Rossby waves in zonally opposing mean flow: Behavior in Northwest Pacific summer monsoon

The interactions between monsoon circulations and tropical disturbances in the Northwest Pacific, where the low-level mean flow is westerly in the wes t and easterly in the east, are studied with a barotropic model. The author s' model results suggest that the scale contraction by the confluent backgr ound flow, the nonlinear dynamics, the beta effect, and the large-scale con vergence are important for the energy and enstrophy accumulation near the r egion where the zonal flow reverses. The energy/enstrophy accumulation can be maintained with a continuous Rossby wave emanation upstream. The largest accumulation occurs when the emanating zonal wavelength is around 2000 km. Longer Rossby waves experience less scale contraction and nonlinear effect s while shorter Rossby waves cannot hold a coherent structure against dispe rsive effects. The nonlinear energy/enstrophy accumulation mechanism is significantly diff erent from previous linear energy accumulation theories. In the linear theo ries this is primarily accomplished by the slowdown of the Doppler-shifted group velocity through the convergence of mean zonal advection, while in no nlinear dynamics the contraction of the zonal wave scale plays the crucial role. More importantly, after the initial energy increase by the wave accum ulation, linear dynamics will lead to an eventual loss of wave energy to th e mean flow due to the increase of zonal wavenumber near the critical longi tude. Thus, without the presence of other forcing processes such as diabati c heating, the disturbances will decay. In nonlinear dynamics, the sharpeni ng of the vorticity gradient as the waves approach the confluence zone lead s to the development of disturbance asymmetries with respect to the central latitude. This effect is through the nonlinear interaction of Rossby waves with the planetary vorticity gradient. This development leads to a pair of vorticity centers that straddles the central latitude with the cyclone (an ticyclone) in the north (south), and an elongated, weak westerly flow along the central latitude. This elongated westerly flow, which possesses a zona l wavenumber smaller than that in the linear cases, reverses the sign of th e Reynold's stress and allows the energy to grow near the critical longitud e, leading to intensified disturbances. With a more realistic monsoonlike background flow, a northwestward propagat ion pattern with an approximately 8-day period and 3000-km wavelength is pr oduced, in general agreement with observed disturbances in the Northwest Pa cific. The intensified disturbance may disperse energy upstream, leading to a series of trailing anticyclonic and cyclonic cells along the northwestwa rd propagation path. When an opposing current is present, the energy disper sion leads to the formation of new disturbances in the confluence zone by v ortex axisymmetrization dynamics. Thus, our results indicate that the scale contraction and nonlinear effects may cause a succession of tropical distu rbances to develop without disturbance-scale diabatic effects.