The Energetics and Propagation Dynamics of Tropical Summertime Synoptic-Scale Disturbances

Periods of enhanced synoptic activity in the tropical western Pacific, Bay of Bengal-northeastern India, and African-Atlantic regions are identified by extended empirical orthogonal function analysis. Composite mete-orological fields for such active periods at various sites are constructed using European Centre for Medium-Range Weather Forecasts (ECMWF) analyses for the northern summers of 1980–1987. These composite data form the basis for evaluating the contributions of different dynamical processes to local balances of beat, moisture, vorticity, enstrophy, and energy, so that the propagation dynamics and principal energy sources of the tropical disturbances may be studied in detail. In all three tropical regions considered here, the westward propagation of the synoptic-scale disturbances is attributed mostly to vorticity advection by both the time-mean flow and the transient fluctuations. In the western Pacific and Indian sectors, condensation heating associated with cumulus convection is seen to be the most significant energy source for the tropical disturbances. The stretching effect associated with large-scale convective activity is the most important mechanism for the generation of eddy enstrophy in these maritime disturbances. There is substantial barotropic conversion of enstrophy and kinetic energy from the time-mean flow to the transient fluctuations. In the African-Atlantic sector, the disturbances along the more prominent northern track at approximately 20°N are accompanied by dry desert-type convection. Vortex stretching associated with the dry convection is still the most important process for the generation of eddy enstrophy in these disturbances. However, the main source of available potential energy for these North African disturbances is the baroclinic conversion from the time-mean flow to the transient fluctuations along the zone of strong temperature gradients south of the Sahara. The dynamics and energetics of the weaker southern disturbances along 10°N in the African-Atlantic sector are similar to the moist disturbances found in the western Pacific and Indian sectors.