Asymmetry of two types of ENSO in the transition between the East Asian winter monsoon and the ensuing summer monsoon

This study investigates the transition from East Asian winter monsoon (EAWM) to following summer monsoon (EASM) under two types of El Niño and La Niña events. A robust out-of-phase transition from weak EAWM to strong EASM is related to El Niño events, which is more distinct in eastern Pacific (EP) El Niño than that in central Pacific (CP) El Niño due to the stronger and wider western North Pacific (WNP) anticyclone (WNPAC) as a persistent atmospheric bridge. The WNPAC differences result from the combined impacts of the warming over northern Indian Ocean (NIO) remotely, the dipolar sea surface temperature (SST) anomalies and the anomalous sinking motion over WNP locally. In terms of La Niña, the out-of-phase strong EAWM to weak EASM transition exists only for CP La Niña. Moreover, this connection is weaker compared to that for El Niño events because of a weaker WNP cyclone (WNPC). Conversely, when EP La Niña occurs, an in-phase transition is detected with a strong EAWM evolving into a strong EASM due to the emergence of WNPAC in summer. For CP and EP La Niña, the cooling SST anomalies over NIO and WNP play opposite roles in affecting WNP summertime circulation anomalies. Observational and model results suggest that the WNPC (WNPAC) is dominated by remote (local) cooling in NIO (WNP) in the summer following CP (EP) La Niña. In addition, the local rising (sinking) flow also contributes to the WNPC (WNPAC) associated with CP (EP) La Niña.