A large fraction of the rain received by continental India is produced by cyclonic vortices with outer radii of about 1000 km that are contained within the larger scale South Asian monsoon flow. The more intense occurrences of these vortices are called monsoon depressions; these consist of bottom-heavy columns of relative vorticity that propagate to the northwest in time-mean low-level eastward flow. Previous studies have argued that this apparent upstream propagation is caused by dynamical lifting west of the vortex centre, with the resulting ascent producing vortex stretching that shifts the vortex to the west. Here, analysis of over 100 Indian monsoon depressions is used to show that low-level vortex stretching has a spatial structure inconsistent with the observed propagation and is balanced by other terms in the low-level vorticity budget. Instead, monsoon depressions are shown to consist of potential vorticity maxima that have peak amplitude in the middle troposphere and propagate westward by nonlinear, horizontal adiabatic advection (i.e. beta drift). The precipitating ascent in monsoon depressions makes a more minor contribution to the total storm motion and primarily acts to maintain the upright structure of the vortex. These results suggest a new view of Indian monsoon depressions as potential vorticity columns that propagate primarily by adiabatic dynamics.

An idealized, three-dimensional, cloud-system-resolving model is used to investigate the influence of surface enthalpy flux variations on tropical depression (TD) spinup, an early stage of tropical cyclogenesis in which the role of surface fluxes remains incompletely understood. A range of simulations supports the hypothesis that a negative radial gradient of surface enthalpy flux outside the storm center is necessary for TD spinup but can arise from multiple mechanisms. The negative radial gradient is typically created by the wind speed dependence of surface enthalpy fluxes, consistent with some previous theories for tropical cyclone intensification. However, when surface enthalpy fluxes are prescribed to be independent of wind speed, spinup still occurs, albeit more slowly, with the negative radial gradient of surface enthalpy flux maintained by an enhanced air–sea thermodynamic disequilibrium beneath the cold core of the incipient vortex. Surface enthalpy flux variations seem more important for intensification than initial conditions. For example, a vortex forms and intensifies even from a state of rest when the center of the domain is initialized to be nearly saturated with water vapor, but this intensification is modest in amplitude and transient, lasting less than 12 h, without interactive surface enthalpy flux. Sustained spinup on time scales longer than a day does not occur when surface enthalpy fluxes are horizontally homogeneous or constant, even when fixed at the high value of 200 W m−2. In the ensemble of simulations presented here, the vortex intensification rate scales linearly with the storm-scale surface enthalpy flux anomaly relative to the undisturbed environment.

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 midtropospheric 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.

Potential vorticity (PV) has been used to understand the intensification and motion of a variety of tropical vortices. Here, atmospheric reanalyses and idealized models are used to understand how the vertical structures of moist convective heating and adiabatic advection jointly shape the vertical structures of PV in tropical depressions. Observationally based estimates reveal a top‐heavy PV structure in tropical depressions, contrasting with bottom‐heavy structures of absolute vorticity and diabatic PV generation. These distinct vertical structures are reproduced in an axisymmetric model which employs the weak temperature gradient approximation for conceptual simplicity and is forced by stratiform and deep convective heating. When applied in isolation, the stratiform and deep convective heatings produce PV maxima at 500 hPa and near the surface, respectively. When these two heatings are applied simultaneously, interactions between the stratiform and deep convective modes enhance the adiabatic advective tendencies produced by the transverse circulation, making the PV distribution more top‐heavy. In the lower and middle troposphere, radial advection also greatly reduces the radius of the PV structure relative to that of the imposed heating, consistent with structures in observed tropical depressions; the implications of these differences in radial structures for using the flux form of the relevant conservation equations (e.g. for PV substance or absolute vorticity) are discussed.

South Asian monsoon depressions are convectively coupled cyclonic vortices that form and intensify in a region of easterly vertical shear of the horizontal wind. Observations of maximum precipitation downshear of the cyclonic center have led to prior theories of quasigeostrophic (QG) control of moist convection in these storms. This study examines the interaction between adiabatic QG lifting and moist convection in monsoon depressions using an atmospheric reanalysis and idealized model. Inversion of the QG omega equation in the reanalysis shows that in the downshear, heavily precipitating region, adiabatic QG ascent, due to advection of vorticity and temperature, is comparable to diabatic ascent in the lower troposphere, while diabatic ascent dominates in the middle and upper troposphere. The causal influence of adiabatic QG lifting on precipitating ascent in monsoon depressions is then examined in the column QG modeling framework, where moist convection evolves in the presence of vorticity and temperature advection. The heavy observed precipitation rates are only simulated when moist convective heating amplifies QG ascent, with this interaction accounting for roughly 40% of the increase in precipitation relative to the basic state. Another 40% of this increase is produced by enhanced surface wind speed in the surface enthalpy flux parameterization, which represents the effect of cyclonic winds in the monsoon depression. Horizontal advection of the mean-state poleward moisture gradient accounts for the remaining 20% of the precipitation increase. In the upshear region, adiabatic QG subsidence and horizontal moisture advection both suppress precipitation, and are opposed by wind-enhanced surface enthalpy fluxes.