Westward-propagating SST Anomaly Features in the Sargasso Sea, 1982–88

Abstract

Sea surface temperature (Ts) maps of the region from 59.5° to 75.5°W, 22.5° to 33.5°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 ≥ 220 km and periods ≥ 50 days. Warm and cold anomaly features were observed cast of 71°W between 26° and 32°N that propagated westward at 3?4 km day?1 and that occasionally exceeded ±1°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 ?800 km in the minor axis direction and periods of ?200 days. Local forcing by synoptic atmospheric variability alone could not amount 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 hand of large mean meridional Ts 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 an 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.