Energetics of zonal waves during different phases of monsoon

Energetics of lower tropospheric zonal waves during onset, established and withdrawal phases of monsoon have been studied for 1994, 1995 and 1996. The analysis show that energetics of wave 0 over R1 (10°S-10°N), long waves (waves 1-2) over R2 (10°N - 30°S) and short waves (waves 3-10) over R3 (30° N - 50° N) influence the monsoon activity over India on intra-seasonal scale. The weekly analysis of the energetics of zonal waves indicates that the momentum transport of wave 0 over latitudinal belt L0 (12° S - 3° N), wave 1 over the belt L1(10° N - 15° N) and wave 2 over the belt L2 (33° N - 45° N) is related to all India rainfall on a weekly scale. Larger southward momentum transport of wave 0 over L0 and larger northward momentum transport of wave 1 over L1 and wave 2 over L2 enhance the monsoon activity over India.

A numerical study of quasi-geostrophic flow over isolated topography

An extensive set of numerical simulations is performed to synthesize the behaviour of a barotropic flow over isolated topography on an f-plane and on a β-plane. The model is based on the quasi-geostrophic vorticity equation, where the dissipation terms have been retained. The use of open boundary conditions. following the method described by Orlanski (1976), allows detailed simulation of time-dependent flows over long periods.On the f-plane, the ultimate solution is always characterized by a typical vorticity field with an anticyclonic vortex trapped over the topography, but different transient regimes occur, related to the importance of advection versus topography effect: direct advection of the positive vortex for strong flows; eddy interactions and double-vortex-structure appearance for weaker flows; oscillatory regimes with topographic trapped-waves generation for very strong vorticity-interaction cases.On the β-plane, and for prograde flows, the situation is complicated by a Rossby wave pattern extending mainly downstream but also having an upstream component corresponding to zonal waves. For retrograde flows the obstacle does not excite Rossby waves but a transient response with zonal waves whose lifetime depends on the nonlinearity.