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 integrations of an atmospheric general circulation model (AGCM) forced with differently configured SSTs. An attempt is made to separate the atmospheric response to local SST forcing from internal atmospheric variability, using various statistical analyses. The analyses reveal a robust pattern of atmospheric response to SST forcing. The dominant response is largely confined within the tropical Atlantic sector and may be associated with the variation in location and intensity of the intertropical convergence zone (ITCZ) in response to changes in SST gradient near the equator. Within the deep Tropics, particularly in the western tropical Atlantic warm pool region, there is an indication of a positive feedback between surface heat flux and SST anomalies. In this warm SST region, the latent hear flux tends to dominate surface heat flux variability, and the positive feedback takes place between the wind-induced Aux and SST. Outside the deep Tropics, surface heal flux generally tends to dampen the SST, indicating a negative feedback. A strong negative feedback is found off the coast of west Saharan Africa, where the mean 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 Atlantic sector. An ENSO-like response in the tropical Atlantic is also identified. It appears as the second dominant forced response and has its strongest manifestation during the boreal fall. By comparing the responses of different sets of ensemble experiments, it is found that the dominant near-surface atmospheric response in the tropical Atlantic sector primarily comes from the local SST forcing. A significant remote influence of the Pacific ENSO on the tropical Atlantic variability is also noted. The strongest remote influence from the tropical Pacific occurs during the boreal spring. The companion paper by Saravanan and Chang further explores the effect of ENSO using an index-based regression analysis.