Tuesday, August 10, 2010

Paper: No Increase in El Niño Events since 1856

The alternating equatorial warm/cold Pacific Ocean temperatures, known as the El Niño–Southern Oscillation (ENSO), have large influences upon worldwide weather/climate. According to Wikipedia, "the studies of historical data show that the recent El Niño variation is most likely linked to global warming." However, a paper published yesterday in JGR finds no such link to global warming, with no change in frequency or magnitude detected since 1856, and finds a normal distribution instead. 

El Niño–Southern Oscillation: Magnitudes and asymmetry
By David H. Douglass, Department of Physics and Astronomy, University of Rochester, Rochester, New York, USA

Abstract: The alternating warm/cold phenomena in the Pacific, known as El Niño–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 NL 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 NL 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 Niño 25% larger will occur once in approximately 700 ± 200 years. (3) The distribution of negative values of NL deviates from Gaussian because of a deficiency of large La Niña events. (4) Examination of the 20 largest El Niño events since 1856 shows that there is no increase in the frequency of such events with time.

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