Interdecadal shifts in tropical sea surface temperature modulate autumn tropical cyclone genesis over the western North Pacific

Abstract

Using satellite era data from 1979 to 2019, we find an abrupt change in autumn (September–November) tropical cyclone (TC) activity over the western North Pacific (WNP) in the late 1990s. Autumn TC genesis frequency (TCGF) has significantly decreased in 2000–2019 relative to 1979–1999. Specifically, the decreased number of autumn WNP TCs forming south of 15°N, referred to as the southern WNP (SWNP), accounts for ∼80% of the overall autumn reduction and ∼ 70% of the annual reduction. Interdecadal shifts in large-scale sea surface temperature (SST) patterns have led to a weakening of the El Niño-Southern Oscillation (ENSO)–WNP TCGF relationship and an enhancement of the relationship between SST in the tropical western Pacific Ocean and TCGF during 2000–2019. From 1979 to 1999, ENSO significantly influenced the interannual variability of autumn TCGF by affecting the lower-level vorticity associated with monsoon trough over the WNP basin. Since 2000, the tropical western Pacific SST has contributed to the coupling of lower-level cyclonic circulation and an upper-level anticyclonic circulation, thus enhancing the interannual associations between vertical motion and autumn TCGF. The recent development of a Pacific Meridional Mode-like SST configuration appears to intensify the relationship between dynamic conditions for TCGF and tropical western Pacific SST. Sensitivity experiments conducted with the Community Earth System Model version 1.2.2 further support these findings and highlight the role of changes in tropical SST patterns in influencing WNP TC activity throughout the study period.