Location of the Preferred Region for Tropical Cyclogenesis in Strong Monsoon Trough Pattern over the Western North Pacific

We examined the processes of tropical cyclogenesis in strong monsoon trough pattern over the western North Pacific (WNP) using reanalysis data and numerical experiments. Composite analysis showed that more tropical cyclones are likely to form in the central WNP (130-165 degrees E) and fewer tropical cyclones appear in the western (120-130 degrees E) and eastern (165-180 degrees E) WNP when monsoon trough extends southeastward. Numerical experiments with the same weak artificial vortices inserted into eight different regions of the monsoon trough showed that weak tropical disturbances tend to develop more rapidly in the central WNP near 140-160 degrees E, particularly near 150-155 degrees E when the monsoon trough extends eastward, whereas weak tropical disturbances tend to develop more slowly in the eastern WNP near 165-170 degrees E and do not form in the western WNP near 120-137.5 degrees E. Our modeling results are consistent with the observational analyses. The failure of tropical cyclogenesis in the western WNP is due to the decrease of the moisture and heat (including the sensible and latent heat) from the underlying ocean, whereas large vertical wind shear and dry conditions in the upper-level of the vortex reduce the gradient of intensification of tropical disturbances in the eastern WNP when the vortices have a similar initial intensity.