A RECENT CHANGE IN THE MEAN STATE OF THE PACIFIC BASIN CLIMATE - OBSERVATIONAL EVIDENCE AND ATMOSPHERIC AND OCEANIC RESPONSES

The first half of the present decade has been characterized by anomalo us conditions in a number of Pacific basin atmospheric and oceanic var iables. The sea surface temperature (SST), in particular, has been war mer than normal over vast areas of the subtropics and the Gulf of Alas ka. The SST anomaly pattern is shown to be part of a long-term trend w hich began in the early 1970s. This trend is the dominant source of SS T variability over significant portions of the basin. Experiments with atmospheric models demonstrate that the atmospheric anomalies of the 1990s are consistent with this change in SST and the mechanisms for th is are analyzed. Finally, experiments with an ocean general circulatio n model are used to investigate which features of the atmospheric anom alies are responsible for the observed SST anomalies. It is determined that wind stress changes and changes in heat flux due to wind speed r eductions are the most likely causes. Two potentially important mechan isms for positive ocean-atmosphere feedback leading to the climate ano malies of the 1990s have thus been identified.