Diverse onset pathways of South China Sea summer monsoon driven by SSTA across Indian-Pacific-Atlantic Oceans

In climatology, the South China Sea (SCS) summer monsoon (SCSSM) onset features rapid establishment of low-level westerlies and convection across the entire basin, in contrast to Indian summer monsoon onset which begins with northward migration of the Intertropical Convergence Zone (ITCZ). Diverse SCSSM onset pathways complicate subseasonal rainfall forecasts in the surrounding region during boreal spring. Besides simultaneous onset in the SCS, this study recognizes two other types of SCSSM onset: northern-initiated and southern-initiated pathways. In the northern-initiated type, the low-level westerly wind begins over northern SCS, accompanied by a positive Tripole mode in the North Atlantic and decaying El Niño SSTAs in the Pacific. This SSTA configuration regulates the extra-tropical meridional cell and tropical Walker circulation to shift the western Pacific subtropical high (WPSH) southward, suppressing (intensifying) westerly wind in the southern (northern) SCS. In contrast, the southern-initiated type features initiation of westerly wind in the southern SCS. This type is ascribed to the tropical southern Indian Ocean cooling and negative Pacific Meridional Mode (PMM)-like pattern. The colder Indian Ocean boosts northward migration of ITCZ over Arabian Sea, which strengthens low-level westerly wind over the southern SCS by enhancing cross-equatorial Somali jet and shifts the WPSH northward, thereby suppressing the monsoon onset over the northern SCS. This pattern is developed by the PMM-like related easterly wind in the tropical Pacific Ocean. These findings shed new light on SCSSM onset behavior and show potential implications for prediction regional precipitation. • A Zero-U850 Profile Index (ZUPI) is proposed to quantify the SCSSM onset pathways. • Distinct onset pathways are induced by Indian-Pacific-Atlantic SSTAs through diverse physical mechanisms. • The zonal and meridional position of WPSH directly modulates the onset pathway of the SCSSM.