Diverse Interannual Variability of Asian Summer Monsoon Onset Process

The onset of the Asian summer monsoon (ASM) starts the rainy season in one of the most world populous regions. The present study found that the stepwise ASM onset process (ASMOP) in the Bay of Bengal, South China Sea (SCS), and India exhibits three distinct interannual modes induced by different sea surface temperature anomalies (SSTAs). The first mode features are in-phase variation of ASMOP in southern Asia, which originates from the ENSO-related SSTAs and anomalous circulations in the tropical Indo-Pacific Ocean. The second mode indicates the anomalous ASMOP in India and depends on the southwestern Indian SSTAs that alter the low-level cross-equatorial flow over the Arabian Sea. In the third mode, the western North Pacific SSTAs and their resultant circulation anomalies regulate ASMOP in the northern SCS. These extra-equatorial SSTAs and their associated air-sea interaction serve as an additional source of seasonal predictability for ASMOP besides El Nino-Southern Oscillation events.