Some remarks on the westward propagation of the monsoon depression

Abstract

Several westward propagation properties of the Indian monsoon depression were neglected byprevious studies. They include: (1) the slower propagation speed of the depression depicted bya quasi-geostrophic model, (2) the initiation of the asymmetric secondary circulation withrespect to the depression center, and (3) the absence of the depression perturbation in the uppertroposphere. Some further insights into these neglected propagation properties of the depressionare obtained from the streamfunction budget analysis with the ECMWF (European Centre forMedium RangeWeather Forecasts) reanalysis data. (1) The inclusion of relative vorticity stretching, which is neglected in a quasi-geostrophic model, increases the depression’s westward propagationspeed. (2) Within the large-scale environment of the summer monsoon, the coupling ofthe east–west differentiation of the meridional absolute vorticity advection with the CISKmechanism is conducive to the initiation and development of the asymmetric secondary circulationassociated with the depression. (3) The Tibetan high is formed by summertime global-scalestationary waves which are maintained by a Sverdrup balance. The positive streamfunctiontendency induced by the upper-tropospheric vortex stretching over the monsoon region suppressesthe development of the monsoon depression in the upper troposphere.