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 Niño–Southern Oscillation events. Plain Language Summary: The onset of the Asian summer monsoon (ASM) begins the rainy season in South and East Asia, one of the world's most populous regions. In general, the ASM onset process (ASMOP) follows a stepwise progression over the Bay of Bengal (BOB), South China Sea (SCS), and India, in turn. The year‐by‐year relationship between the monsoon onset dates over these areas helps to predict the arrival of the monsoon in Asian countries. However, there is no consensus on the year‐by‐year relationship of ASMOP among these areas. The present study used a unified definition to revisit the distinct modes of interannual variation in ASMOP. The El Niño–Southern Oscillation (ENSO)–associated tropical sea surface temperature anomalies (SSTAs) in the Indian and Pacific oceans can modify ASMOP over BOB, the southern SCS, and India simultaneously. Besides the influence of ENSO, the SSTAs in the southwestern Indian Ocean can regulate ASMOP in India. At the same time, the SSTAs modulate ASMOP in the northern SCS in the western North Pacific. Results from numerical experiments verified that air–sea interaction is critical for the effect of extra‐equatorial SSTAs on ASMOP, which provides an additional source of seasonal predictability of ASMOP besides ENSO factors. Key Points: The Asian summer monsoon onset process (ASMOP) shows distinct interannual modes in the tropicsEl Niño–Southern Oscillation determines the interannual variability in ASMOP over India, the Bay of Bengal, and the southern South China Sea (SCS)The southwestern Indian and western Pacific sea surface temperature anomalies modulate ASMOP over India and the northern SCS via air–sea interaction, respectively [ABSTRACT FROM AUTHOR]