Interannual Relationship between the Tropical SST and Summertime Anticyclone over the Western Pacific

This study aims to clarify the processes in maintaining the low-level anomalous anticyclone (AAC) over northwestern Pacific (NWP) in different El Niño decaying summers (denoted as JJA(1)). Observed El Niño events from 1950-2016 are categorized into short-decaying, long-decaying, and prolonged types based on Oceanic Niño Index (ONI) in JJA(1). Composite method is applied to analyze the sea surface temperature anomalies (SSTA), low-level atmospheric circulation and rainfall.

The composite fields of the short-decaying events in El Niño developing fall SON(0), are featured with a strong low-level quadrupled circulation, driven by tripole heating distribution (heating-cooling-heating) over tropical Eastern Pacific (EP)-Eastern Indian Ocean (IO)-Western IO that correspond to warm-cold-warm SSTA. The AAC in the northwestern quarter of the quadruple is located over IO. From SON(0) to DJF(1), the cooling center and the AAC migrate eastward from Eastern IO to Western Pacific (WP) because of the abrupt demise of the Eastern IO pole and Wind-Evaporation-SST (WES) feedback. By MAM(1), the local WES feedback over NWP is still clear, but the EP SSTA decays so dramatically that the IO basin heating forces an anomalous easterly over WP (IO Capacitor Effect). An forced upwelling Kelvin wave over WP turns the equatorial Pacific to a La Niña condition in JJA(1), and the AAC is largely maintained by equatorial EP cooling and partly by IOCE.

The feature of prolonged events in SON(0) is similar to that of the short-decaying events but of weak amplitude and AAC is insignificant due to the anomalous cooling over WIO. From SON(0) to DJF(1), a band of uniform atmospheric cooling exists south of IO and the AAC remain weak. From MAM(1) to SON(1), warm SSTA persists over equatorial Pacific and IO but the former is accompanied by strong heating while the latter is associated with either cooling or spatially inhomogeneous warming. The AAC appears in MAM(1) and JJA(1) not only over NWP but also extends eastward to central Pacific. They are maintained by atmospheric heating distribution over tropical Pacific with IO playing a passive role.

The CP- and EP- type El Niños in the short-decaying category are further separated and examined. It is found that beside the heating position over Tropical Pacific, there is no distinct difference over NWP generally.