El Nino and the related phenomenon Southern Oscillation (ENSO): The largest signal in interannual climate variation

El Nino and the related phenomenon Southern Oscillation (ENSO) is the stron gest signal in the inter annual variation of ocean-atmosphere system, It is mainly a tropical event but its impact is global. ENSO has been drawing gr eat scientific attention in many international research programs. There has been an observational system for the tropical ocean, and scientists have k nown the climatologies of the upper ocean, developed some theories about th e ENSO cycle, and established coupled ocean-atmosphere models to give encou raging predictions of ENSO for a 1-year lead. However, questions remain abo ut the physical mechanisms for the ENSO cycle and its irregularity, ENSO-mo nsoon interactions, long-term variation of ENSO, and increasing the predict ive skill of ENSO and its related climate variations.