Influence of Westerly Wind Events Stochasticity on El Niño Amplitude: The case of 2014 vs. 2015.

Short-lived wind events in the equatorial Pacific strongly influence the El Niño/Southern Oscillation (ENSO). Contrasting the tropical Pacific evolution in 2014 against that of 2015 (or 1997) provides a compelling illustration of the key role of westerly wind events (WWEs) on ENSO. In late march, the years of 1997, 2014 and 2015 displayed relatively similar oceanic conditions in the tropical Pacific. Those three years were characterized by higher than normal (> 1 std) equatorial Pacific heat content, and an abnormal extension of the warm pool towards the central Pacific following the occurrence of one or several strong WWEs during winter. Yet, 1997 and 2015 developed into some of the strongest observed El Niño events on record while only a weak warming occurred in 2014. One major difference between 2014 and 1997 / 2015 was however a series of strong WWEs during the summer of the two later years while almost none occurred in 2014.

In this study, we investigate the role of summer WWEs in the El Niño development using the CNRM-CM5 coupled model. A 100-member ensemble simulation initialized with early-spring equatorial conditions analogous to those observed in 2014 and 2015 demonstrates that early-year elevated WWV and strong WWEs preclude the occurrence of a La Niña but lead to El Niños that span the weak (with few WWEs) to extreme (with many WWEs) range. Sensitivity experiments confirm that numerous/strong WWEs shift the El Niño distribution toward larger amplitudes, with a particular emphasis on summer/fall WWEs occurrence which result in a five-fold increase of the odds for an extreme El Niño. A long simulation further demonstrates that sustained WWEs throughout the year and anomalously high WWV are necessary conditions for extreme El Niño to develop. In contrast, we find no systematic influence of easterly wind events (EWEs) on the El Niño amplitude in our model. Our results demonstrate that the weak amplitude of El Niño in 2014 can be explained by WWEs stochastic variations without invoking EWEs or remote influences from outside the tropical Pacific and therefore its peak amplitude was inherently unpredictable at long lead-time.