Interdecadal variability of meridional tropical cyclone genesis over the western North Pacific and associated modulating processes

During the period 1970-2020, evident interdecadal variability of meridional tropical cyclone (TC) genesis occurs in the western North Pacific (WNP) from September through October (SO). From the periods 1970-1996 (IP1) to 1998-2020 (IP2), TC genesis during SO is enhanced (suppressed) in the region north (south) of 20 degrees N. The large-scale modulating processes feature a general warming of sea surface temperature (SST) over the WNP and systematic northward displacement in atmospheric circulations. The northward-shifted Pacific subtropical high and monsoon trough result in an anomalous anticyclone across the subtropical region and an anomalous cyclone across the South China Sea and the northern WNP east of Taiwan. This anomalous cyclone facilitates TC genesis to increase in the northern WNP. Meanwhile, the southeastward-extending monsoon trough retreats northwestward, leading to an anomalous anticyclone over the tropical eastern WNP and a decrease in TC genesis over the southern region. Correlation analyses indicate that the Pacific Decadal Oscillation (PDO) is more effective than global warming in modulating interdecadal increases of TC genesis in the northern region due to inclusions of both trend and oscillation features. Interdecadal decreases of TC genesis in the southern region exhibit a major oscillation feature and are thus mainly affected by interdecadal oscillation components of PDO. More TCs forming in the oceans east of Taiwan are steered by the overlying anomalous cyclone to recurve northward toward oceans to the northeast and north of Taiwan. TC-induced rainfall over Taiwan thus increases by 25% from IP1 to IP2, which accounts for about 82% of increased total rainfall in Taiwan, showing the notable impact of interdecadal TC genesis on local climate in the WNP.