On the Atmospheric Response to SST Anomalies Associated with the Atlantic Warm Event during 1984

Abstract

The impact of sea surface temperature (SST) anomalies observed during the Northern Hemisphere spring of 1984, which include the growing phase of an intense Atlantic warm event on the atmospheric circulation over the tropical Atlantic and Pacific is investigated using the nine-layer, low resolution version of the UCLA general circulation model. This impact is contrasted with that for the same period during 1983, when SST anomalies include the decaying phase of the strongest Pacific El Niño on record. Results obtained in control and anomaly simulations, consisting, respectively, of extended integrations with and without the observed SST anomalies, are analyzed. It is found that simulated anomalies in the atmospheric circulation corresponding to 1984 include low-level westerlies over the equatorial Atlantic and easterlies over the equatorial Pacific. There are centers of anomalous low-level convergence and divergence off the northeast coast of Brazil and equatorial Brazil, respectively, which are associated with positive and negative precipitation anomalies. Differences between results corresponding to 1984 and 1983 show the impact of El Niño over the Pacific. Further, positive precipitation anomalies over the equatorial Atlantic shift from generally north of the equator in 1983 to south of the equator in 1994 (dry and wet years for northeast Brazil, respectively). These simulated anomalies and interannual differences in the atmospheric circulation are in good general agreement with those observed. This agreement strongly suggests that the atmospheric anomalies observed during the northern springs of 1984 and 1983 over the tropical Atlantic and Pacific were primarily due to the SST anomalies.