What Controls the Decadal Variation of Tropical Cyclone Genesis Frequency over the Western North Pacific?

The decadal variation and its cause of tropical cyclone (TC) activity over western North Pacific (WNP) are examined based on data from 1970 to 2009. The results show that there is a significant decadal variability, which accounts for 55.14% of the total variance in the TC genesis frequency during July-October (JASO). The highest TC frequency occurred in the period of 1989-1997 with 21.67 per year, above climatological mean during 1970-2009 by 3.9. It is found that the anomalously high TC frequency in this period was strongly controlled by the large-scale modes of decadal variability in sea surface temperature (SST) over both the North Atlantic and Pacific Ocean. The North Atlantic and Pacific SSTs show a decadal oscillation, which alters the large-scale low-level vorticity and vertical wind shear, and thus modulating the TC genesis frequency over the WNP. Much higher TC frequency occurred when the SST anomalies over the North Atlantic are negative and the North Pacific PDO-like mode was at its positive phase. It is shown that SST anomalies in the North Atlantic contributed much more to the anomalously high TC frequency than those in the Pacific during the period of 1989-1997 (32.67% versus 14.08% of the total variance). The regression and teleconnection analyses show that the large-scale vertical wind shear together with the low-level vorticity east of 150oE over the WNP exhibits a significant response to SST anomalies in the North Atlantic and the North Pacific. This response is conveyed through the Eurasian continent via the East Atlantic (EA)/West Russia (WR) teleconnection pattern.