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%                    H Y D / I N T R O . T E X 
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%                    dlm: Wed Jun  7 22:00:58 2000
%                    (c) 2000 A.M. Thurnherr
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In this chapter the hydrographic and velocity data collected during the
two quasi-synoptic surveys of the 1997 \F \rsec{F-methods} and the 1998
\FF \rsec{F2-methods} cruises are analyzed and interpreted. The main
goal is to provide the hydrographic context for the analysis of the
current-meter records \rchap{CM} and of the hydrothermal plume
signatures \rchaps{plume}{NESPI}.

In \rsecNP{HYD-1997} the 1997 survey is presented. First, the regional
hydrography above the MAR is analyzed, including the signatures of the
Mediterranean water tongue and of the Azores Current. Within the rift
valley the water column was less dense and stratified than both eastern
and western off-ridge water, with $T/S$ characteristics consistent with
inflow across a sill from the east. The data from the rift valley west
of \RR contain signatures consistent with interleaving of western
off-ridge water above \m{2000}. Below \m{2000}, the rift-valley
hydrography and dynamics were dominated by a hydraulically controlled
NE-ward along-valley flow across \RS. Large-amplitude internal waves
consistent with tidal forcing were observed near the sill. Above
\m{2000} the mean flow across the topography generated lee waves which
radiated energy upwards and downstream.

In \rsecNP{HYD-1998} the 1998 survey is presented. A comparison of the
\ThetaS properties of the two surveys suggests that the rift-valley
water was of the same origin in both years. No interleaving of western
water was observed in 1998. Below \m{3000} the water had cooled by
\degC{0.06} but at the depth of the current meters (\m{1800$--2100$})
no systematic temperature trend rises above the variability encountered
during each survey. Direct velocity measurements indicate that the flow
below \m{2100} was similar in 1997 and 1998 but that it had reversed
direction at \m{1800}. Along-segment distributions of temperature,
salinity and density below \m{2500} are characterized by unidirectional
gradients consistent with along-valley mixing but not with geothermal
heating. Near the southern end of the segments hydrographic signatures
of inflow from the eastern ridge flank were observed. The chapter
concludes with a discussion in \rsecNP{HYD-discu}.

