Contribution of Low Pressure Systems Rainfall to Interannual Variability of the Indian Summer Monsoon

Abstract Monsoon Low-Pressure Systems (LPS) are significant rain-bearing storms during the Indian summer monsoon, yet their impact on the interannual variation of Indian Summer Monsoon Rainfall (ISMR) remains uncertain. This study revisits the relationship between LPS and ISMR variations and extreme rainfall events, emphasizing the role of LPS-generated rainfall. A strong correlation is observed between LPS-generated rainfall and ISMR variations; years with an excess of ISMR correspond to increased LPS-induced rainfall, whereas years with a decrease in ISMR show a decrease in LPS rainfall. The increase in LPS rainfall during excess years is primarily linked to higher rainfall rates, which arise from elevated background humidity and stronger vertical motions associated with LPS, thus making a substantial contribution to excess ISMR. Traditional LPS intensity indices indicate that the dynamical intensity exhibits only minor variation despite the higher rainfall rates, suggesting an apparent decoupling between precipitation rate and dynamical strength. In contrast, our analysis reveals that increased rain rates coincide with strengthened horizontal pressure gradients and higher wind speeds in the vicinity of the LPS vortex center. These conditions correspond to LPS that are more intense and spatially more confined during excess monsoon years than during drought years, pointing to a certain degree of coupling between the dynamical intensity of monsoon LPS and their rainfall rates.