Large-Scale Extreme Rainfall-Producing Synoptic Systems of the Indian Summer Monsoon

In recent years India has been increasingly experiencing widespread floods induced by large-scale extreme rainfall events (LEREs). LEREs are mainly associated with monsoon low-pressure systems (LPS). The forecast of these high-flood-potential events, however, has remained challenging. Here, we compare LPSs of the summer monsoon that led to LEREs (LPS-Lg) and strong LPSs that did not result in LEREs (LPS-noLg) over central India for the period 1979-2012. We show that having a strong LPS is not a sufficient condition to produce LEREs, and the LPS-Lgs are accompanied by secondary cyclonic vortices (SCVs). The simultaneous existence of an LPS and an SCV creates a giant midtropospheric vortex. SCVs enhance dynamic lifting, static instability, and moisture transport from the Arabian Sea that precondition the atmosphere for deep convection. SCVs also slow down the propagation of LPSs. We show that the interaction of synoptic-scale systems can lead to LEREs even if individual systems are not strong enough.