North–South Asymmetry of Pre‐Monsoon Rainfall Over the Western Ghats in India

ABSTRACT The pre‐monsoon rains break a long dry spell over many regions of India, bringing relief from the summer heat, reducing air pollution, aiding agriculture and water resources. However, the pre‐monsoon rainfall shows a strong contrast along the Western Ghats, with the southern portion receiving higher and more consistent rainfall and the northern Ghats experiencing sporadic pre‐monsoon rains. Here, we examine the causes for this disparity using ERA‐5 reanalysis and observed precipitation from two stations, Mumbai and Kochi, in the northern and southern Western Ghats, respectively. We find that during the pre‐monsoon, south‐westerly wind bursts occur occasionally, bringing moisture to the southern Western Ghats, which build up moist convective instabilities, causing pre‐monsoon rainfall over Kochi. However, during the periods when it rains over Mumbai, these winds are stronger and shift northwards. With increased moisture loading and stronger winds, rainfall days over Mumbai are associated with increased moist static energy over the northern Western Ghats. We further show that the northward shift of the south‐westerly winds is forced by a Rossby wave propagation pattern in the upper troposphere, which causes a surface cyclonic circulation and warm sea surface temperatures over the northern Arabian Sea. Our findings demonstrate that the episodic northward expansion of south‐westerly winds over the Arabian Sea also spreads the rainfall northward over the Western Ghats during the pre‐monsoon. While our results shed light on the potential mechanisms, further studies are needed to understand the feedbacks driving this northward shift and its consequences for the monsoon season, if any. This is especially urgent since the northern Arabian Sea warming is now shown to affect extreme rainfall events over the northern Western Ghats, northwestern India and into Pakistan.