The Thermal Effect of the Tropical Western Pacific on the Interannual Variability of Monsoon Trough Tropical Cyclone and Typhoon Activity over the Northwest Pacific

The results analyzed by using JTWC and reanalysis data show about 70% or more of tropical cyclones geneses occur in monsoon trough over the Northwest Pacific (NWP). And the interannual variability of the monsoon trough position is very obvious, which can cause a significant interannual variability of tropical cyclone and typhoon (TC in short) activities (including geneses and tracks) over the Northwest Pacific (NWP) are very obvious. These variabilities are closely associated with the thermal states of the tropical western Pacific (TWP) especially the OHC of the TWP. When the TWP is in a warm state, then the monsoon trough and convergent region of zonal wind field shifts westward and northward over the NWP. In this case, the region of the transition from the MRG wave to the TD type disturbance also shifts westward and northward, which leads to the westward and northward shift of TC geneses and tracks over the NWP. Therefore, more TCs can influence China. Oppositely, when the TWP is in a cold state, the monsoon trough and the convergent region of zonal wind field shifts eastward and southward over the NWP. In this case, the region of the transition from the MRG wave to the TD type disturbance also shifts eastward and southward, which leads to the eastward and southward shift of TC geneses and tracks, i.e., TC tracks are easy to turn northeastward near 130°E. Therefore, less TCs influence China, but more TCs can affect Japan and South Korea.

Moreover, the physical process of effect of the monsoon trough on the TC activities mentioned above is dynamically discussed further and is simply simulated with shallow-water wave model.