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 meridional structure of the Pacific Warm Pool’s annual mean state is fundamental to ENSO’s peak in boreal winter.
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