Intraseasonal Variation of the Western Pacific Subtropical High during the El Niño Decaying Summers

In this study, we analyzed the intraseasonal variation of the western Pacific subtropical high (WPSH) during the El Niño decaying summers and observed two distinct modes, i.e., west and east modes. The west mode exhibits a consistent westward extension from June to August. The east mode shows a westward extension during June and July but retreats eastward in August. The anomaly in the west mode is larger than that in the east mode. For the west mode, because of forcing from a positive sea surface temperature (SST) anomaly in the tropical North Atlantic (TNA), a teleconnection pattern from the TNA via the high latitude of Eurasia to East Asia is observed. As a result, the geopotential height of East Asia tends to increase in association with suppressed convection in the warm pool region, inducing the westward extension of the WPSH with strong intensity. The SST anomaly in the tropical Indian Ocean for the east mode is similar to that for the west mode but with a relatively smaller magnitude. Moreover, the positive SST anomaly in the TNA reaches its maximum in April and tends to decay afterward. Therefore, the warm pool convection in August cannot be suppressed sufficiently by SST forcing over the two oceans. By contrast, the warm pool convection begins to develop from June to July because of the local air–sea interaction associated with the increase in SST. In this case, the WPSH tends to retreat eastward with the decrease in geopotential height. The results indicate that the SST anomaly in the TNA plays a major role in the WPSH anomaly in August. When predicting the WPSH during the El Niño decaying summer, we must consider the influence of the SST anomaly over the two oceans.