Seesaw of Tropical Cyclone Frequency between Eastern and Western Regions of the Western North Pacific

Abstract Because of the limited length of observed tropical cyclone (TC) data and low confidence in modeling TC genesis frequency, it has been difficult to understand why the genesis frequency of global TCs has remained nearly invariant while regional variations are highly pronounced. Investigating the covariability of TC genesis frequency between different regions may shed light on this question. Here, we identify that TC genesis frequency varies out of phase between the eastern and western regions of the western North Pacific (WNP). Such a seesaw relationship could be explained by the east–west dipole patterns of low-level vorticity and midtropospheric relative humidity in the WNP, associated with variations in atmospheric conditions. Composite analyses and numerical experiments show that a combination of cooling in the WNP and warming in the north Indian Ocean (NIO) exerts a significant influence on the seesaw pattern. Our study adds more evidence to believe on number conservation of worldwide TC genesis. Significance Statement Little had been known about why the genesis frequency of global tropical cyclones has remained nearly invariant in a changing climate. To find out whether there is any covariability of tropical cyclone genesis frequency between different regions may provide a chance to understand such a steady trend. The western North Pacific is the ocean basin where tropical cyclones are the most active in the world. In this study, it is observed that tropical cyclone genesis frequency between the eastern and western regions of the western North Pacific has a negative correlation. It is demonstrated that the zonal contrast of sea surface temperature in the western North Pacific to the north Indian Ocean plays a significant role in driving the seesaw relationship of tropical cyclone genesis frequency by moderating the large-scale atmospheric circulation.