Westward-propagating SST Anomalies and Baroclinic Eddies in the Sargasso Sea

Abstract

Previous studies have documented the existence of a zonal band of strong, persistent, westward-propagating sea surface temperature (T(s)) anomalies with zonal wavelengths of almost-equal-to 800 km and periods of almost-equal-to 200 days that are confined to the subtropical convergence zone (STCZ, roughly 26-degrees-32-degrees-N). Two years of satellite-derived sea surface temperature (T(s)) and sea surface elevation anomaly (eta) maps of the Sargasso Sea (22.5-degrees-33.5-degrees-N, 71.5-degrees-59.5-degrees-W) are analyzed to determine how these anomalies are forced and why they are confined to the STCZ. A simple anomaly model forced by horizontal eddy currents and damped by a linear feedback mechanism explains many properties of the anomaly response. At wavelengths exceeding several hundred kilometers, forcing by horizontal eddy currents becomes less important relative to atmospheric forcing with increasing wavelength. The anomalies are confined to the STCZ partly because the large mean T(s) gradient there enables the horizontal eddy currents to be relatively effective at forcing anomalies. Also, the eddies that force these anomalies, wavelike features with wavelengths of approximately 800 km and periods of approximately 200 days, are themselves confined to the STCZ. These wavelike eddies were not detected during earlier experiments such as MODE because the domains within which they were conducted were too small. Within the STCZ, zonal dispersion properties of the eddy field are consistent with baroclinic Rossby wave variability. To the north and south of the STCZ, however, zonal dispersion properties differ substantially from the properties observed within the STCZ. The eddy dispersion properties change abruptly across transition zones 1-2 degrees wide centered at 32.5-degrees and 25.5-degrees-N. A simple linearized reduced-gravity model is used to demonstrate that interaction between eddies and zonal mean currents can qualitatively account for the change is dispersion properties south of the STCZ, but not to the north within the Gulf Stream recirculation region.