The behavior of Rossby waves in summer monsoon-like flow

The interactions between tropical monsoon circulations and equatorial wave disturbances in the western North Pacific, where the low-level mean flow shows a rapid zonal variation, are studied. Our barotropic calculations suggest that the scale contraction by the confluent background flow, the nonlinear dynamics, the $\beta$ effect, and the large scale convergence are important for the energy and enstrophy accumulation near the region where the zonal flow reverses. The energy/enstrophy accumulation can be maintained with a continuous Rossby wave emanation upstream. The vorticity gradient in the waves is sharpened by the scale contraction as the waves approach the confluence zone. The nonlinear dynamics become dominate as a result of the scale contraction. Disturbance asymmetries with respect to the central latitude develop due to the nonlinear interaction of waves with the planetary vorticity gradient. A pair of vorticity centers eventually straddle the central latitude with the cyclone (anticyclone) in the north (south). The vorticity pair is associated with an elongated westerly flow along the central latitude. The zonal scale of the westerlies is about 3000 km and its speed is about 1 $ms^{-1}$. A zonal wavelength of around 2000 km for the emanating waves is favored for the largest enstrophy/energy accumulation. For Rossby waves with zonal wavelengths longer than 3000 km, it appears that there will be less scale contraction and nonlinear effect. On the other hand, a Rossby wave with a shorter zonal wavelength and no diabatic effects on the disturbance scale, can not hold itself in a coherent fashion against dispersive effects. It is argued that the energy/enstrophy accumulation near the critical longitude is important for the initiation of synoptic tropical disturbances in the western North Pacific. Neither convergence nor convection within the wave is required for the dynamics of accumulation.

In a more realistic monsoon-like background flow calculation, the northwestward propagation of disturbances is simulated. This northwestward propagation is due to the vortex interaction with the planetary vorticity gradient and due to the background flow advection. The intensified disturbance may disperse energy upstream, which leads to a series of trailing anticyclonic and cyclonic cells along the northwestward propagation path. When a monsoon-like flow with a flow reversal is present, the energy dispersion leads to the formation of new disturbances in the confluence zone. The successive formation of new disturbances has a period of approximately 8 days, and the spatial scale between them is about 3000 km. Scale contraction in monsoon-like background flows and nonlinear effects appear to be important in the initiation of new disturbances by the energy dispersion process.