Weakening of Equatorial Zonal Wind Stress during the Mature Phase of ENSO Tied to the Meridional Structure of the Pacific Warm Pool

Abstract The evolution of El Niño–Southern Oscillation (ENSO) is known to be phase locked to the calendar year, yet the underlying mechanisms are not well understood. Previous studies have documented that the southward shift of zonal wind anomalies is an important factor contributing to the timing of ENSO’s peak. This study revisits the southward wind shift from the perspective of the weakening of anomalous equatorial zonal wind stress. Diagnostic analysis of the steady-state surface zonal momentum budget indicates that the weakening of equatorial zonal wind stress results from the competition between the zonal pressure gradient force and meridional advections. The annual mean state of the Pacific warm pool, with warm sea surface temperatures (SSTs) in the Southern Hemisphere, amplifies meridional advections in winter and spring, enabling their intensification during winter to offset the increase in zonal pressure gradient force and result in the weakening of equatorial zonal wind stress. Simple atmospheric model experiments further distinguish the weakening of equatorial zonal wind stress from the intensification of zonal winds south of the equator. Although the intensification of zonal winds south of the equator resembles the southward wind shift, it is accompanied by a weakening of equatorial zonal wind stress only when the intensification of negative meridional advections is strong enough to offset the increase in zonal pressure gradient force. Our findings suggest that the asymmetric meridional structure of the Pacific warm pool’s annual mean state is fundamental to ENSO’s peak in boreal winter. Significance Statement Despite the evolution of El Niño–Southern Oscillation (ENSO) having long been recognized to be tied to the annual cycle, the underlying physical dynamics remain incompletely understood. Current state-of-the-art climate models often struggle to accurately simulate this characteristic of ENSO. This study elucidates the roles of the annual mean state and annual cycle of the Pacific warm pool in causing the winter decrease in equatorial wind stress associated with the timing of ENSO’s peak. We demonstrate that the meridional distribution of the Pacific’s annual mean sea surface temperatures (SSTs) is a key factor contributing to ENSO’s winter peak. The findings of this study enhance our understanding of the mechanisms driving ENSO evolution, therefore having potential for improving the simulation accuracy of ENSO.