The Role of the North Atlantic Blocking High during Large-Scale Heavy Rainfall Events over Central India

Large-scale extreme rainfall events (LEREs) over central India are produced by monsoon low pressure systems (LPSs) when assisted by a secondary cyclonic vortex (SCV). Both the LPS and the SCV are embedded in a monsoon trough and form mainly during the positive phase of the boreal summer intraseasonal oscillation. Here, we observe that tropical-extratropical interactions exist during LEREs. Using ray tracing, we show that extratropical Rossby waves propagate to the Indian subcontinent during the summer monsoon season. Stationary Rossby wave rays originating over the North Atlantic Ocean reach India following approximately a great circle path at midtropospheric levels. This pathway appears to play an important role in tropical-extratropical interactions during LEREs. Seventy-seven percent of LEREs are preceded by a North Atlantic blocking high and 90% by a quasi-stationary central Asian high. The Atlantic blocking high triggers a quasi-stationary Rossby wave response and strengthens the downstream central Asian high. In turn, the quasistationary central Asian high facilitates Rossby wave breaking, transporting high-PV streamers and cutoffs equatorward. The central Asian high is in close proximity to the monsoon trough in the mid- and lower troposphere. It interacts with the monsoon trough over the northwest Indian subcontinent. The equatorial monsoon trough is strengthened due to the supply of dynamic forcing and static instabilities from the extratropics. This additional forcing from the extratropics creates an environment that is conducive for LEREs.