The equation for the mean tangential wind in a storm-following cylindrical coordinate system is used to investigate the intensity changes of tropical cyclones and monsoon depressions. In this equation, the symmetric, the resolved asymmetric and the subgrid-scale contributions are fully separated. The tangential wind changes can be interpreted in terms of absolute vorticity fluxes, angular momentum fluxes, PV fluxes or the Eliassen-Palm flux-divergence. The corresponding eddy fluxes are compared, and the impact of convection on the intensity changes as well as the interaction between a monsoon depression and upper level troughs and ridges are investigated. Analyses of observational data for a monsoon depression that formed near the coast of northwestern Australia in February 1994 show that all terms in the tangential wind equation are important, except for the drift term. This includes the subgrid-scale terms which are presumed to be largely due to deep convection.
In the lower and middle troposphere, the main terms contributing to the cyclonic intensification of the monsoon depression are the radial absolute vorticity fluxes (cyclonic inward fluxes of relative and planetary vorticity; or the corresponding terms that include the Eliassen-Palm fluxes, the radial PV-fluxes, or the angular momentum fluxes) and the vertical advection of tangential wind. In the upper troposphere, the cyclonic tangential wind change can be attributed mainly to the relative vorticity fluxes (anticyclonic outward fluxes of relative vorticity), the vertical advection of tangential wind, and the subgrid-scale eddy contributions. The weakening of the upper-level anticyclone associated with these effects in connection with the channelled, widespread outflow is related to the strengthening of the monsoon depression. The inward movement of the eddy PV-flux-maxima in the upper troposphere reflects the approach of PV anomalies associated with passing mid-latitude troughs. We assume that the convection near the monsoon depression is enhanced in front of the approaching PV anomalies by the reduction of static stability and by the vacuum cleaner effect, and this influences the intensification of the depression. As a check on the data quality, the results using BMRC analyses have been compared with those using ECMWF analyses. Despite the high diurnal variability, the patterns of the daily-mean symmetric and resolved-asymmetric contributions agree relatively well, although the figures from the higher resolution ECMWF-analyses show more details with more distinct peaks