INFLUENCE OF THE INTERANNUAL VARIATION OF CROSS-EQUATORIAL FLOW ON TROPICAL CYCLOGENESIS OVER THE WESTERN NORTH PACIFIC

Abstract:

The influence of the interannual variation of cross-equatorial flow (CEF) on tropical cyclogenesis over the western North Pacific (WNP) is examined in this paper by using the tropical cyclone (TC) best track data from the Joint Typhoon Warning Center and the JRA-25 reanalysis dataset. The results showed that the number of TCs forming to the east of 140°E over the southeastern part of the western North Pacific (WNP) is in highly positive correlation with the variation of the CEF near 125°E and 150°E, i.e., the number of tropical cyclogeneses increases when the cross-equatorial flows are strong. Composite analyses showed that during the years of strong CEF, the variations of OLR, vertical wind shear between 200-850 hPa, 850 hPa relative vorticity and 200 hPa divergence are favorable for tropical cyclogenesis to the east of 140°E over the tropical WNP, and vice versa. Moreover, it is also discussed from the view of barotropic energy conversion that during the years of strong CEF, an eastward-extended monsoon trough leads to the rapid growth of eddy kinetic energy over the eastern part of WNP, which is favorable for tropical cyclogenesis; but during the years of weak CEF, the monsoon trough is located westward in the western part of the WNP, consistent with the growth area of eddy kinetic energy. As a result, there are fewer TC geneses over the eastern part of WNP. Besides, the abrupt strengthening of a close-by CEF 2-4 days before tropical cyclogenesis may be the one of its triggers.