A Numerical Simulation of the Variability in the Tropical Atlantic Ocean, 1980–88

Abstract

The authors have examined a numerical simulation of the tropical Atlantic Ocean circulation forced by the European Centre for Medium-Range Weather Forecasts (ECMWF) surface wind stress during 1980?88. The mean state and annual cycle of the ocean are realistically simulated by the model. The simulated interannual variability of sea surface temperature (SST) is also remarkably consistent with the observations, particularly the observed patterns of the basinwide warm/cold periods and the variation of dipole pattern and the associated meridional SST gradient. Discrepancies between observed and simulated SST anomalies are large early in the simulation, which seems caused by errors in the ECMWF wind analysis during that period. The low-frequency fluctuations of the meridional SST gradient associated with the dipole pattern during 1980?88 were caused by opposite SST anomalies between hemispheres, forced by out of phase fluctuations of the trade winds. Specifically, the southeast trade winds were anomalously strong during 1981?83 and weaker than normal during 1985?86 and 1987?88. The northeast trade winds, on the other hand, showed nearly opposite variation, being weak in 1980?83 and strong in 1985?86. In the northern ocean, SST was higher during 1980?83 but lower during 1985?86 as the local trade winds were weak and strong. On the other hand, as the southeast trades and the equatorial easterlies were strong in 1981?83, the slope of the thermocline was anomalously steep along the equator and both the South Equatorial Current and the Equatorial Undercurrent intensified. Forced by the anomalous equatorial easterlies, warm water diverged from the equator into the Tropics in the western ocean. In the southeastern portion of the basin, the thermocline was shallow and SST was anomalously low. When the southeast trade winds were weakened in 1984, warm water converged toward the equator from both hemispheres, and then shifted into the southeast part of the ocean. The heat anomalies were maintained there during 1985/86, when the southeast trades were weak, deepening the thermocline and causing anomalously high SST. Therefore, unlike those in the northern Tropics, SST fluctuations in the southeastern part of the ocean are related to the basinwide adjustment of the ocean and net heat transport across the equator in response to the equatorial wind anomalies.