Indian Ocean Sea Surface Temperature and El Niño-Southern Oscillation: A New Perspective

International audienceHere we show that the 1976-1977 climate regime shift was accompanied by aremarkable change in the lead-lag relationships between Indian Ocean Sea SurfaceTemperature (SST) and El Niño evolution. After the 1976-1977 regime shift, acorrelation analysis suggests that southern Indian Ocean SSTs observed during lateboreal winter are a key precursor in predicting El Niño evolution as the traditionaloceanic heat content anomalies in the equatorial Pacific or zonal wind anomalies overthe equatorial western Pacific. The possible physical mechanisms underlying this highlysignificant statistical relationship are discussed. After the 1976-1977 regime shift,southern Indian Ocean SST anomalies produced by Mascarene High pulses duringboreal winter trigger coupled air-sea processes in the tropical eastern Indian Oceanduring the following seasons. This produces a persistent remote forcing on the Pacificclimate system, promoting wind anomalies over the western equatorial Pacific andmodulating the regional Hadley cell in the southwest Pacific. These modulations, inturn, excite Rossby waves, which produce quasi-stationary circulation anomalies in theextratropical South Pacific, responsible for the development of the southern branch ofthe “horseshoe” El Niño pattern.The change of the background SST state that occured in the late 1970s over the IndianOcean may also explain why ENSO evolution is different before and after the 1976-1977 regime shift. These results shed some light on the possible influence of globalwarming or decadal fluctuations on El Niño evolution through changes inteleconnection patterns between the Indian and Pacific Oceans