QBW Dynamics and Multiscale Interactions in Contrasting Indian Summer Monsoon Years

Previous studies have extensively examined the intraseasonal and synoptic-scale variability of the Indian summer monsoon, but the Quasi-Biweekly (QBW) mode remains less explored. This study investigates the key modes of subseasonal variability in the homogeneous rainfall regions of India over the past 73 summer monsoon seasons, with a particular focus on the QBW scale. By analysing scale energetics in the frequency domain, the study finds that QBW variability over Northeast India is mainly driven by Rossby wave-like atmospheric disturbances from the Western North Pacific (WNP), which are triggered by diabatic heating and the resulting generation of available potential energy. The strength of QBW variability varies significantly between different monsoon years, with stronger variability during deficit monsoons and weaker variability during excess monsoons. The enhanced (or reduced) available potential energy over the WNP during deficit (or excess) monsoons is responsible for the stronger (or weaker) QBW activity. Wave–wave interactions are identified as the primary mechanism for the formation and propagation of QBW oscillations, while mean–wave interactions play a secondary role, though with contrasting effects over the Indian monsoon region. The interaction between QBW, intraseasonal oscillations, and synoptic systems reveals a multiscale exchange of kinetic energy that impacts the formation and clustering of low-pressure systems over the Bay of Bengal. These findings underscore the significant role of QBW-scale dynamics in shaping the variability and extremes of the Indian summer monsoon.