Mixed layers in gulf stream rings

Abstract

Immediately after formation, warm and cold Gulf Stream rings are visible in satellite infrared images of sea-surface temperature (SST) as pools of anomalous water. It is now well documented that the surface expressions of cold core rings quickly disappear, while warm rings, summertime excepted, remain visible to satellite infrared sensors throughout their lifetimes. From XBT data, one sees evidence of strong air-sea exchange and deep mixed layers in warm rings, but no unusual mixed layer development in cold rings. We apply a simple one-dimensional annual mixed layer model to Gulf Stream rings to explain these phenomena. The key ingredients of the model are the different atmospheric states over the Sargasso Sea and the Slope Water, the relative density structures of warm and cold rings, and the difference in mixed layer response to heating and cooling. Several features of the surface water behavior in warm and cold rings are qualitatively reflected in the model results. Quantitative comparisons between model predictions and observations are also encouraging, suggesting that the SST evolution of a ring can be attributed to local air-sea exchange.