The influence of surface heat fluxes on the growth of idealized monsoon depressions


This study explores the effect of surface sensible and latent heat fluxes on monsoon depressions using a series of idealized convection-permitting simulations. Each experiment is initialized with a small amplitude wave that is allowed to grow within an environment representative of the South Asian monsoon. Comparing experiments with and without interactive surface heat fluxes, it is found that these fluxes enhance the growth of the simulated vortices. Without interactive surface fluxes, the strengthening period is short and the vortices fail to reach intensities characteristic of stronger monsoon depressions. Using a large set of experiments in which the vertical and meridional shear are systematically varied, it is found that surface heat fluxes enhance intensity the most when upper-level shear is weak, the low-level shear and associated moist static energy gradient are sufficiently steep, and the meridional shear is strong. These experiments reveal two different regimes of monsoon depression growth: one in which convection is driven by moist static energy (MSE) advection and one in which it is driven by surface heat fluxes and quasi-geostrophic forcing for ascent. Both regimes require sufficiently strong meridional shear to achieve initial growth by barotropic instability.