Changing Northeast India Summer Monsoon Onset Dynamics in a Warming World

The planetary-scale onset of the Indian Summer Monsoon over Northeast India (NEI) typically occurs about two weeks earlier than the monsoon the onset over Kerala, extending the length of the rainy season in NEI to approximately 150 days. The dynamics of the May onset over NEI are strongly linked to interactions with extra tropical Rossby waves, which through downward and eastward propagation of potential vorticity (PV) drive low-level relative vorticity and initiate the onset process. Additionally, the quasi-biweekly oscillation (QBWO) also plays a significant role in modulating the onset dynamics over NEI. While climate change is known to weaken the seasonal mean monsoon over NEI, its role in amplifying extra tropical influences on the onset and the trend of early onset over NEI remains unclear. Here we assess these changes by examining tropical circulation patterns and extra tropical influences, specifically focusing on the intrusion of potential vorticity, over two distinct periods: 1940–1960 and 2002–2023. Our analysis reveals a notable increase in extra tropical influence in recent decades. In the 2002–2023 period, with clear evidence of downward PV intrusion and the generation of low-level cyclonic vorticity, both of which are largely absent during the earlier period. Lead-lag composites of PV anomalies at 200hPa further support these findings, showing prominent eastward-propagating PV anomalies around the onset in recent years. In contrast, during the 1940–1960 periods, monsoon onset over NEI was predominantly governed by local meso-scale convective activity and self-organization, as indicated by the strengthening of low-level cyclonic vorticity two days after the 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 in May compared to recent decades. Furthermore, the study highlights the role of air-sea interactions in influencing the onset dynamics. In recent decades, the Bay of Bengal Sea surface temperature show a stronger correlation with the onset process, indicating that changes in SST patterns and associated heat fluxes play a crucial role in facilitating the extra tropical influences. These findings underscore a significant shift in the mechanisms driving monsoon onset over NEI, with increasing extra tropical influences and altered air-sea interactions in recent decades. This evolving dynamic, likely influenced by climate change, has critical implications for understanding and predicting monsoon variability in NEI.