Weakening of Regional Hadley Circulation Delays Indian Summer Monsoon Rainfall Onset

Abstract The timing of the Indian Summer Monsoon Rainfall (ISMR) onset significantly influences agriculture, food security, livelihoods, and the Indian economy. While the drivers of climatological ISMR onset are well discussed, the causes of delayed ISMR onset remain unclear. This study shows that the delay is primarily driven by anomalous high pressure over the Arabian Sea and anomalous low pressure over the Mascarene High, which weakens the regional Hadley circulation and, consequently, the low‐level moist southwesterly winds over the Arabian Sea. This weakening of southwesterlies directly delays the ISMR onset. When the ocean forces the atmosphere over the Arabian Sea, it alters the earlier delaying conditions and creates a favorable environment for the ISMR onset. The key novelty of this study is understanding the combined role of the Mascarene High, regional Hadley circulation, and Arabian Sea air–sea interactions in regulating delayed ISMR onset, offering new insights for improving ISMR onset prediction.