Distinct Influences of the ENSO-Like and PMM-Like SST Anomalies on the Mean TC Genesis Location in the Western North Pacific: The 2015 Summer as an Extreme Example

Abstract

Abstract This study reports the different effects of tropical and subtropical sea surface temperature anomalies (SSTAs) on the mean tropical cyclone (TC) genesis location in the western North Pacific (WNP), a TC–SSTA relationship that has been largely ignored. In the Pacific, the interannual variability of the tropical SSTA in the boreal summer is characterized by an El Niño–Southern Oscillation (ENSO)-like pattern, whereas the subtropical SSTA exhibits a Pacific meridional mode (PMM)-like structure. Partial correlation analysis reveals that the ENSO-like and PMM-like SSTAs dominate the south–north and east–west shift of mean TC genesis location, respectively. The 2015/16 El Niño was a strong event comparable with the 1997/98 event in terms of Niño-3.4 SSTA. However, the mean TC genesis location in the WNP during the summer of 2015 exhibited an unprecedented eastward shift by approximately 10 longitudinal degrees relative to that in 1997. Whereas the ENSO-like SSTAs in 1997 and 2015 were approximately equal, the amplitude of the PMM-like SSTA in 2015 was approximately twice as large as that in 1997. Numerical experiments forced by the ENSO-like and PMM-like SSTAs in June–August 2015 reveal that the positive PMM-like SSTA forces an east–west overturning circulation anomaly in the subtropical North Pacific with anomalously ascending (descending) motion in the subtropical central (western) Pacific. The mean TC genesis location in the WNP therefore shifts eastward when warmer SST occurs in the subtropical eastern Pacific. This finding supports the hypothesis that the extremely positive PMM-like SSTA in the summer of 2015 caused the unprecedented eastward shift of the TC genesis location in the WNP.