Origins of synoptic and intraseasonal variabilities in the Northwestern Pacific monsoon trough

The monsoon trough (MT) in the western North Pacific (WNP) exhibits strong synoptic and intraseasonal variabilities. The origins of the variabilities were investigated through idealized numerical model experiments and a theoretical model. To demonstrate to what extent the synoptic and intraseasonal variabilities in MT arise from signals outside of the region, idealized numerical model experiments in the presence and the absence of propagating synoptic and intraseasonal signals from the outside of the region were carried out without lateral forcing signals. The simulation results indicate little change in the intensity and structure of the synoptic and intraseasonal variabilities over MT. This implies that the origin of these variabilities arises from internal atmospheric dynamics in the region. A 2.5-layer theoretical model was further constructed, in which an idealized background mean state derived from the observed moisture and zonal wind profiles in the MT region is specified. The model extends the traditional 2-level quasi-geostrophic model framework by including a prognostic moisture tendency equation and an interactive planetary boundary layer. The eigenvalue analysis of this theoretical model shows two most unstable modes. The first has a preferred zonal wavelength of 2700 km and a westward phase speed of 1.5 m s− 1, consistent with the observed synoptic mode characteristics. The second has a much larger (12000 km) zonal wavelength and near-zero zonal phase speed, resembling the observed intraseasonal mode characteristics.