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 Ocean summer monsoon often develops in association with the southeast Arabian Sea warm pool and a monsoon onset air pressure vortex, giving rise to a complex air-sea coupled system, though their precise interactions and impacts remain unclear. In this study, our analysis covering 1992-2017 demonstrates that the vortex triggers an earlier onset due to vortex-induced rainfall when the monsoon system has not yet developed to its climatological intensity. The weaker monsoon at the onset time means the large-scale moisture transport is expected to be lower over the Indian subcontinent, and rainfall analysis confirms a drier central India, where agriculture is mostly rain-fed, at the monsoon onset time in the vortex years. These results help to understand the transition from localized synoptic activity to the large-scale monsoonal system, highlighting the crucial role of the vortex. More importantly, when a vortex is pre-observed, rainfall available for agricultural irrigation is expected to be lower, providing guidance for agricultural irrigation.