Important role of the ENSO combination mode in the maintenance of the anomalous anticyclone over the western North Pacific in boreal summer

This study demonstrates the main physical mechanism for the maintenance of the western North Pacific (WNP) anomalous anticyclone (WNPAC) during the El Nino decaying summer by analyzing the respective effects of the cold sea surface temperature (SST) anomalies in the WNP, the warm SST anomalies in the Indian Ocean (IO), and the El Nino and Southern Oscillation (ENSO) combination mode. We find that the WNPAC is usually accompanied by significant cold WNP SST anomalies in the El Nino mature winter and following spring, which almost disappear in the decaying summer and cannot explain the maintenance of the WNPAC in summer. The influence of the IO warm SST anomalies on the WNPAC exhibits conspicuous decadal differences. Before the 2000s, the IO warm SST anomalies play a role in the WNPAC maintenance through the response of the baroclinic atmospheric Kelvin wave; however, this effect cannot be evidently detected after the 2000s. This decadal difference may be related to changes in the decaying speed of ENSO events. In contrast to El Nino events before the 2000s, El Nino events after the 2000s decay more rapidly, and the associated tropical central-eastern Pacific SST features a La Nina-like condition in the El Nino decaying summer. Concomitantly, no significant warm SST anomalies appear over the tropical Indian Ocean, exerting a weak influence on the WNPAC. Relative to the cold WNP SST anomalies and warm IO SST anomalies, the ENSO combination mode, originating from the nonlinear interaction between ENSO and the annual cycle, has a relatively more stable relationship with the WNPAC during the El Nino decaying summer, which exhibits a crucial role in the maintenance of the WNPAC. Considering the persistence of the ENSO combination mode, the WNPAC and associated climate variability during the El Nino decaying summer can be skillfully predicted at least one season in advance based on the ENSO combination mode.