Strengthening linkage between the Boreal Summer Intraseasonal Oscillation and extreme rainfall events over India

Extreme rainfall events (EREs) over India are strongly influenced by the Boreal Summer Intraseasonal Oscillation (BSISO), yet how this relationship
has evolved in recent decades remains poorly understood. Using observational datasets and reanalysis from the past four decades, we examine the changes in BSISO characteristics in the recent past and their role in modulating EREs over the Indian monsoon region. We find a marked strengthening of BSISO-associated rainfall over central India (15N–25N), along with a spatially coherent increase in rainfall accumulation from EREs as well as in seasonal mean monsoon rainfall.

Our results suggest that these trends mainly stem from an increase in the number of active BSISO days. Increased BSISO activity creates a more favourable environment, which supports the occurrence and persistence of extreme rainfall. A dynamic-thermodynamic decomposition of the BSISO precipitation shows that the dynamic component, associated with the BSISO circulation, dominates the changes in precipitation. However, increased vertical velocity is limited to areas with increased background moisture, indicating a strong connection between dynamic forcing and thermodynamic conditions.

In summary, our findings highlight a linkage between BSISO variability and extreme rainfall in India over recent decades. The mechanisms we identified provide a physical framework for understanding observed changes in monsoon rainfall and offer insights into how intraseasonal variability might impact future monsoon extremes in the warming climate.