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Many point indices have been developed to describe the El Niño/Southern Oscillation but the Multivariate ENSO Index (MEI) is considered the most representative since it links different meteorological parameters measured over the tropical Pacific. The monthly values of the positive feature of MEI (interval: 1950-2008) have been ranked as a catalogue of El Niño events according to a semi-quantitative strength index ranging from 1 to 6. Such a catalogue is shown to be scale invariant in respect both to strength index and to times of occurrence, suggesting that the ocean-atmosphere interaction phenomenon at the basis of El Niño belongs to the class of dynamical processes which are in a self-organized critical state.
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