The Critical Role Of ENSO Combined Modes On The Maintenance Of Anomalous Anticyclones In The Northwest Pacific In Summer

This study demonstrates the main physical mechanism for the maintenance of the western North Pacific anomalous anticyclone(WNPAC)during the El Ni?o decaying summer by analyzing the respective effects of the local air-sea interaction,Indian Ocean capacitor effect,and the El Ni?o and Southern Oscillation(ENSO)Combination mode(C-mode).The main conclusions are as follows:(1)The WNPAC is usually accompanied by significant WNP cold SST anomalies in the El Ni?o mature winter and following spring,which almost disappear in the decaying summer and therefore cannot explain the maintenance of the WNPAC in summer.(2)The influence of the IO warm SST anomalies exhibits conspicuous decadal differences in maintaining WNPAC in summer.Before the 2000 s,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 2000 s.This decadal difference may be related to changes in the decaying speed of ENSO events.In contrast to El Ni?o events before the 2000 s,El Ni?o events after the 2000 s decay more rapidly,and the associated tropical central-eastern Pacific SST features a La Ni?a-like condition in the El Ni?o decaying summer.Concomitantly,no significant warm SST anomalies appear over the tropical IO,exerting a weak influence on the WNPAC.(3)Relative to the WNP and IO SST anomalies,the C-mode,originating from the nonlinear interaction between ENSO and the annual cycle,has a stable relationship with the WNPAC during the El Ni?o decaying summer for both pre-and post-2000 periods.The feasibility of the mechanism appears invulnerable to changes in the ENSO evolution feature(i.e.,ENSO decaying speed),suggesting a crucial role of the C-mode in the maintenance of the WNPAC.Considering the persistence of the ENSO C-mode,the WNPAC and associated climate variability during the El Ni?o decaying summer can be skillfully predicted at least one season in advance based on the C-mode.