Unprecedented rainfall intensification over Western India during the 2019 summer monsoon

<p>Large areas of Western India (WI) recorded the highest seasonal (June&#8211;September) monsoon precipitation during 2019 since 1901, resulting in disastrous floods claiming hundreds of human lives. Our analysis illustrates that widespread heavy rainfall in this region markedly came from three intense rain episodes (IREs), viz. 1) 28 June to 12 July, 2) 24 July to 11 August, and 3) 1&#8211;14 September, in the backdrop of strong positive Indian Ocean Dipole (pIOD) conditions evolving at the equatorial Indian Ocean. Based on the station count of heavy rainfall over WI, the IREs were defined in such a way that they captured the essence of both spatial and temporal coherences of heavy rainfall during those periods. The IREs favorably stemmed from northward propagating bands of organized convection embedded with stratiform precipitating systems. Analysis of satellite swaths brings out the prevalence of stratiform precipitation and elevated latent heating in the precipitating systems. Regression analyses of various atmospheric fields were performed to understand the dominant spatial structures during the IRE days. The continual top-heavy stratiform latent heating, in synergy with the pIOD-generated equatorial heating anomalies, forced a Rossby-wave pattern of high mid-tropospheric potential vorticity with the maximum over WI and an associated massive cyclonic vortex stretching across South and Southeast Asia (SSEA). It is found that the pIOD-induced enhanced cross-equatorial moisture transport fostered large-scale moisture convergence and deep convective ascent over much of SSEA, thereby leading to extensive heavy rainfall. This study highlights the ramifications of intense pIOD manifestations and their potential consequences for increasing the occurrence of hydrological extremes in WI as well as SSEA in the 21st century.</p>