ENSO, Atlantic Climate Variability, and the Walker and Hadley Circulations

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This chapter describes and discusses the Walker and Hadley circulations associated with the El Nino/Southern Oscillation (ENSO), the Atlantic “Nino”, the tropical Atlantic meridional gradient variability, the Western Hemisphere warm pool (WHWP), and the North Atlantic Oscillation (NAO). During the warm phase of ENSO, the Pacific Walker circulation, the western Pacific Hadley circulation, and the Atlantic Hadley circulation are observed to be weakened, whereas the eastern Pacific Hadley circulation is strengthened. During the peak phase of the Atlantic Nino, the Atlantic Walker circulation weakens and extends eastward and the Atlantic Hadley circulation strengthens. The tropical Atlantic meridional gradient variability corresponds to a meridional circulation in which the air rises over the warm sea surface temperature (SST) anomaly region, flows toward the cold SST anomaly region aloft, sinks in the cold SST anomaly region, then crosses the equator toward the warm SST region in the lower troposphere. During periods when the NAO index is high, the atmospheric Ferrei and Hadley circulations are strengthened, consistent with surface westerly and easterly wind anomalies in the North Atlantic and in the middle to tropical Atlantic, respectively. The chapter also discusses a tropo-spheric bridge by the Walker/Hadley circulation that links the Pacific El Nino with warming of the tropical North Atlantic (TNA) and the WHWP.