A Southern Hemisphere booster of super El Nino

A distinct class of El Nino events with extreme magnitude (termed super El Nino events in this study) is identified after removing decadal variation. These events occurred in 1972/1973, 1982/1983, and 1997/1998. They are distinguished not only by their size but also by associated features such as a Southern Hemispheric transverse circulation that is not similarly robust in other El Nino events. This transverse circulation is characterized by a low-level equatorward flow, which spins off from a high sea-level-pressure anomaly around Australia and then merges into the deep convection anomalies over the central Pacific, and by an upper-level southward divergent flow, which branches off from the convection center and connects to the subsidence of the Australia high. It is suggested that this transverse cell, peaking in boreal summer, serves as an effective booster during the developing stage of a super El Nino by intensifying tropical Pacific low-level westerly winds.