The effect of local sea surface temperatures on atmospheric circulation over the tropical Atlantic sector

The effects of tropical Atlantic sea surface temperature (SST) anomalies on atmospheric circulation are examined by analyzing several ensembles of int egrations of an atmospheric general circulation model (AGCM) forced with di fferently configured SSTs. An attempt is made to separate the atmospheric r esponse to local SST forcing from internal atmospheric variability, using v arious statistical analyses. The analyses reveal a robust pattern of atmosp heric response to SST forcing. The dominant response is largely confined wi thin the tropical Atlantic sector and may be associated with the variation in location and intensity of the intertropical convergence zone (ITCZ) in r esponse to changes in SST gradient near the equator. Within the deep Tropic s, particularly in the western tropical Atlantic warm pool region, there is an indication of a positive feedback between surface heat flux and SST ano malies. In this warm SST region, the latent hear flux tends to dominate sur face heat flux variability, and the positive feedback takes place between t he wind-induced Aux and SST. Outside the deep Tropics, surface heal flux ge nerally tends to dampen the SST, indicating a negative feedback. A strong n egative feedback is found off the coast of west Saharan Africa, where the m ean SST is cold and the surface heat Bur variability is largely induced by the air-sea temperature difference. The forced response exhibits a seasonal dependence. The structure throughout the year bears a close resemblance to the winter pattern, suggesting that the anomalies during the boreal winter contribute to much of the year-to-year variability in the tropical Atlanti c sector. An ENSO-like response in the tropical Atlantic is also identified . It appears as the second dominant forced response and has its strongest m anifestation during the boreal fall. By comparing the responses of differen t sets of ensemble experiments, it is found that the dominant near-surface atmospheric response in the tropical Atlantic sector primarily comes from t he local SST forcing. A significant remote influence of the Pacific ENSO on the tropical Atlantic variability is also noted. The strongest remote infl uence from the tropical Pacific occurs during the boreal spring. The compan ion paper by Saravanan and Chang further explores the effect of ENSO using an index-based regression analysis.