Muted ENSO Modulation of Tropical Cyclone Outer Size Over the Western North Pacific in Recent Decades

Abstract

Utilizing a data set of objectively estimated tropical cyclone (TC) size based on deep learning algorithms, this study investigates the relationship between the interannual variation of TC outer size over the western North Pacific and El Niño-Southern Oscillation (ENSO) during July–September from 1981 to 2017. The size of TCs is measured by the mean radius of gale-force winds at their lifetime maximum intensity. Our results reveal an abrupt decadal change in the ENSO-TC size relationship: the annual mean TC size exhibited a strong correlation with the Niño 3.4 SST index before 1998, but this correlation has significantly weakened since then. This change is primarily attributed to the more uniform distributions of cyclone expansion rate (CER) across ENSO phases during the past two decades. Climatically, environmental conditions favorable for TC size expansion weaken with increasing latitude, resulting in a dominant meridional gradient of CER. Before 1998, TC activity displayed a pronounced north-south contrast between El Niño and La Niña years, leading to a significantly higher mean CER for TCs during El Niño episodes. In recent decades, however, interannual variations in TC genesis density have shifted to a southeast-northwest dipole pattern. This shift, along with changes in TC tracks, has substantially increased the latitudinal overlap of TC occurrences between warm and cold phases, thereby narrowing differences in CER distributions. Concurrently, changes in environmental conditions have become more favorable for TC size expansion during La Niña years, further reducing disparities in TC size distributions across ENSO phases.