Hydrography and high‐temperature heat flux of the Rainbow hydrothermal site (36°14′N, Mid‐Atlantic Ridge)

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Abstract
On the basis of an extensive set of conductivity‐temperature‐depth, lowered acoustic Doppler current profiler (LADCP), and nephelometry profiles and tow‐yos the hydrography, flow field, and particle plume of the Rainbow hydrothermal site on the Mid‐Atlantic Ridge are analyzed. In the rift valley the water column is 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 buoyant hydrothermal plumes rise into a strong boundary current flowing along the slope of a topographic high which partially blocks the rift valley below 1950 m. The bulk of the neutrally buoyant plume is advected across a sill which forms both the narrowest and the shallowest part of the valley and acts as a hydraulic control point for the flow below 2000 m. Large‐amplitude internal waves consistent with tidal forcing are observed near the sill, but LADCP measurements suggest that the tidal signal is not strong enough to lead to flow reversal at plume depth. Above 2000 m the mean current across the topography generates lee waves which radiate energy upward and downstream. Density‐averaged light‐scattering profiles show the hydrothermal particle plume to be Gaussian in depth, even in the near field, where many of the individual profiles are characterized by multiple peaks and the horizontal variability is highest. The temperature anomalies associated with the mean near‐source particle plume are of order −5×10−3°C; that is, the plume is cold/fresh as expected from the background hydrography of the deep Atlantic. Using the flow field, light‐scattering, and hydrographic anomaly observations, the heat flux associated with the hydrothermal particle plume at Rainbow is estimated to lie between 1 and 5 GW.