Dynamics of the inland penetrating Indian monsoon depressions

Abstract

Despite the decisive role of Bay of Bengal Monsoon Depressions (MDs) in contributing to the Indian summer monsoon mean rainfall, the exact mechanisms and processes contributing to their prolonged residence over the Indian landmass remain still elusive. So, a comprehensive analysis on the factors affecting the inland penetration length of MDs after landfall over the Indian landmass, is presented here using MDs identified from Indian Meteorology Department weather reports over 3 decades (1991–2020). Exploration of large-scale atmospheric circulation features using ERA-5 reanalysis data shed new insights on the processes contributing to the maintenance and sustenance of land penetrating MDs. Notably, all Higher Inland Propagating (HIP) MDs were preceded by Mid-Tropospheric Cyclones (MTCs, with cyclonic circulation in the lower and mid-troposphere) over North-West (NW) India, with this entire region experiencing increased moisture flux convergence, thus facilitating the deep intrusion of MDs into the Indian landmass. In contrast, the Lower Inland Propagating (LIP) MDs experience dry air intrusion from the adjacent desert region, creating unfavorable moisture conditions and leading to their rapid dissipation. Intriguingly, the HIP MDs were further found to cluster with the westward propagating large-scale intraseasonal mode (i.e.,10–20 days). Using the updated GLDAS soil moisture reanalysis, it is further revealed that the precipitation caused by both the MTC, and the 10–20 days intraseasonal oscillation mode saturates the land surfaces before the arrival of MDs in the northwestern central Indian region during HIP events. In short, our results show that the presence of cyclonic vorticity over NW India, in association with the large-scale westward propagating (10–20 days) mode of variability, creates favorable conditions for the inland penetration of HIP events. Further, the results highlighting the importance of 10–20 days mode and its propensity to cluster with the land penetrating MDs, assume significance in a changing climate, as the subseasonal Indian monsoon rainfall variability is projected to increase in the future.