(In revision) An irregularly occurring pattern of abnormal warming of the surface coastal waters off Ecuador, Peru and Chile This coupled atmosphere-ocean phenomenon is associated with the fluctuation of inter-tropical surface pressure pattern and circulation in the Indian and Pacific oceans, called the Southern Oscillation. There have been a number of attempts to define El Ni–o, both quantitatively and qualitatively, but none has achieved universal recognition. This phenomenon triggers a shift in seasonal patterns of weather systems over many subtropical and mid-latitude parts of the globe.