The annual-cycle modulation of meridional asymmetry in ENSO's atmospheric response and its dependence on ENSO zonal structure

Abstract

Previous studies documented that a distinct southward shift of central Pacific low-level wind anomalies occurring during theENSOdecaying phase is caused by an interaction between thewestern Pacific annual cycle and El Niño-Southern Oscillation (ENSO) variability. The present study finds that the meridional movement of the central Pacificwind anomalies appears only during traditional eastern Pacific El Niño (EP ElNiño) events rather than in central Pacific El Niño (CP El Niño) events in which sea surface temperature (SST) anomalies are confined to the central Pacific. The zonal structure of ENSO-related SST anomalies therefore has an important effect on meridional asymmetry in the associated atmospheric response and itsmodulation by the annual cycle. In contrast toEPEl Niño events, the SST anomalies ofCPElNiño events extend farther west toward thewarmpool region with its climatologicalwarm SSTs. In thewarm pool region, relatively small SSTanomalies are thus able to excite convection anomalies on both sides of the equator, even with a meridionally asymmetric SST background state. Therefore, almost meridionally symmetric precipitation and wind anomalies are observed over the central Pacific during the decaying phase of CP El Niño events. The SST anomaly pattern of La Niña events is similar to CP El Niño events with a reversed sign. Accordingly, no distinct southward displacement of the atmospheric response occurs over the central Pacific during the La Niña decaying phase. These results have important implications for ENSOclimate impacts over EastAsia, since the anomalous low-level anticyclone over the western North Pacific is an integral part of the annual cycle-modulated ENSO response.