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%                    H Y D / D I S C U . T E X 
%                    doc: Tue Feb 29 15:10:30 2000
%                    dlm: Wed Jun  7 22:31:26 2000
%                    (c) 2000 A.M. Thurnherr
%                    uE-Info: 75 36 NIL 0 0 72 67 2 4 ofnI
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% obs consistent with S+F

The two hydrographic surveys carried out in 1997 and 1998 are fully
consistent with the observations of
\citeN{Progress/saunders+francis85}. Within the rift valley of the \A
segments the density and stratification are reduced in comparison to
the water column on the ridge flanks. The \ThetaS properties of the
rift-valley water suggest inflow from the east where the water is
denser than at the same depth on the western ridge flank. In 1998 we
observed along-segment hydrographic gradients consistent with inflow
near the southern end of the segments. A density section from the most
likely candidate sill connecting \sA to the eastern ridge flank
indicate inflow of the right magnitude to account for the flow
observations at \RS (see also \rsecNP{CM-asflow}).

% inter-annual hydrographic variability

While the 1997 survey did not allow along-segment gradients to be
calculated, the mutual consistency of the rift-valley \ThetaS
properties during the two surveys indicates that the situation was most
likely similar in both years. Whereas the \ThetaS properties did not
change, there are indications for a cooling of approximately
\degC{0.06} at \m{3200}. Such a change is consistent with time-varying
inflow, e.g.\ caused by eddies or variable boundary currents
\cite{Progress/saunders+francis85}. Nevertheless, the weekly averaged
current-meter records from the beginning and from the end of the
mooring deployment show similar flow across \RS below \m{2100}
indicating that the relationship between forcing and response should be
investigated (c.f.\ \rchapNP{BOF}). Above \m{2500} the high-frequency
hydrographic variability is too high to determine any change between
the two surveys.

% along-valley gradients explained

The availability of high-quality hydrographic data along the entire \A
segments allows the conceptual model of
\citeN{Progress/saunders+francis85} \rsec{BG-MAR} to be extended. The
rift-valley \ThetaS properties indicate that the observed along-segment
temperature gradient is not (primarily) caused by geothermal heating
(c.f.\ \rchapNP{mixing}). There are two different processes which can
account for the observations: \begin{description}

\item[Topographic Blocking.] It is unlikely that topographic blocking
plays a significant role because the gradient calculations are based on
measurements at \m{2550} which is shallower than the rift valley
everywhere except for a \km{7} stretch near \latN{35}{55}
\rfig{AMAR-bridge}. 

\item[Diapycnal Mixing.] The rough bathymetry of the \MAR is known to
be associated with high rates of diapycnal mixing
\citeEG{Science/polzin+97,Nature/ledwell+00}. It has been hypothesized
that much of the energy available for diapycnal mixing on a global
scale is dissipated by breaking internal waves \citeEG{JPO/polzin+95}.
The isopycnal variability on tidal time scales and the topographic wake
observed in 1997 are therefore consistent with high rates of diapycnal
mixing. Our observations furthermore suggest that hydraulically
controlled sill flows may be common in the rift valley of the \MAR.
Direct measurements elsewhere have shown greatly enhanced dissipation
in such overflows \citeEG{JGR/wesson+gregg94,Nature/polzin+96},
providing an additional mechanism for enhanced diapycnal mixing on the
\MAR.

\end{description} The mutual consistency between the hydrographic data
from the two surveys indicates that the rift-valley flow had most
likely not changed significantly between 1997 and 1998. This is
supported by the average current-meter velocities recorded during the
first and the last weeks of mooring deployment which show similar
NE-ward flow below \m{2000} across \RS. Hydrographic data collected in
the \A segments in 1992 \cite{JGR/wilson+95} are consistent with the
along-segment gradients of the \FF data set. Additional evidence for
persistent NE-ward flow near \RR is available for 1994
\cite{GRL/german+96} and 1996 \cite{EOS/german+96}, suggesting that the
along-segment flow and hydrographic gradients may not have changed
significantly for years.

% transition region

\REM{Hydrographic and flow observations above \m{2000} indicate that
the transition region between the rift-valley and the overlying water
column can be complex. Not surprisingly, there were no interleaving
signatures of western off-ridge water observed in 1998 when the flow at
\m{1800} was SW-ward. The most likely inflow of western water in 1997
was across the sill near \latN{36}{25}/\lonW{34}{10} \rfig{AMAR-SS97}.}

% generalization

Unidirectional along-valley hydrographic gradients have been observed
elsewhere, e.g.\ by \citeN{Progress/saunders+francis85} in the \MAR
rift valley near \degN{45} and by \citeN{Nature/ledwell+00} in a canyon
on the western flank of the MAR near \degS{20}, raising the possibility
that insights gained from a study of the \A segments may be valid at
other sites as well.

