Westward-Propagating Sst Anomaly Features in the Sargasso Sea, 1982-88

Sea surface temperature (T(s)) maps of the region from 59.5-degrees to 75.5-degrees-W, 22.5-degrees to 33.5-degrees-N containing the western North Atlantic Subtropical Convergence Zone (STCZ) were derived from AVHRR/2 images. The 7-year mean annual cycle was removed and the maps were filtered in space and time to represent anomaly variability with wavelengths greater-than-or-equal-to 220 km and periods greater-than-or-equal-to 50 days. Warm and cold anomaly features were observed east of 71-degrees-W between 26-degrees and 32-degrees-N that propagated westward at 3-4 km day-1 and that occasionally exceeded +/- 1-degrees-C in amplitude. They are generally strong and persistent from fall to spring, and are only marginally detectable during summer. During 1981-82, 1982-83, and 1985-86, individual features could be followed through the entire fall-spring interval. During 1983-84, 1986-87, and 1987-88, they could typically be followed for 2-4 months, and during 1984-85, for only 1-2 months. The features were anisotropic during all fall-spring intervals except 1986-87, and they had characteristic wavelengths of approximately 800 km in the minor axis direction and periods of approximately 200 days. Local forcing by synoptic atmospheric variability alone could not account for the existence of these features. Anomaly features propagated westward in a manner consistent with theoretical zonal dispersion properties of first-mode baroclinic Rossby waves, suggesting that the anomalies may be coupled to a field of wavelike eddies. Since the anomalies were confined to the zonal band of large mean meridional T(s) gradients associated with the STCZ, where meridional eddy currents are relatively effective at forcing anomalies these eddy currents could be largely responsible for their existence. In one case, however, the influence of eddies on vertical heat flux at the mixed layer base appeared to be important. The relatively strong and persistent 1985-86 anomaly features appeared during a several-day interval at the onset of relatively stormy fall weather and (presumably) rapid mixed-layer deepening.