Indian monsoon depressions are synoptic-scale storms that form primarily over the Bay of Bengal and propagate westward over the subcontinent, producing a large fraction of India's total summer precipitation. We recently showed that, contrary to long-standing ideas, the westward propagation of Indian monsoon depressions is accomplished primarily by horizontal adiabatic advection of potential vorticity (PV), not by vortex stretching or diabatic PV generation that occurs in the region of quasi-geostrophic ascent southwest of the vortex center. This chapter extends that work by using several reanalysis products to examine case studies of Indian monsoon depressions. In all reanalyses examined, monsoon depressions have maximum PV in the middle troposphere, at higher altitudes than the level of maximum relative vorticity. The horizontal structure of mid-tropospheric PV suggests that the axial asymmetry of the vortex that produces the nonlinear westward advection may result at least partly from diabatic heating. Thus, although storm motion is produced primarily by horizontal adiabatic advection, diabatic heating can play an indirect role by shaping the PV field that produces this advection.

Global constraints on momentum and energy govern the variability of the rainfall belt in the intertropical convergence zone and the structure of the zonal mean tropical circulation. The continental-scale monsoon systems are also facets of a momentum- and energy-constrained global circulation, but their modern and palaeo variability deviates substantially from that of the intertropical convergence zone. The mechanisms underlying deviations from expectations based on the longitudinal mean budgets are neither fully understood nor simulated accurately. We argue that a framework grounded in global constraints on energy and momentum yet encompassing the complexities of monsoon dynamics is needed to identify the causes of the mismatch between theory, models and observations, and ultimately to improve regional climate projections. In a first step towards this goal, disparate regional processes must be distilled into gross measures of energy flow in and out of continents and between the surface and the tropopause, so that monsoon dynamics may be coherently diagnosed across modern and palaeo observations and across idealized and comprehensive simulations. Accounting for zonal asymmetries in the circulation, land/ocean differences in surface fluxes, and the character of convective systems, such a monsoon framework would integrate our understanding at all relevant scales: from the fine details of how moisture and energy are lifted in the updrafts of thunderclouds, up to the global circulations.