Intensification and Northward extension of Northwest Pacific anomalous anticyclone in El Niño decaying mid-summer: an energetic perspective

The Northwest Pacific (NWP) anomalous anticyclone (AAC) intensifies and extends northward from El Niño decaying early to mid-summer, despite decaying El Niño-induced sea surface temperature (SST) anomalies in the North Indian Ocean, North Atlantic and subtropical NWP. The present study suggests the intra-seasonal variations of AAC are induced by local mean state changes and investigates the underlying mechanisms from the perspective of energetics. Compared with early summer, the efficiency of dry energy conversion from the mean flow to El Niño-excited AAC pattern increases in El Niño decaying mid-summer. The moist feedback over subtropical NWP is also enhanced in El Niño decaying mid-summer due to the onset of the climatological NWP summer monsoon. Both of them contribute to the intensification of El Niño-excited AAC pattern. Moreover, mean state changes over East Asia–NWP from early to mid-summer are found in favor of the northward shift of the preferred latitude of the circulation anomalies. Thus, the El Niño-excited AAC pattern are more northward-extended in El Niño decaying mid-summer. Empirical orthogonal function analyses further confirm that the northward extension of El Niño-excited AAC pattern stems from changes of local internal dynamic mode. The present study highlights that both the El Niño-induced SST anomalies and local atmospheric internal dynamics are of paramount importance for the intra-seasonal variations of AAC in El Niño decaying summer.