Changing Northeast India Summer Monsoon Onset Dynamics in a Warming World

ABSTRACT The Indian Summer Monsoon typically begins over Northeast India (NEI) about 2 weeks earlier than monsoon onset over Kerala, extending the rainy season in NEI to around 150 days. This early onset is strongly linked to planetary‐scale processes, particularly interactions with extratropical Rossby waves. These waves facilitate the downward and southward intrusion of potential vorticity, which triggers low‐level cyclonic vorticity and initiates monsoon onset. The quasi‐biweekly oscillation further facilitates this process. While climate change is associated with a weakening of the seasonal mean monsoon over NEI, its impact on extratropical influences and the trend of earlier onset remains uncertain. This study examines changes in tropical circulation and extratropical influences by comparing two periods: 1940–1960 and 2002–2023. Results reveal a marked increase in extratropical influence in recent decades. In 2002–2023, clear signs of downward PV intrusion and enhanced low‐level cyclonic vorticity are observed, in contrast to the earlier period where such features are largely absent. Lead–lag composites of PV anomalies at 200 hPa show prominent eastward‐propagating signals during onset in the recent period. In 1940–1960, monsoon onset was primarily driven by local mesoscale convection and self‐organisation, with cyclonic vorticity strengthening 2 days post‐onset. During the earlier period, heat sources were more likely driven by local convective instability, as indicated by higher convective available potential energy, which was approximately 500 J/kg greater at the time of monsoon onset. Additionally, air‐sea interactions have become more influential in recent decades. While stronger Sea surface temperatures (SSTs) over the Bay of Bengal (BOB) during pre‐onset facilitates the onset through enhanced moisture flux, SSTs over BOB tend to cool once the onset process begins. These findings highlight a significant shift in monsoon onset mechanisms over NEI, with increasing extratropical control and evolving ocean–atmosphere interactions likely shaped by climate change.