El Niño-Southern Oscillation: Magnitudes and asymmetry

Abstract

The alternating warm/cold phenomena in the Pacific, known as El Nio-Southern Oscillation (ENSO), is characterized by large perturbations to the worldwide climate. Indices have been defined to characterize this phenomenon. However, the commonly used indices contain an unwanted effect from the annual cycle that can be reduced by digital filtering. Using a filtered ENSO index N L on data from 1856 to the present allows more accurate calculations of various quantities to be made. New results are (1) the distribution of positive values of N L is Gaussian. Thus, large-magnitude El Niño events come from the tail of this distribution and not from some rare external perturbation. (2) The probability of occurrence of an El Niño of any magnitude can be calculated. A 1997-1998 El Niño will occur once in approximately 70 20 years, while an El Nio 25% larger will occur once in approximately 700 200 years. (3) The distribution of negative values of N L deviates from Gaussian because of a deficiency of large La Nia events. (4) Examination of the 20 largest El Nio events since 1856 shows that there is no increase in the frequency of such events with time. Copyright 2010 by the American Geophysical Union.