The variability of the tropical Atlantic

Abstract

In the present work we simulate the equatorial Atlantic variability at annual and interannual timescales using a coupled mixed layer - isopycnal ocean general circulation model (OGCM) forced with observations for the period 1980-1989. Surface features such as the seasonal variations of the equatorial system of currents or of the temperature are quite satisfactorily simulated with a relatively coarse model. For the interannual variability, the model reproduces quite well the observed warmings and coolings of the Gulf of Guinea, except for the 1981 event. The anomalies appear in the simulation earlier than in observations, and their magnitude is overestimated. These characteristics of the simulated interannual variability are shared with other simulations of the tropical Atlantic using a very different OGCM and different setup. The generation of the events is monitored through the anomalous heat content field. Although this generation can be explained in a first approximation as "the displacement to the east of warm water accumulated in the west", there are other mechanisms at work that account for the differences in the generation of the 1984 and 1988 events. Through statistical analysis the main characteristics of this field are related to other output variables and to signals in the filtered forcings. Meridional convergence in surface velocities plays a significant part in the appearance of anomalies at the Gulf of Guinea. Transition from onset stage to peak phase is connected to extraequatorial signals in the northwestern tropical Atlantic. Differences between the two simulated events (1984 and 1988) can be related to a signal in the wind forcings north of 18°N, and near the coast of Africa. Copyright 1998 by the American Geophysical Union.