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 anomalous conditions in a number of Pacific basin atmospheric and oceanic variables. The sea surface temperature (SST), in particular, has been warmer than normal over vast areas of the subtropics and the Gulf of Alaska. The SST anomaly pattern is shown to be part of a long-term trend which began in the early 1970s. This trend is the dominant source of SST 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 this are analyzed. Finally, experiments with an ocean general circulation model are used to investigate which features of the atmospheric anomalies are responsible for the observed SST anomalies. It is determined that wind stress changes and changes in heat flux due to wind speed reductions are the most likely causes. Two potentially important mechanisms for positive ocean-atmosphere feedback leading to the climate anomalies of the 1990s have thus been identified.