Can monsoon low pressure systems propagate even under reduced mean monsoon precipitation?

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. Earlier studies have found a decrease in mean monsoon precipitation with a decrease in the height of THO. 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 meridional wind is found to increase with a decrease in height of THO, providing energy for LPS formation. However, in the later stages, horizontal advection of dry static energy (DSE) is found as the major energy source for LPS propagation. The decrease in height of THO leads to an increase in dry air intrusion into the Indian mainland and an increase in surface temperature. This leads to an increase in horizontal DSE advection, which in turn induces vertical motion and moistens the atmosphere to the west of LPS. The moist ascent over the west of LPS maintains the precipitation and leads to 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.