Introduction

ABSTRACT

The phase relationships of the western North Pacific (WNP) summer monsoon (WNPM) with the Australian monsoon (AM) and Indian monsoon (IM) are investigated using observational rainfall, SST and NCEP reanalysis data for the period of 1979-2005. It is found that a strong WNPM often follows a strong AM but leads a weak AM, and a significant simultaneous negative correlation appears between WNPM and IM. The in-phase relationship from AM to the succeeding WNPM occurs often during the El Nino decaying phase when the warm eastern Pacific SST anomaly (SSTA) weakens AM through anomalous Walker circulation and the persistence of an anomalous WNP anticyclone from the boreal winter to summer leads to a weak WNPM. The out-of-phase relation from WNPM to the succeeding AM occurs either during the El Nino early onset year when the warm SSTA in JJA is strong enough to force a low-level cyclonic flow anomaly in WNP and in DJF the same warm SSTA forces a weak AM, or during the El Nino decaying phase when the persistence of the WNP anomalous anticyclone causes a weak WNPM and the transition of a warm to a cold episode causes a strong AM in DJF. The simultaneous negative correlation between WNPM and IM often appears either during the El Nino early onset years when the warm eastern Pacific SSTA induces the cyclonic wind shear that strengthens WNPM but suppresses convection over India, or during the El Nino decaying summer when a weak WNPM results from the persistence of the local anomalous anticyclone and a strong IM results from the El Nino-to-La Nina transition or a basin-wide Indian Ocean warming.