Intrinsic Ocean–Atmosphere Variability of the Tropical Atlantic Ocean

Abstract

The tropical Atlantic variability is composed of three major patterns of significant importance for variability and predictability of climate in the Atlantic sector. They are the southern tropical Atlantic (STA) pattern with anomalous sea surface temperature (SST) fluctuations expanding from the Angolan coast to the central equatorial ocean, the northern tropical Atlantic (NTA) pattern centered near the northern African coast, and the southern subtropical Atlantic (SSA) pattern in the open subtropical ocean. Previous studies have suggested that both the regional air?sea coupling and remote forcing from outside the basin may affect the formation of these patterns and their variability. A specially designed global coupled ocean? atmosphere general circulation model, which eliminates air?sea feedback outside the Atlantic, reproduces the major features of these observed patterns realistically. This suggests that these patterns originate from air?sea coupling within the Atlantic Ocean or by the oceanic responses to atmospheric internal forcing, in which there is no anomalous forcing external to the Atlantic Ocean. The effect of the Pacific El Niño?Southern Oscillation (ENSO) seems to modulate their temporal evolution through influencing atmospheric planetary waves propagating into the basin. One of the problems of the model simulation is that the STA pattern as represented by the SST fluctuations centered at the Angolan coast is weak in the equatorial waveguide. Unlike the observations, the model SST fluctuations around the equator are largely unconnected with the changes in the southeastern part of the ocean. This lack of connection between these two parts of the tropical ocean is related to a model systematic bias of excessive southward shift of the model intertropical convergence zone to around 10°S in boreal spring. In the coupled model, the air?sea feedback forms an artifical ?warm pool? to the south of the equator extending from the Brazilian coast nearly to the eastern boundary. This warm pool blocks the connection between the fluctuations in the equatorial and the southern part of the ocean. Due to this systematic bias, this model did not simulate the STA pattern adequately. Several sensitivity experiments have been conducted to further examine the mechanisms of the anomalous SST patterns. The results demonstrate that both the NTA and SSA patterns are mainly associated with the thermodynamic air?sea interactions, while the STA pattern is likely more closely associated with the dynamical response of the equatorial and tropical ocean to the surface wind forcing. Moreover, results from a simulation with a time-independent correction term of the surface heat flux show that the simulated STA mode can be significantly strengthened and have a more realistic spatial structure if the model mean SST errors are reduced.