notES And corrESPondEncE Influence of the Sea Surface temperature in the Indian ocean on the In-Phase transition between the South Asian and north Australian Summer Monsoons

In this study we investigate the physical processes underlying maintenance of an in-phase transition between the South Asian summer monsoon (SASM) and the succeeding North Australian summer monsoon (NASM). In particular, our attentions were focused on identification of the roles of sea surface temperature anomaly (SSTA) in the Indian Ocean in contrast to that of the El Niño-Southern Oscillation (ENSO). To realize this goal, we partition the in-phase years into two groups, those being ENSO and non-ENSO groupings, and then we conduct a composite study. For the non-ENSO-year composite, the associated SST changes resemble a dipole structure characterized by a SST warming in the western Indian Ocean and a cooling in the southeastern Indian Ocean in the boreal summer and fall. The warm SSTA, along with a stimulated ascent, expands eastward in the boreal fall and arrives in the eastern Indian Ocean around northern Australia, concurrent with a cold SSTA to its north with a descent. Therefore, a strong ascent in northern Australia associated with the regional Hadley circulation occurring in the Philippine Sea-northern Australia region is responsible for a wet NASM. Our study also identifies the enlarged influence of the SSTA in the Indian Ocean being likely associated with change in the background SST state that occurred around 1976 77 over the Indian Ocean.