It is found that there is a close relationship between the early/late onset of the summer monsoon and cold/warm sea surface temperature anomaly (SSTA) in the central-eastern Pacific during boreal spring. Robust precursory signals for the early onset include the cold SSTA in the central-eastern Pacific and warm SSTA in the western Pacific in the preceding winter. No close relationship exists between the late onset and the preceding winter SSTA in the central-eastern Pacific. However, the cold SSTA in the western Pacific is a precursory signal for the late onset.
It is noted that the anomalous low-level convergence over Indo-china in the preceding winter and spring, which is mainly resulted from the convergence between anomalous northeasterly along the coastal of East Asia and southwesterly in the northeastern Indian Ocean, plays an essential role in causing the early monsoon onset. The land-sea thermal contrast between cold East Asia and the warm western Pacific results in a strong pressure gradient along coastal of East Asia and thus an enhanced anomalous northeasterly that converges into Indo-china. The enhancement of the southwesterly south of Indo-china is possibly associated with the overturning of the Walker circulation caused by the warm SSTA in western Pacific north equator and cold SSTA in central Pacific. The weak western Pacific subtropical high in the preceding winter and spring is also found as a robust precursory signal, which favors an early monsoon onset. The situation is reversed for a late monsoon onset case.
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