A Study on the Factors causing the Enhanced Inland Penetration of Monsoon Depressions into the Indian Landmass

Abstract

Monsoon Depressions (MDs) are low-pressure systems that form over the Bay of Bengal (BoB) during boreal summer, thus constituting more than half of the ISMR. The inland penetration and duration of MDs over Indian landmass are significant as they can regulate the spatiotemporal variability of ISMR. We utilized GLDAS reanalysis data for soil moisture to understand its role in the persistence of landfalling MDs, by a comprehensive examination of the long-lived (TOP 10 cases) and short-lived MDs (BOTTOM 10 cases). This is the first qualitative application of the soil moisture data for ascertaining the relationship of soil moisture with the long residence time of MDs over Indian landmass. Our result reveals that long-lived MDs had higher soil wetness over the area ahead of the MD, compared to short-lived MDs. We also analyzed different dynamical and thermodynamical meteorological parameters using ERA-5 reanalysis data. Interestingly, all 10 TOP cases had preceding cyclonic circulation at mid-troposphere over the northeastern Arabian Sea (i.e., Mid Tropospheric cyclones, MTCs), whereas only 1 MD is accompanied by MTC in the BOTTOM group. In BOTTOM cases, dry air intrusion from heat-low regions creates background moisture conditions unfavorable for the sustenance of the MD structure. The cyclonic vorticity associated with MTC transports moist Arabian sea air to the Indian subcontinent and thus explains the observed increase in convergence of vertically integrated moisture flux over northwest India for TOP cases. Overall, our results provide the first seminal evidence that the presence of MTC over the northeastern Arabian sea, which in conjunction with wet soil conditions create favorable moisture and circulation conditions for a prolonged duration as well as deeper penetration of MD over the Indian subcontinent.