Effects of orography and surface heat fluxes on the South Asian summer monsoon


A high-resolution (40km horizontal) global model is used to examine controls on the South Asian summer monsoon by orography and surface heat fluxes. In a series of integrations with altered topography and reduced surface heat fluxes, monsoon strength, as indicated by a vertical wind shear index, is highly correlated with the amplitude of the maximum boundary layer equivalent potential temperature ($\theta_{eb}$) over South Asia. Removal of the Tibetan Plateau while preserving the Himalayas and adjacent mountain ranges has little effect on monsoon strength, and monsoon strength decreases approximately linearly as the height of the Himalayas is reduced. In terms of surface heat flux changes, monsoon strength is most sensitive to those in the location of the $\theta_{eb}$ maximum just south of the Himalayas. These results are consistent with the recent idea that topography creates a strong monsoon by insulating the thermal maximum from dry extratropical air. However, monsoon strength is found to be more sensitive to variations in the $\theta_{eb}$ maximum when topography is altered than when surface heat fluxes are reduced, and it is suggested that free-tropospheric humidity changes lead to deviations from strict convective quasi equilibrium and cause this difference. When topography is reduced, dry extratropical air intrudes into the troposphere over the $\theta_{eb}$ maximum and is entrained by local deep convection, requiring a higher $\theta_{eb}$ to achieve convective equilibrium with a given upper-tropospheric temperature and associated balanced monsoon flow. These results illustrate potential complexities that need to be included in simple theories for monsoon strength built on strict convective quasi equilibrium.

Journal of Climate