Some remarks on the westward propagation of the monsoon depression

Abstract

Several westward propagation properties of the Indian monsoon depression were neglected by previous studies. They include: (1) the slower propagation speed of the depression depicted by a quasi-geostrophic model, (2) the initiation of the asymmetric secondary circulation with respect to the depression center, and (3) the absence of the depression perturbation in the upper troposphere. Some further insights into these neglected propagation properties of the depression are obtained from the streamfunction budget analysis with the ECMWF (European Centre for Medium Range Weather Forecasts) reanalysis data. (1) The inclusion of relative vorticity stretching, which is neglected in a quasi-geostrophic model, increases the depression's westward propagation speed. (2) Within the large-scale environment of the summer monsoon, the coupling of the east-west differentiation of the meridional absolute vorticity advection with the CISK mechanism is conducive to the initiation and development of the asymmetric secondary circulation associated with the depression. (3) The Tibetan high is formed by summertime global-scale stationary waves which are maintained by a Sverdrup balance. The positive streamfunction tendency induced by the upper-tropospheric vortex stretching over the monsoon region suppresses the development of the monsoon depression in the upper troposphere.