An observational study on the energetics of the onsent monsoon vortex, 1979

Abstract

AbstractTime variations, of the vertically integrated, and vertical distribution of time‐mean zonal and eddy components of the kinetic and available potential energy, and the associated energy conversions, are examined in order to understand the formation and growth mechanisms associated with the monsoon onset vortex during 14‐17 June 1979. For the computations, the area 0‐22.5°N and 48.75°E‐78.75°E and the vertical layer 1000‐300 hPa are considered. The dynamical characteristics of the large‐scale monsoon current surrounding the onset vortex are also investigated. Strengthening of the meridional and vertical shear of the mean zonal wind is noticed during the period. The vortex is formed to the north of the westerly jet, in the zone of maximum cyclonic shear. The northward shift of the large‐scale monsoon current is noticed in association with northward movement of the onset vortex.It is identified from the mean energetics that both barotropic and baroclinic energy conversions, and also the boundary terms, maintain the vortex. Baroclinic energy transfer to eddy kinetic energy (KE) takes place not only in the upper levels but also in the lower levels. Further, the energy conversion from zonal kinetic energy (KZ) to KE dominates over the energy conversion from eddy available potential energy (AE) to KE. The barotropic energy conversion due to the down‐gradient eddy transport of meridional momentum is more than that due to zonal momentum transport. The transfer of the vertical wave‐energy flux, due to the eddy pressure interaction, away from the middle level also contributes significantly to the maintenance of KE in the upper and lower levels. The generation rate of the zonal and eddy available potential energy and the frictional dissipation of KE are computed b the residue method; the values are 0.54 W m‐2, 0.25 W m−2 and 0.65 W m−2, respectively.The meridional and vertical transports of momentum and heat are examined in relation to the structure of the vortex. The requirement of minimum vertical resolution for the vortex modelling studies is indicated.