How has the North Pacific Gyre Oscillation affected peak season tropical cyclone genesis over the western North Pacific from 1965 to 2020?

The North Pacific Gyre Oscillation (NPGO) is an important mode of decadal variability in North Pacific sea surface temperature (SST) and sea surface height. This study investigated the potential influence of the NPGO on spatial characteristics of peak season (July to October) tropical cyclone genesis (TCG) number over the western North Pacific (WNP) from 1965 to 2020. We show that the NPGO was the first leading empirical orthogonal function mode of North Pacific SST during the peak tropical cyclone season in that time. On a decadal time scale, the NPGO has opposite impacts on TCG in the west and east WNP. The relatively weak positive correlation west of 140° E and the strong negative correlation east of 140° E result in an overall significant negative correlation between the NPGO and WNP total TCG number ( r = −0.49), which is much more robust than the relationship between the Pacific Decadal Oscillation and TCG. The critical factors of the NPGO that affect TCG are vertical motion in the west WNP and vertical wind shear (VWS) in the east WNP. The positive NPGO pattern could induce an anomalous off-equatorial vertical circulation, resulting in an upward motion and increased convective precipitation in the west WNP, favoring local TCG. The anomalous convective precipitation enhances the zonal gradient of the atmospheric heat source in the east WNP, increasing VWS. The North Pacific low-level anticyclonic and upper-level cyclonic associated with the NPGO further enhance the VWS in the east WNP and lead to the negative low-level relative vorticity, inhibiting local TCG. This study emphasizes the importance of the climate impact of the NPGO in recent decades. The findings here have significant implications for the decadal prediction of change in WNP TCG.