Interannual Relationship between ENSO and Tropical Cyclone Genesis over the Southwest Indian Ocean and Its Modulation by the Interdecadal Pacific Oscillation

Abstract

This study investigates the interannual relationship between El Niño-Southern Oscillation (ENSO) and tropical cyclone (TC) genesis potential (measured by the Dynamic Genesis Potential Index or DGPI) over the Southwest Indian Ocean (SWIO) and its modulation by the Interdecadal Pacific Oscillation (IPO) during 1959-2020 based on observation and reanalysis. Results show that DGPI can well capture the spatial distribution and seasonal cycle of TC genesis over the SWIO. A positive correlation is found between ENSO and the SWIO-DGPI on interannual timescales, with higher (lower) DGPI and in-creased (decreased) TC occurrence frequency during El Niño (La Niña) years, indicating an important role of ENSO in modulating TC activity in the SWIO. Further analyses show that ENSO favors the SWIO TC genesis primarily through modulating vertical wind shear and upward motion. Furthermore, the interannual relationship between ENSO and the SWIO DGPI also exhibits re-markable interdecadal variations that are likely modulated by the IPO with strong (weak) correlation in positive (negative) IPO phases. This is because the ENSO-driven anomalies of vertical wind shear, upward motion, and low-level vorticity are larger under positive IPO phases than its negative phas-es, which favor the sensitivity of the SWIO TC genesis to ENSO and thus promote a higher correlation between the two. These findings highlight the importance of considering interdecadal variability when assessing the EN-SO-TC relationship over the SWIO and contribute to improving seasonal pre-diction of TCs in the region.