%======================================================================
%                    H Y D / F L A M E 2 . T E X 
%                    doc: Tue Nov 16 14:02:23 1999
%                    dlm: Wed Jun  7 22:22:21 2000
%                    (c) 1999 A.M. Thurnherr
%                    uE-Info: 280 33 NIL 0 0 72 3 2 4 ofnI
%======================================================================

\Subsection{HYD-FF2}{Changes in the Rift-Valley Hydrography and
Flow Regime Between 1997 and 1998}

A plot of the pressure-averaged \ThetaS properties of the rift-valley
stations occupied in 1998, the corresponding eastern background
station, and the western background station occupied in 1997 (no
western background station was occupied during \FFp{}{)} results in a
figure which is visually indistinguishable from \rfigNP{F-tscompare}
except for slight changes in the Mediterranean signature of the eastern
background station. The near-surface temperature was higher in 1998
(\degC{23} vs.\ \degC{19}) while the corresponding salinity had
decreased (from \psu{36.3} to \psu{36.2}). Because the depth of the
\degC{15} isotherm had decreased as well (from \m{280} to \m{205}), the
Azores Front is inferred to have been situated south of \A during the
1998 survey (c.f.\ \rsecNP{F-regional}). This is supported by the
observation (from the uncalibrated hull-mounted ADCP) that the strong
currents in the top \m{300} of the water column observed in 1997
had disappeared in 1998.

%--------------------
\begin{figure}[t]

\placeFig{HYD/tscompare}

\caption[1997/98 rift-valley \ThetaS comparison]{\ThetaS comparison of
the \F and \FF data below \m{\approx\!1600}; the rift-valley profiles
are derived from pressure-averaged CTD stations in the 1997 sampling
region; the eastern background profiles of the two surveys are shown as
well; \SigmaTwo contour interval is 0.025.}

\labelFig{tscompare}

\end{figure}
%--------------------

\rFigNP{tscompare} shows the pressure-averaged rift-valley \ThetaS
characteristics of the \R region in 1997 and 1998 (10 stations) as well
as those of the eastern background station occupied in both years.
There is a slight difference between the eastern background properties
of the two surveys at densities $>$36.9 (i.e.\ below
\m{\approx\!1750}). Interestingly, the rift-valley hydrography of the
1998 data set appears more consistent with the eastern background
properties of the 1997 survey. Below \degC{4} the hydrographic
properties of the 1997 and 1998 rift-valley data are identical (within
the measurement uncertainties). Above \degC{4} the \ThetaS properties
of the 1997 data set diverge from the linear trend because of the
interleaving signatures observed in the SW basin \rsec{F-waterorigin}
whereas no similar effect is seen in the corresponding properties of
the 1998 data set. Inspection of the individual profiles confirms that
no interleaving structures were observed in 1998. The similarity of the
corresponding \ThetaS properties below \degC{4} indicates that it is
unlikely that the origin of the rift-valley water had changed between
the surveys.

%--------------------
\begin{figure}[t]

\placeFig{HYD/tempcompare}

\caption[1997/98 rift-valley temperature comparison]{Comparison of the
potential temperature ranges (minimum and maximum values are plotted)
observed during the 1997 and 1998 surveys; the horizontal lines
indicate the nominal depths of the moored current meters
\protect\rsec{CM-methods}.}

\labelFig{tempcompare}

\end{figure}
%--------------------

To assess possible changes in the rift-valley hydrography the density,
temperature and salinity ranges of the two surveys were compared;
\rfigNP{tempcompare} shows the temperature profiles. Depths \m{>\!2500}
are not shown because the 1998 data set contains only 3 profiles from
the \R region extending below this limit. The temperature ranges are
indistinguishable within the variability encountered during each
survey. The same observation holds for the salinity and for the density
data.

%--------------------
\begin{figure}[t]

\placeFig{HYD/deeptscompare}

\caption[1997/98 deep-basin \ThetaS comparison]{\ThetaS comparison of a
1997 and a 1998 profile taken in the deep SW basin; both profiles are
truncated at \m{3200}; \SigmaTwo contour interval is 0.01.}

\labelFig{deeptscompare}

\end{figure}
%--------------------

While these comparisons indicate that changes in the rift-valley
hydrography above \m{2500} between 1997 and 1998 were small compared to
the short-time-scale variability, \rfigNP{F-sigcompare} reveals that
comparing the hydrographic properties at greater depths can reveal more
precise answers. In the SW basin there is one CTD profile extending
below \m{3200} from each survey (the horizontal distance between the
stations is \km{1.4}); the corresponding deep \ThetaS properties are
shown in \rfigNP{deeptscompare}. Extrapolating the temperature profiles
to the seafloor at \m{3300} yields minimum temperatures of \degC{3.655}
\Fp{(}{)} and \degC{3.597} \FFp{(}{)}. This implies a cooling of the
bottom water of order \degC{0.06}, a value significantly greater than
the temperature calibration uncertainties
\rsecs{F-methods}{F2-methods}. The near-perfect agreement between the
salinities is fortuitous (observed salinity differences between
neighboring profiles of the 1998 data set are of order \psu*{3}{-3},
consistent with the salinity sensor accuracy).

\rFigNP{F2-CM} shows the average velocities recorded during the last
full week of current-meter deployment. Comparison with \rfigNP{F-CM}
indicates that the cross-sill flow below \m{2100} was similar in 1997
and 1998 while it had reversed direction and increased in strength at
\m{1800}. The current meters from mooring ``F'', which is not shown in
the figure, indicate NE-ward mean flow of \mps{0.02$--$0.03} below
\m{2100} (c.f.\ \rfigNP{sigsec}) and SW-ward flow of \mps{0.02} at
\m{1800}. Strong weekly-averaged SW-ward currents across \RS at
\m{1800} had been recorded during the 2 preceding weeks as well,
peaking during the penultimate week of mooring deployment (c.f.\
\rfigNP{CM-tempvarflow}). The lack of interleaving signatures in the
1998 data is consistent with the reversed flow at \m{1800} and provides
further support for the hypothesis that the origin of the interleaving
water in 1997 was west of \RR \rsec{F-waterorigin}.

\begin{figure}[t]

\placeFig{HYD/F2_CM}

\caption[1998 current-meter velocities (last week)]{Average
current-meter velocities in the vicinity of \RR from the last full week
of mooring deployment; shading of the arrows is determined by the
instrument depths with black, gray and white indicating \m{2300},
\m{2100} and \m{1800}, respectively; bathymetric shading is the same as
in \rfigNP{AMAR-bridge}; contour interval is \m{250}.} 

\labelFig{F2-CM}

\end{figure}

In the 1998 data set the mean density profiles from the NE and the SW
basin do not show any patterns which can be related to the flow field.
This is most likely because of the small number of stations occupied in
the vicinity of \RS \rfig{F2-stations} which does not allow the
calculation of representative mean hydrographic profiles.

%----------------------------------------------------------------------

\Subsection{F2-grad}{Along-Segment Hydrographic Gradients}

\begin{figure}[t]

\placeFig{HYD/F2_tgrad}

\caption[1998 along-segment temperature gradient]{Scatter plot and
linear fit of the 1998 temperature measurements at \m{2550}; the
regression coefficient ($\pm1\sigma$) and RMS error are
\protect\degCpkm*{1.0(\pm0.1)}{-3} and \protect\degC*{1.3}{-2},
respectively; the correlation coefficient is $r=0.96$.}

\labelFig{F2-tgrad}

\end{figure}

During the 1998 survey CTD stations were occupied along the entire
length of the \A segments between \latN{35}{30} and \latN{36}{40}
\rfig{F2-stations}. To assess along-segment gradients in the deep water
all profiles extending below \m{2550} were selected (north of \RS there
is only one profile extending below \m{2600}). These deep stations are
marked with black-and-white stars in \rfigNP{F2-stations}. One profile
with anomalous hydrographic properties below \m{2175} \rsec{F2-inflow}
was removed from the data set. The \ThetaS properties of the remaining
stations are indistinguishable within the measurement uncertainties 
below \m{2000}, i.e.\ there are no indications for geothermal heating
(the temperature differences are compensated by corresponding salinity
differences). \rFigNP{F2-tgrad} shows a scatter plot and linear fit of
the along-segment temperature observations at \m{2550}. The regression
coefficient ($\pm1\sigma$) and RMS error are \degCpkm*{1.0(\pm0.1)}{-3}
and \degC*{1.3}{-2}, respectively. (The data are plotted against
latitude for easy location but the gradient is calculated in terms of
distance to aid in comparison with other sites; total segment length is
\km{150}; \rsecNP{AMAR}.\REM{dist("35:30", "-34:20", "36:40", "-33:30")
= 150km; 36:40-35:30=1.16666667} For comparison, the temperature
gradient of segment ``C'' surveyed by
\citeN{Progress/saunders+francis85} is \degCpkm*{\approx\!0.8}{-3},
estimated from the \degC{0.1} warming over \km{120} horizontal
distance.) The magnitude of the RMS error is consistent with the
high-frequency variability at \m{2300} recorded by the moored
temperature sensors around \RR \rsec{CM-tempvar}.\REM{Only two
profiles\REM{28 & 12} from the 1998 survey extend below \m{3000}; the
horizontal distance between the two stations is
\km{70}.\REM{dist("36:12.59","-33:56.95","35:40.04","-34:21.15") =
70km} The resulting potential temperature gradient at \m{3200} is
\degCpkm*{10.9}{-4}, similar to but less well constrained than the
gradient at \m{2550}.} 

Applying the same analysis to the salinity measurements results in a
gradient estimate and RMS error of \psupkm*{7.8(\pm2.6)}{-5} and
\psu*{1.5}{-3}, respectively. The RMS error is consistent with the
salinity fluctuations associated with the high-frequency variability,
estimated from the temperature fluctuations and the \ThetaS properties.
Finally, the along-segment density gradient and RMS error are
\kgpmmmpkm*{-1.0(\pm0.1)}{-4} and \kgpmmm*{1.8}{-3}, respectively.

%----------------------------------------------------------------------

\Subsection{F2-inflow}{Inflow Into \sA}

\begin{figure}[t]

\placeFig[1]{HYD/inflow}

\caption[1998 inflow-sill density section]{Density section from the CTD
tow-yo on the rift-valley side of the sill near
\latN{35}{35}/\lonW{34}{15} \protect\rfig{F2-stations}; the bathymetry
is taken from the CTD altimeter; the shaded area (\SigmaTwo$\geq
36.9449$) shows isopycnal downsloping and spreading consistent with
inflow from the southeast; contour levels were selected for uniform
spacing with depth in the final upcast.}

\labelFig{F2-inflow}

\end{figure}
%--------------------

As discussed in sections~\ref{sec:F-waterorigin} and~\ref{sec:HYD-FF2},
the rift-valley \ThetaS properties of both surveys suggest inflow from
the eastern ridge flank. The most likely location of the inflow is near
the southern end of the \A segments where the densest water is found
\rsec{F2-grad}. This is consistent with the observed unidirectional
NE-ward flow across \RS (\rsecNP{LADCP} and \rchapNP{CM}). The
available bathymetry (\rfigsNP{AMAR-bridge}{AMAR-SS97},
\citeN{JGR/detrick+95}, and an unpublished \emph{IFREMER}
high-resolution bathymetric data set) indicate that the most likely
inflow sill is located near \latN{35}{35}/\lonW{34}{15}.
\rFigNP{F2-inflow} shows a density section from the rift-valley side of
that sill; total sampling time of the CTD tow-yo was approximately
\h{4}. The down-sloping and spreading of the isopycnal surfaces in the
shaded area is consistent with inflow from the southeast. The densest
water observed in this section has a \SigmaTwo value of
36.970\REM{inflow/Y.2db} which corresponds to a depth of \m{2200} in
the eastern off-ridge profile, located approximately \km{50} NE of the
sill.

To estimate the volume flux into the \A segments the hydraulic model of
\fullciteN{misc/whitehead+74} is used (c.f.\ \rsecNP{hydraulics}).
Evaluating the non-dimensional parameter of \citeN{JPO/pratt86} yields
a value of $P\approx0.02$ (using \km{l=5} and \m{h=300}, estimated from
\rfigNP{F2-inflow}), indicating that frictional effects may be ignored
at this sill as well. In addition to the tow-yo section there was only
one CTD station occupied near the sill \rfig{F2-stations}. Therefore,
there is some uncertainty as to the bifurcation depth and the
upstream/downstream density difference; the best estimates for the
upstream interface height and reduced gravity are \m{\hu=250$--$300}
and \mpss*{g'=1.8}{-4}, respectively. At the bifurcation depth the sill
is wide (\km{12}\REM{dist("35:32", "-34:08", "35:32", "-34:16")})
compared to the Rossby radius of deformation (\km{\approx\!2.5}) and
the volume flux estimate becomes \mmmps*{65$--$95}{3}. Because of the
good agreement between a similar calculation and direct flow
observations at \RS \rsec{hydraulics}, the inflow velocity is estimated
from the interfacial gravity wave speed of the \OneAndAHalfLayer
hydraulic model resulting in a value of \mps{U\approx0.2}.

Not all of the rift-valley water must necessarily flow in across a
single sill. The \ThetaS properties below \m{2175} recorded at a
station near \latN{35}{40}/\lonW{34}{20} \rfig{F2-stations} are
characterized by a positive salinity offset of order \psu*{3}{-3} with
respect to the remaining profiles of the 1998 survey. (Because the
salinity offset is apparent in both the down- and the up-cast
instrument problems can be excluded.) Whether the deep water at this
station has a different origin or whether it is a remnant of an earlier
inflow event across the same sill cannot be determined from our
data.\REM{The Mn distribution increases linearly with depth in that
basin indicating that the water may be stagnant.} There are no similar
signatures in any of the other profiles from either survey. We also
occupied stations near two sills at the northern end of the \A segments
\rfig{F2-stations}. The respective \ThetaS properties show no
indications for different water masses which could indicate additional
inflow across these sills (see also \rchapNP{mixing}).

%----------------------------------------------------------------------

\REM{\Subsection{F2-longsill}{\sA Overflow}

%--------------------
\begin{figure}[t]

\placeFig[1]{HYD/longsill}

\caption[1998 \sA density section]{Density section between \latN{35}{40}
and \latN{36}{12}; the bathymetry has been interpolated at
\protect\km{1} intervals between the stations; shaded areas and contour
levels are the same as in \protect\rfigNP{F2-inflow}.}

\labelFig{longsill}

\end{figure}
%--------------------

Within the \sA segment the bathymetry shallows towards the north before
descending into the deep SW basin near \RR \rfig{AMAR-bridge}. To
investigate the density distribution across this elongated ``sill'' a
number of CTD stations were manually selected along a line representing
the along-valley bathymetry. \rFigNP{longsill} shows the resulting
along-valley density section. Contouring was done without taking the
bathymetry into consideration because the valley is wide enough so that
the water can flow around isolated bathymetric highs. The bathymetry,
interpolated at \km{1} intervals, is shown to indicate the approximate
bathymetry and roughness of the valley floor. Total sampling time of
this data set is \h{36}. Similar to observations from \RS
\rfig{sigBGT07} and the inflow sill \rfig{F2-inflow}, there are
indications for downsloping and spreading of the isopycnal surfaces
downstream of the ``sill saddle''.}
