Consistent Late Onset of the Western North Pacific Tropical Cyclone Season Following major El Nino Events

Most studies have focused on variations of tropical cyclone (TC) frequency, intensity, and track over the western North Pacific (WNP), but variability of WNP TC season onset date (TCSO) has been less studied. Recent research has indicated a close association between WNP TCSO and sea surface temperature (SST) over the tropical Indian Ocean and the tropical central-eastern Pacific. This study has found that relationship between TCSO and SST underwent an inter-decadal change in the late 1990s, likely due to a climate shift that occurred around that time. An observed significant correlation between TCSO and SST before the late 1990s has been insignificant since that time. This was confirmed by the fact that El Nino Southern Oscillation (ENSO) at 0.46 positively correlates with TCSO from 1965-1999 (significant at the 95 % level), and the correlation becomes insignificant (0.16) during 1998-2016. Further analysis suggests that the close association between TCSO and SST is robust only for major El Nino events, with consistently extreme late TCSO following major El Ninos during the satellite era. Accompanying the decay of major El Ninos, tropical equatorial easterly anomalies in the WNP are driven by a Matsuno-Gill-type response to the specific SST anomaly pattern over the tropical Indo-Pacific sector. This in turn induces an anomalous anticyclone, anomalous westerly vertical wind shear, reduced mid-level moisture and suppressed convection over the WNP basin-all of which are unfavorable for WNP TCs, resulting in delayed TCSO following major El Nino events. These inter-decadal changes in the inter-annual correlation between TCSO and ENSO are largely due to the changing influence of moderate El Nino events on TCSO before and after the late 1990s. This study improves understanding of the ENSO-TC relationship, which should aid seasonal outlooks of WNP TC activity.