National Oceanic and
Atmospheric Administration
United States Department of Commerce


FY 2016

Extreme noise–extreme El Niño: How state-dependent noise forcing creates El Niño-La Niña asymmetry

Levine, A.F.Z., F.-F. Jin, and M.J. McPhaden

J. Climate, 29(15), 5483–5499, doi: 10.1175/JCLI-D-16-0091.1 (2016)

A major open question about El Niño-Southern Oscillation (ENSO) is what causes ENSO amplitude asymmetry, with strong El Niños generally larger than strong La Niñas. We examine a leading hypothesis, that the ENSO state modifies the fetch and/or wind speed of westerly wind bursts that create asymmetric forcing and an asymmetric ENSO response. Further, in El Niño forecasts, the number of westerly wind bursts (WWBs) expected increases in the month following a strong WWB when compared with the month preceding it. Using a conceptual model, we derive a relationship between the magnitude of the westerly wind burst state-dependence on ENSO and ENSO asymmetry. We find that this relationship between the magnitude of the state-dependence and ENSO asymmetry holds in both the observations and the 21 coupled climate models. Westerly wind burst state-dependence also links the relationship of eastern Pacific El Niño events and extreme El Niño events.

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