Influences of Tropical Western and Extratropical Pacific SST on East and Southeast Asian Climate in the Summers of 1993–94

This study emphasizes the importance of sea surface temperature (SST) over the tropical western Pacific and the ocean–atmosphere coupling in the extratropical Pacific for the climate in East and Southeast Asia. Specifically, it demonstrates that the anomalies of tropical SST explain many features of the climate variability in those regions during the summers of 1993 and 1994. Very different atmospheric circulation patterns appeared in East and Southeast Asia between 1993 and 1994. Many regions including northern China, Korea, and Japan suffered from extremely high temperatures and severe droughts in the summer of 1994 but experienced reverse climate anomalies in the summer of 1993. To the south of these regions, the opposite climate patterns occurred. These climate features do not really resemble those associated with the El Niño–Southern Oscillation, which usually exerts a moderate impact on the East Asian climate. However, different SST anomalies have been observed in the tropical western and extratropical Pacific in the spring and summer between these two years. The authors carried out a series of simulations using an atmospheric circulation model and a slab oceanic model to understand the influences of these SST anomalies on the climate features. Both the uncoupled atmospheric and coupled oceanic–atmospheric experiments indicate that the tropical western Pacific SST affects the East and Southeast Asia climate significantly. Warming in the tropical western Pacific produces hot, dry conditions in northern China, Korea, and Japan, and opposite climate signals to their south. These climate anomalies produced by the local SST resemble the observed climate difference between the summers of 1994 and 1993 when positive and negative SST anomalies, respectively, existed in the tropical western Pacific. The coupled experiment also shows that the changes in extratropical atmospheric circulation caused by tropical SST anomalies generate changes in the extratropical Pacific SST, which, in turn, reinforces the climate signals produced by the tropical SST. On the other hand, the uncoupled experiments forced by the extratropical Pacific SST anomalies show that the extratropical SST exerts an insignificant impact on the East and Southeast Asian climate. The change in this SST between 1994 and 1993 generates unrealistic climate patterns in some East Asian regions, accompanying an unnatural shift of the atmospheric circulation.