Processes Establishing the Anomalous Philippine Sea Anticyclone During a Developing El Nino

During a developing phase of a strong El Nino, a low-level anomalous anticyclone forms over the Philippine Sea, which maintains from winter to the ensuing summer and exerts a prolonged impact on East Asian monsoon. This work investigates when and how the anomalous Philippine Sea anticyclone (PSA) is established. It was found that the occurrence of the anomalous PSA leads the Nino 3.4 index peak by 2-4 months, and on average, it occurs in mid-September. The abrupt establishment of the anomalous PSA is attributed to a set of factors. The intraseasonal oscillation and associated local air-sea interaction play a critical role in the rapid development of the anomalous PSA. The monsoon seasonal mean flows determine the nature of the air-sea interaction and whether the anomalous anticyclone persists. The strong remote forcing from the central-eastern Pacific warming displaces the upper level jet southward during the retreat of East Asian summer monsoon, which favors for cold air outbreak toward tropical western North Pacific, triggering the air-sea interaction and accelerating the advance of the winter monsoon in the western North Pacific.