How the vortex over the Arabian Sea warm pool triggers an early Indian summer monsoon

The Indo-Pacific region is vulnerable to changes in the Indian summer monsoon and its onset. The associated monsoon rainfall strongly affects agriculture, water resources, and human security across the densely populated regions of South Asia. The Indian summer monsoon often develops in association with the southeast Arabian Sea warm pool and a monsoon onset air pressure vortex, extracting energy from the warm pool and giving rise to a complex air-sea coupled system, though their precise interactions and impacts remain unclear. In this study, our analysis covering 1983–2024 demonstrates that the vortex years correspond to an earlier onset date due to the localized convection when the monsoon system has not yet developed to its climatological intensity. A weaker monsoon at the onset time is associated with reduced large-scale moisture transport over the Indian subcontinent, further indicating drier conditions over central India, where agriculture is predominantly rain-fed, during monsoon onset in the vortex years. Therefore, when a vortex occurs prior to monsoon onset, the rainfall available for agriculture is likely to be reduced. These results help elucidate the transition from localized synoptic activity to the large-scale monsoonal system, highlighting the distinction between a rainfall-based earlier onset date and the lagged development of the large-scale monsoon system and providing new insights for the monsoon onset definition.