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Abstract The south Asian summer monsoon (SASM) is a crucial weather system in South Asia, characterized by three main types according to the Asia-Pacific monsoon split: East Asian Monsoon (EASM), Indian Monsoon (ISM), and Western North Pacific Summer Monsoon (WNPSM). The Mainland-Indochina (MIC) study region lies at the transition between ISM and WNPSM, displaying a unique climate influenced by these monsoon systems. Understanding the SASM's onset, particularly the Mainland-Indochina Southwest Monsoon (MSwM), is vital for agricultural planning and water resource management. This study examines the onset dates of the MSwM over Mainland Indochina region using various meteorological datasets and indices. An objective, non-threshold definition of the MSwM onset is established based on surface winds and moisture flux transport changes. The study reveals significant interannual variability in MSwM onset dates, impacting agricultural activities and water resources. Spatial patterns of MSwM onset dates are analyzed, highlighting localized variations within the MIC region. The study correlates MSwM onset dates with May rainfall across Asia, showing significant relationships with moderate to strong correlations, particularly over MIC. Atmospheric circulation anomalies associated with late and early MSwM onsets are investigated, revealing distinct patterns in wind, vertical velocity, and sea surface temperature anomalies. Convective activity and atmospheric heating play crucial roles in driving circulation anomalies, affecting rainfall distribution across MIC. Overall, this study provides valuable insights into the dynamics of the MSwM onset, its impact on regional rainfall, and the underlying atmospheric processes driving these phenomena. Understanding these complexities is essential for improving seasonal monsoon predictions and enhancing resilience in agriculture and water management in South Asia.
Oo et al. (Fri,) studied this question.