Energetics of monsoon low-pressure systems in stylized simulations of reduced mean monsoon precipitation through orography forcing

Abstract

Monsoon low pressure systems (LPS) are synoptic scale disturbances that form over South Asia during the summer monsoon season and often produce extreme precipitation events, causing disastrous floods. Numerous modelling and observational studies have confirmed the role of convection as a major energy provider for the propagation of LPS. Here, using NCAR’s Community Earth System Model (CESM1.2.2), we investigate the major energy providers for LPS propagation under a reduced mean monsoon precipitation. Four simulations are performed in which the height of the Tibetan and Himalayan Orography (THO) is altered by 1.5, 1.0, 0.5, and 0.0 times its original height. With a decrease in the height of THO, the mean monsoon precipitation decreases. However, even with reduced precipitation and convective activity, the number, intensity, and lifetime of LPS are higher when the height of THO is decreased. Barotropic instability associated with the horizontal shear of mean zonal wind is found to increase with a decrease in the height of THO, providing energy for LPS formation. In the initial stages of LPS propagation, when THO is completely removed, even with a reduced mean monsoon precipitation, the enhanced meridional wind enhances horizontal advection of Moist static Energy (MSE), which induces isentropic ascent and vertical advection over the western quadrant of LPS and helps in LPS intensification. Once sufficient moisture has been built up, in the later stages, enhanced mean zonal winds associated with an increase in surface temperature enhance horizontal advection of MSE and the intensification of LPS. This idealized study suggests that monsoon LPS can form and propagate in scenarios of reduced mean monsoon precipitation, potentially leading to extreme precipitation events even in drought years.