Exploring the Origins of Monsoon Onset Variability in the South Asia Monsoon through MSE Dynamics

The South Asia monsoon is a globally significant meteorological phenomenon, delivering approximately 80% of India’s annual rainfall from June to September. Given that this precipitation is concentrated within a few months, variations in the onset of the monsoon—whether early or late—can lead to severe outcomes such as flash floods or droughts in the region. Therefore, accurate forecasting of the monsoon’s onset is of great societal importance.

However, the mechanisms governing the onset timing remained poorly understood. This study aims to investigate the differences between early and late monsoon onsets through the lens of Moist Static Energy (MSE) recharge-discharge processes. Preliminary analysis indicates that the transition into the monsoon season is characterized by the development and northward movement of shallow circulation, which vertically transports moist static energy from the boundary layer to the free troposphere, around 500 hPa. While the timing of shallow circulation development varies, the overall spatial structure appears consistent between the early and late onset cases. These findings suggest that the onset of the monsoon may be linked to the development of shallow circulation, which reduces mid-atmospheric gross-moist stability and subsequently promotes deep convection. The further analysis will explore the origins of shallow circulation using the WK99 space-time decomposition. By decomposing circulation/MSE into different components of tropical disturbance, we aim to uncover relationships between eddy moist static energy flux and the monsoon timing.