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%                    N E S P I / I N T R O . T E X 
%                    doc: Mon Feb  7 15:37:33 2000
%                    dlm: Wed Jun  7 23:24:09 2000
%                    (c) 2000 A.M. Thurnherr
%                    uE-Info: 116 0 NIL 0 0 72 3 2 4 ofnI
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In this chapter the hydrographic anomalies associated with the \R
hydrothermal plume are analyzed in detail. The main goal is to quantify
the temperature anomalies which are used to calculate the
high-temperature convective heat flux from the hydrothermal plume
observations of 1997 \rchap{HFLUX}.

Both conductive heating and hydrothermal circulation contribute to the
geothermal heat flux to the ocean, with the latter dominating near the
axes of mid-ocean ridges \citeEG{JGR/stein+stein94}. Conductive
geothermal heating increases the water temperature, which implies a
decrease in density. Hydrothermal circulation introduces heat into the
water column as well but the resulting hydrographic signatures are
often complicated by salinity anomalies in the hydrothermal effluents.
Depending on the buoyancy flux, the background stratification and the
flow field, the effects of geothermal heating can be distributed in the
water column by a combination of turbulent diffusion and convective
plumes. Entrainment of background fluid into buoyant plumes results in
vertical transport of heat and salt, further complicating the
hydrographic signatures.

% GEOTHERMAL HYDROGRAPHIC ANOMALIES (definition, use)

Geothermal hydrographic anomalies have been used to detect hydrothermal
vent fields \citeEG{EOS/rona+74,Nature/weiss+77}, to calculate heat
fluxes \citeEG{EPSL/baker+massoth87,EPSL/thomson+92,GRL/helfrich+98},
to estimate effluent dilutions in hydrothermal plumes
\citeEG{Nature/lupton+85}, to determine mid-depth circulation pathways
\citeEG{GRL/reid82}, to estimate age differences of abyssal water
\cite{DSR/joyce+86}, and to assess geothermal effects on basin-scale
circulation patterns
\citeEG{EPSL/stommel82,JGR/joyce+speer87,JPO/hautala+riser93,DSR/thompson+johnson96}.

% ESTABLISHING THE HYDROGRAPHIC BACKGROUND INCL TIME-INTEGRATED EFFECTS

In order to isolate the geothermal anomalies a \emph{hydrographic
background} must be defined. In regions of small temporal and spatial
variability this background can be taken from stations which are free
from the geothermal signatures being studied \citeEG{GRL/helfrich+98}.
Where this is not feasible the background must be estimated from the
geothermally modified profiles. Two methods which have been used for
this purpose are based on the observation that geothermal processes
produce anomalies in otherwise linear background $T/S$
\cite{Nature/weiss+77} and $T/\rho$ \cite{Nature/lupton+85}
relationships. The resulting hydrographic anomalies will be called
\emph{isohaline} (in salinity space) and \emph{isopycnal} (in density
space), respectively. Interpretation of isopycnal and isohaline
anomalies in the vicinity of hydrothermal vent fields is often
difficult because the anomalies can be caused by the integrated effects
of a range of geothermal processes, both local and distant in space and
time. \citeN{DSR/joyce+86}, for example, use the basin-scale
distribution of isohaline temperature anomalies in the abyssal North
Pacific to derive water-age differences of the order of 100 years while
closely related isopycnal anomalies \rsec{NESPI-relations} are
sometimes used to derive heat flux estimates for nearby hydrothermal
vent fields, assuming that local effects dominate
\citeEG{EPSL/baker+massoth87}. Even if the observed anomalies are
caused entirely by processes associated with the nearby vent fields,
the potential for plume-fluid re-circulation complicates their
interpretation \citeEG{EPSL/thomson+92}.

% ISOPYCNAL ANOMALIES => MCDOUGALL90

The total geothermal heat content of the water column is equal to the
spatially integrated \emph{isobaric} (in pressure or depth space)
temperature anomalies. Spatially integrated isopycnal and isohaline
anomalies, on the other hand, are not equal to the heat content,
because of the geothermal effects on the density and salinity fields,
respectively \citeEG{EPSL/mcdougall90}. Sometimes, isohaline anomalies
are used as a substitute for isobaric anomalies
\citeEG{DSR/joyce+86,JGR/thomson+95}. Isohaline and isobaric anomalies
are equal if there are no geothermal effects on the salinity field,
i.e.\ in the absence of salinity anomalies in the hydrothermal
effluents (if any) and without vertical salinity fluxes in convective
plumes. Therefore, isohaline temperature anomalies are particularly
useful for the calculation of conductive geothermal fluxes.

% OUTLINE

In \rsecNP{NESPI-mcd90} the hydrographic effects caused by hydrothermal
plumes are discussed using an expression derived by
\citeN{EPSL/mcdougall90} which indicates that both cold/fresh and
warm/salty anomalies are possible independent of the sign of the
background salinity stratification. In \rsecNP{NESPI-relations}, it is
shown that methods based on $T/S$ and $T/\rho$ linearity are
equivalent, and that isohaline temperature anomalies are equal to the
corresponding isopycnal anomalies corrected with the expression of
\citeN{EPSL/mcdougall90}. In \rsecNP{NESPI-bracket}, a new anomaly
definition (and an associated method) is presented which uses the
limits of the hydrothermal particle anomalies to determine the
hydrographic background properties of individual profiles. Applying the
new method to the 1997 CTD data, the corresponding light-scattering and
isopycnal temperature anomalies are found to be linearly correlated
within individual profiles. This observation is used to develop a
second new method which determines the hydrographic background from
measurements within the hydrothermal particle plume \rsec{NESPI-bilin}.
Close agreement of the vertical structure of the corresponding
light-scattering and temperature anomaly profiles in the data sets from
the two surveys and the mutual consistency of the near-field anomaly
regressions indicate that the distribution of both properties in the \R
plume is determined by dilution of the hydrothermal effluents
\rsec{NESPI-R}. A comparison of the hydrographic anomalies derived with
the new methods to those defined as deviations from hydrographic trends
in the water column overlying three Pacific plumes indicates that heat
anomalies derived with the ``traditional'' method can be dominated by
signatures not associated with hydrothermal particle plumes
\rsec{NESPI-Pacific}. The chapter concludes with a discussion in
\rsecNP{NESPI-discu}.
