Wintertime convection in warm‐core rings: Thermocline ventilation and the formation of mesoscale lenses

Large heat losses from warm-core ring 82B during February, March, and April 1982 formed a 400-m-thick thermostad in the center of the ring. Two different models were constructed to simulate the formation of the thermostad water mass properties: a one-dimensional vertical model forced with a meteorological time series and a volumetric mixing model with climatological input. Both models indicate that a flux of slope water into the ring is necessary to yield the observed thermostad salinities. The subsequent slow decay of the ring core in the slope water suggests that such winter-convected rings can easily maintain the “warm band” water identified by Wright and Parker (1976). The large loss of core water during interactions with the Gulf Stream suggests that the reabsorption of such winter-cooled warm rings is an important source of high-salinity, high-oxygen anomalies in the main thermocline of the Sargasso Sea. The volume of water in a typical ring reabsorption is sufficient to generate many (>20) intrathermocline eddies with T-S properties close to those of recently reported lenses. This suggests that many of these small eddies are much more local in origin and may be more highly dissipative than has previously been suggested. An estimate of the flux of high-salinity, high-oxygen water into the main thermocline by this mechanism leads to ventilation at a rate that is a significant fraction of the Ekman pumping at these density levels. Thus the reabsorption of winter-cooled warm-core rings by the Gulf Stream is an important eddy ventilation mechanism for the Sargasso Sea.