Current theories on El Ni o-Southern Oscillation: A review

The current state of knowledge on the physics of the El Ni o-Southern Oscillation (ENSO) phenomenon is presented. The main observed features are described, and the basic physics that control the phenomenon are discussed with results from models of various levels of complexity. ENSO is a coupled ocean-atmosphere phenomenon in which feedback mechanisms are very important; therefore clear causality relationships are difficult to establish, and despite 20 years of great advances in observing, explaining and predicting it, important questions still remain.