Thermal Controls on the South Asian Monsoon through Boundary Layer Equivalent Potential Temperature

The Tibetan Plateau (TP) was previously considered as an elevated heat source that drives the large-scale boreal summer South Asian monsoon (SAM). Recent studies suggested the Himalayas alone can produce strong SAM by insulating warm and moist air south of the Himalayas from extratropical air. Almost all of these studies, however, used fairly coarse resolution models that barely resolve the Himalayan mountain range. Here we used the high-resolution Weather Research and Forecasting model to conduct a series of integrations exploring the thermal controls on SAM. Our results confirmed the recent studies, and show that SAM weakens as the height of the Himalayas decreases. We also conducted simulations with surface heat flux perturbations in different areas. It is shown that the strength of SAM, indicated by the Webster and Yang Index (WYI), is highly correlated with the boundary layer equivalent potential temperature (θeb) south of the Himalayas, collocated with the θeb maxima. The results are interpreted under convective quasi-equilibrium: surface heat flux contributes to θeb, and WYI is determined by the θeb maxima. We also assessed the relative importance of surface heat flux perturbations in different parts of South Asia.; JJA θeb. Black contours denote the elevation with intervals of 1500m. Dotted contour shows the coastline. ; Correlation coefficient between the WYI and θeb (significance level 0.05).