals to be evidence of negatively
buoyant fluid near the MEF plume tops, high above its ultimate equilibration
depth. 

\begin{figure} \begin{center}
\includegraphics*[width=.95\textwidth]{figs/setting/bkgrd_vs_stn10good.th-s.z_sigt2.eps}
\caption[$\theta$--$S$ and $\sigma_\theta$ profile with buoyant plumes: Station
10]{ Raw 0.5\,Hz data from CTD station~10, both the VOT portion along a
cross-axis plane just north of the MEF, and the horizontal tow portion that
passed through a buoyant plume near Hulk.  Left panel shows the $\theta$--$S$
trends over $\sigma_\theta$ isopycnals.  Right panel shows the $\sigma_\theta$
depth profile.  Black trace is the mean of both background stations; blue trace
represents station~10 data.} \label{stn10_th-s-sig-z} \end{center} \end{figure}

Though the scatter of the $\theta$--$S$ data is relatively high in
Figure~\ref{stn10_th-s-sig-z}, it is intriguing to note that a straight line
fit to the points with $\Delta_S\theta>$0.2$^\circ$C constitutes a mixing line
that should point toward the $\theta$ and $S$ of the source fluid.  For the
station~10 data, the line has a slope of about $-0.01$\,psu/$^\circ$C.
Extending this line from a point in the buoyant plume data ($S=34.61$,
$\theta=2.0$) to $\theta_f=$334$^\circ$C observed at Hulk in the year 2000
(D.~Butterfield, pers.\ comm.) suggests that the source
$S\simeq$31.29\,psu.  For comparison, the value of $S$ measured in source
fluid sampled directly from Hulk in 2000 is $\sim$30.56 ([Cl]$=$477\,mM/kg,
D.~Butterfield, pers.\ comm.).  The similarity of the extrapolated and directly
measured values indicates that with enough data from a buoyant plume, source
properties may be accurately inferred.

Station 11 was a horizontal tow that traversed north--south across the MEF,
passed directly over a number of known structures, and intercepted at least 3
distinct, rising hydrothermal plumes (Figure~\ref{stn11}).  It is particularly
noteworthy because it recorded the largest temperature anomaly within a buoyant
MEF plume, 2.71$^\circ$C (Table~\ref{ht_table}), and because the altimeter
trace suggests that steep-sided topography (sulfide structures) underly
\emph{each} major plume.  Station 15 was an almost identical tow and stations
30--32 were horizontal tows that traversed the northern MEF in a roughly
regular grid pattern; both generated results similar to those shown for
station~11, and are therefore not presented graphically.  
 
\begin{sidewaysfigure} \begin{center}
\includegraphics*[width=.45\textwidth]{figs/rising/stn11.hitmap.eps2}\hfill
\includegraphics*[width=.45\textwidth]{figs/rising/stn11.tseries.eps}
\caption[Horizontal CTD tow 11: location map and time series]{ Location map and
time series from horizontal tow through buoyant plumes during CTD station~11,
which proceeded from north to south.  Left panel symbols: dashed line is MEF
control volume perimeter; each $+$ is a known individual high $B$ source;
colored x and * symbols indicate thresholds of $\Delta\theta$ annotated in
upper left corner.  Right panels (top to bottom) show time series of: (1)
$\Delta\theta$, (2) backscatter, ``bks,'' (3) Redox potential, ``Eh'' (values
decrease in reducing solutions, K. Nakamura, pers.\ comm.), (4) difference in
potential density observed at upper and lower CT pairs
(\emph{c.f.}~Figure~\ref{ctd_abe_pix}), indicating fluid instability
\cite{veirs+99}, and (5) depth and bottom traces (based on CTD depth and
altimeter readings).  } \label{stn11} \end{center} \end{sidewaysfigure}


A final feature of the horizontal tows through the interior of the MEF control
volume is the relative uniformity of $\Delta\theta$ \emph{between} the rising
plumes.  While it is clear that the source $\theta$ values rapidly with
elevation in the rising plumes (Table~\ref{ht_table}), the ambient fluid within
the bottom $\sim$75\,m has relatively uniform $\Delta\theta$.  For $\sim$30
short sections taken from the horizontal tow stations ($\sim$2110--2200\,m
depth), the mean $\Delta\theta$ observed between adjacent rising plumes is
$\sim$0.05$\pm$0.02$^\circ$C during.  The $\Delta\theta$ conditions between
many plumes are completely homogeneous, with very low variance relative to the
time series acquired within the buoyant plumes.  In a few cases there are
gradual departures or minor steps from the otherwise constant $\Delta\theta$
values between plumes; these are interpreted as evidence of diffuse plumes
adjacent to focused plumes.  The transition into rising plumes from the steady
ambient $\Delta\theta$ values is generally abrupt, indicating sharp
gradients at the plume edge (\emph{c.f.}~Figure~\ref{stn10}), despite the
expectation of Gaussian time-averaged temperature profiles from laboratory
plume analysis \cite{papanicolaou+list87}.

%\begin{figure} \begin{center}
%\includegraphics*[width=.75\textwidth]{figs/rising/stn11.dTh2.zoom.eps2.eps}
%\caption[]{ } \label{} \end{center} \end{figure}

This tour through the Flow Mow control volume completes the setting of the
stage for calculating and interpreting heat fluxes.  The next 2 chapters
present hydrographic observations on the surfaces of the control volume.  The
$\Delta\theta$ fields on the surfaces greater than $\sim$50--100\,mab are
surprisingly heterogeneous relative to the ambient fluid observed near the
center of the MEF on the horizontal tows.  In contrast to the variability
observed on the MEF perimeter, the uniformity of the $\Delta\theta$ within the
MEF control volume makes it easier to characterize the fluid that is likely to
be entrained by MEF plumes.  This characterization will help to justify the
method of calculating net heat flux from the lower control volume in the next
Chapter.

%Pull together peak anoms as a function of height with inter-plume anom levels
%(also as a function of mas).  This should both summarize this section, provide
%comparison to the dilution predicted by 1-D models, and help justify the
%subtraction in the vertical heat flux calculation.


%\subsubsection{$\theta$ and $S$ character}

%Chance to establish uniformity of fluid in between buoyant plumes.

%Chance to show that dT2/d2x is rarely very steep on field scale, implying that
%diffusive fluxes are negligible...

%\subsubsection{Anomalies of $\theta$ and $S$}

%In their presentation of hydrographic data collected within a 1\,km$^2$ area
%centered on the MEF, \citeN{thomson+92} plot isopycnal anomalies of $S$ and
%$\theta$ against each other and find a linear relationship with a positive
%slope.  They describe the positive slope as ``problematic'' because the
%1-dimensional plume model of \citeN{speer+rona89} predicts profiles of $T$ and
%$S$ that generate a negative slope, even for source salinities less than sea
%water salinity.

%Flow Mow observations, constrained to those data within the same a 1\,km$^2$
%area, show...  Insert figure showing all FM data scatter plot of these
%anomalies.  Do buoyant plumes plot off the linear trend?  To what extent are
%anomalies above 1900m non-zero?  

%All 1-dimensional plume models based on the theory of \citeN{Morton+56} have
%solutions that describe both the rise of the plume and its (infinite)
%oscillation at its equilbration depth.  It is critical to realize, however,
%that many implementations of 1-dimensional plume models only display the
%properties of the fluid at the centerline of the rising plume, and usually
%only consider the character of a plume rising from a source that has near-zero
%initial $\Delta_{\rho}S$.   [Check papers and insert refs to those that take
%this approach in previous sentence...] This is a segue to presenting the McD
%numbers and model results for a full range of source S...

%Because of the massive dilution of the source properties, no matter the value
%of $S_o$, the fluid injected at or near $z^*$ always has positive anomalies of
%$\theta$ and $S$ in the northeast Pacific, no matter the value of $S_o$ or the
%anomaly reference.  Pooling and streaming can concentrate or dilute the
%injected fluid and thereby generate a distribution of hydrothermal $\Delta~S$
%and $\Delta\theta$ that will always have a positive coefficient of
%proportionality.  Furthermore, the relationship will be linear if the injected
%fluxes of $S$ or $\theta$ are constant.  

%This line of reasoning 
%and the differences between the Thomson and FM scatter plots suggests that the
%\citeANP{thomson+92} field data sampled equilibrated plumes, exclusively.
%Even the maximimum anomalies they observed at ``a few isolated points''
%(0.17$^\circ$C, 0.016\,psu) plot in line with the lesser anomalies; if they
%were measurements made in buoyant plumes, then the source $\Delta S_o$ was
%near zero.

%[These differences highlight the extent to which the Flow Mow hydrographic
%data have increased the resolution of plumes, and especially buoyant plumes,
%relative to past studies \cite{thomson+92,veirs+99}.]

%\subsubsection{Density}

%Bill notes (in comment on FM1 discussion sxn) that in numerical models, plumes
%may separate from the bottom when the ambient stratification into which they
%rise has, as is typical, only slight vertical gradients near the sea floor.
%One can imagine steep density profiles near the sea floor in which plume rise
%would be arrested...  
%
%Is there an elegant way to showcase ``typical'' density profiles?  NoSoMEF
%stacks? 


%\subsubsection{Velocity structure}

%Maybe should exclude this section for efficiency?

%cite Thomson ADCP paper?  Rona?  Compare with Jackson (cite?)...

%Bill: Downward velocity at ABE survey depth?

% Compare Bemis values at 20\,m with ABE depth values
%
% ABE w histograms?

 
%\subsubsection{Instabilities and spatial gradients}

%This is a chance to utilize the offset CT pairs a bit... \cite{veirs+99}.

%Where (if at all) should a reference to new field observations (in an appendix?) be made?


%\subsubsection{Plume dimensions and displacements}

%Clearly there is the problem of not knowing if a plume is traversed through
%its center or along a chord...


%%%% DISCUSSION???  (Extra notes...)

%(Interesting to note that \citeN{franks92} defines terms: ``above-plume,''
%``plume-core,'' and ``beneath-plume'' based on the ``characteristic, layered
%structure of the water overlying the [Main Endeavour] vent field.''  In her
%corresponding schematic (Figure II.1), the boundary between plume-core and
%beneath-plume layers is distinctly above the ridge crest... while in the text
%she specifies that the above-plume zone typically extends from 1800\,m up, the
%plume-core is typically centered between 1900--2050\,m, and the below-plume
%layer extends downward to the sea floor.  She also cites Thomson, Gordon,
%Dymond 91 for variable thickness of spreading plume and common tier-like
%structure spanning several hundred meters vertically, as well as observation
%that upper boundary is sharper than lower.  She also cites thomson roth dymond
%and little+87 when suggesting boundaries may shift in depth due to lateral
%flow, topography, and changes in volume and buoyancy of effluent...)

