The interactions between monsoon circulations and tropical disturbances in Northwest Pacific, where the low-level mean flow is westerly in the west and easterly in the east, are studied with a barotropic model. Our model results suggest that the scale contraction by the confluent background flow, the nonlinear dynamics, the b-effect, and the large-scale convergence are important for the energy and enstrophy accumulation near the region where the zonal flow reverses. The largest accumulation occurs when the emanating zonal wavelength is around 2000 km. The intensified disturbance may disperse energy upstream, leading to a series of trailing anticyclonic and cyclonic cells along the northwestward propagation path. When a more realistic monsoon-like background flow is present, this process leads to the formation of new disturbances in the confluence zone by a vortex axisymmetrization dynamics. Thus, our results indicate that the scale contraction and nonlinear effects may cause a succession of tropical disturbances to develop without disturbance-scale diabatic effects.

In this paper we study the interaction of the barotropic energy dispersion mechanism with the nonlinear wave accumulation mechanism. These two nonlinear mechanisms as well as the low-frequency variation of the large-scale flow (monsoon confluent flow) may generate disturbances with different frequencies. The interaction between them may lead to an observed alternation of active and inactive periods of tropical disturbance formation in the Northwest Pacific.