On the evolution of planetary‐scale fields and genesis of monsoon depressions over the Indian region

Abstract

Daily composite Fourier coefficients of height, zonal, meridional and vertical velocity, temperature, vorticity and divergence for zonal wave numbers 0–4, during 7 days of pre‐ and 4 days of post‐depression, from 1000 to 200 hPa in the latitude belt 0–30°N are obtained for six monsoon depressions formed over the Bay of Bengal in the years 2006–2008. The planetary‐scale fields zonally averaged over the Indian region (75–96°E) are examined. Consistent with the quasi‐geostrophic approximation, in the northern sector (15–30°N) the monsoon trough, the low‐level westerlies and the vorticity are strongly intensified and progressively extended upward to 400 hPa preceding depressions. Wave number 1 is the dominant contributor towards the strengthening and vertical upward expansion of these entities linked to an increase of its amplitude in the lower‐ and a large westward phase shift in the mid‐troposphere, respectively. The cyclonic shear zone in the lower troposphere is developed to the north of 15°N due to the combined effect of intensification and sharp decrease of meridional scale of the westerlies. Different northward speeds of the zonal wind reversal and westerly maximum creates a strong cyclonic zone 2 days before depressions at 400 hPa. The planetary‐scale zonal flow in the troposphere satisfies the necessary condition for barotropic instability. The northward (westward) movement of wave number 1 at 400 hPa is 0.73° latitude (5.6° longitude) day−1. An intensification of direct thermal meridional circulation is associated with a southward and upward shift of the primary rising motion zone.The planetary waves have captured the main characteristics of observed vertical and latitudinal profiles of unstable background zonal flow, used earlier for instability studies. It is concluded that wave number 1 is the main contributor to genesis of a depression. The planetary fields weakened in the presence of depressions mainly due to wave numbers 3 and 4.