Institute of Atmospheric Physics Chinese Academy of Sciences
In this study, we analyzed the intraseasonal variation of the western Pacific subtropical high (WPSH) in El Ni?o decaying summers and found two distinct modes, i.e., the west mode and the east mode. The west mode exhibits a consistent westward extension from June to August. The east mode shows a westward extension during June and July, but shifts to an eastward retreat in August. The anomaly in the west mode is much larger than that in the east mode. For the west mode, due to the forcing from a positive sea surface temperature (SST) anomaly in the tropical North Atlantic (TNA), there is a teleconnection pattern from the TNA via high latitude of Eurasia to East Asia. As a result, geopotential high in East Asia tends to increase associated 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. Besides, the positive SST anomaly in the TNA reaches a maximum in April and tends to decay afterwards. Therefore, the warm pool convection in August cannot be suppressed sufficiently by the SST forcing over the two oceans. On the other hand, the warm pool convection begins to develop due to the local air-sea interaction associated with the SST rise from June to July. In this case, the WPSH tends to retreat eastward with reduced geopotential height. The result indicates that the SST anomaly in the TNA plays a major role in the WPSH anomaly in August. When predicting the WPSH in El Ni?o decaying summer, we must consider the influence of the SST anomaly over the two oceans.