Relationship of the Southeast Asian summer monsoon and Mascarene High to the tropical cyclone activity in the western North Pacific

In the present study, a highly positive correlation between the western North Pacific tropical cyclone (TC) genesis frequency and the Southeast Asian summer monsoon (SASM) was found during the June-August period. In order to examine the cause of the positive correlation between the two variables, the 14 years with the highest SASM index (positive SASM years) and the 14 years with the lowest SASM index (negative SASM years) were selected from a total of 39 years (1977-2015) and the mean difference between the two periods was analysed. In the positive SASM years, more TCs were generated in most of the western North Pacific region and tended to move in a northerly direction from the far northeastern sea of the Philippines towards Korea via the East China Sea. In the negative SASM years, TCs moved westward from the eastern sea of the Philippine to the southern coast of China and the Indochinese Peninsula via the South China Sea. A similar result is found in further comparisons in which years with strong El Nino-Southern Oscillations are removed. Anomalous westerlies from the Mascarene High moved eastward, thereby forming anomalous cyclonic circulations in the western North Pacific. Thus, the Mascarene High played a decisive role in strengthening anomalous cyclonic circulations in the western North Pacific. This finding was verified by examining various environmental factors that affected TC genesis and the correlation analyses between the 850-hPa geopotential height and the western North Pacific TC genesis frequency during the June-August period in addition to an analysis of the velocity potential and divergent winds.