FORCING OF ANOMALOUS SEA-SURFACE TEMPERATURE EVOLUTION IN THE TROPICAL ATLANTIC DURING PACIFIC WARM EVENTS

The seasonal evolution of anomalous interhemispheric sea surface tempe rature (SST) gradients in the tropical Atlantic from January to April is studied by composites of the 10 warmest (warm) and 10 coldest (cold ) Januaries during 1948-1993 in the equatorial Pacific using Comprehen sive Ocean-Atmosphere Data Set ship observations. In the warm as compa red with the cold years, an anomalous weakening of the northward SST g radient develops, mainly due to anomalous warming in the tropical Nort h Atlantic. This stems from the combination of three forcings all rela ted to the weakened North Atlantic trade winds during Pacific warm eve nts. Most important are the reduced latent heat flux in much of the tr opical North Atlantic and anomalous downwelling equatorward of 20 degr ees N, with a further contribution from increased net radiation result ing from the reduced cloudiness due to the diminished convergence in t he downstream portion of the North Atlantic trades. In response to the development of warm anomalies in the tropical North Atlantic during J anuary-March, the cross-equatorial northward winds accelerate to April , and this leads south of the equator not only to Ekman downwelling, e nhanced divergence, reduced cloudiness, and increased net radiation, b ut also to enhanced wind speed and evaporation. The result is a modest anomalous warming in the western tropical South Atlantic. Increased a ir temperature over the entire basin, presumably due to atmospheric ad vection from the Pacific, has little effect on the Atlantic SST patter n. The anomalous interhemispheric SST gradient, controlled primarily b y the warm anomaly in the North Atlantic, has a pivotal role to play i n steering the late boreal winter atmospheric circulation in the tropi cal Atlantic sector and thus regional climate anomalies.