Extreme Noise-Extreme El Nino: How State-Dependent Noise Forcing Creates El Nino-La Nina Asymmetry Journal of Climate

A major open question about El Nino-Southern Oscillation (ENSO) is what causes ENSO amplitude asymmetry, with strong El Ninos generally larger than strong La Ninas. The authors examine a leading hypothesis-that the ENSO state modifies the fetch and/or wind speed of westerly wind bursts (WWBs) that create asymmetric forcing and an asymmetric ENSO response. Further, in El Nino forecasts, the number of WWBs expected increases in the month following a strong WWB when compared with the month preceding it. Using a conceptual model, a relationship is derived between the magnitude of the westerly wind burst state dependence on ENSO and ENSO asymmetry. It is found that this relationship between the magnitude of the state dependence and ENSO asymmetry holds in both the observations and 21 coupled climate models. Finally, it is found that because of state-dependent westerly wind burst forcing, extreme El Nino events tend to be of the eastern Pacific variety.