El Nino Dynamics

T El Nino/Southern Oscillation phenomenon (ENSO, for short) is the strongest source of natural variability in Earth's climate system. ' Although ENSO originates in the tropical latitudes of the Pacific Ocean, its climatic impact is felt globally. Variations in major rainfall systems that are attributed to ENSO range from droughts in Indonesia and Australia to storms and flooding in Ecuador and the US. The term "El Nino" was originally used by Peruvian fishermen for a warming of coastal waters that begins around Christmas. [El Nino is Spanish for "the Christ child.") The term is now used to refer to the large-scale warming of the whole tropical Pacific that takes place every four years on average and alternates with an opposite cold phase, sometimes called La Nina. The Southern Oscillation was discovered in 1923 by the British climatologist Gilbert Walker, who sought to explain why the Indian monsoon fails in some years. Walker found irregular standing oscillations in atmospheric surface pressure that span the Pacific from east to west. The term "Southern Oscillation" now also applies to the associated large-scale changes in atmospheric circulation. Climate dvnamicists now recognize that El Nino and the Southern Oscillation are simply aspects of the same coupled mode of the ocean-atmosphere system. Observational evidence for this connection is apparent in figure 1, which shows the anticorrelation of variations in sea surface temperature (SST) and surface pressure gradient. Box 1 on page 34 describes the long-term average state of the Pacific climate system, whose interannual variability constitutes ENSO.