The extreme El Nino of 2015-2016: the role of westerly and easterly wind bursts, and preconditioning by the failed 2014 event

At the beginning of 2015, as oneyear earlier in 2014, the scientific community anticipated that El Nino conditions could develop in the tropical Pacific by year-end. Such projections were related to the occurrence of westerly wind bursts during winter-spring of each year that generated strong downwelling Kelvin waves indicative of an emerging El Nino. However, the event's progression quickly stalled in 2014, but actively continued in 2015, leading to an extreme warm event (comparable to 1997 or 1982). Here, we compare climate evolution during these twoyears using satellite observations and numerical simulations. We show that during 2014, El Nino development was interrupted mid-year by an exceptionally strong easterly wind burst, whereas during the second year it continued through the summer. Further, we show that the failed 2014 event created favorable conditions for El Nino development during the next year, as it kept ocean heat content recharged and the western Pacific warm pool extended eastward. Subsequently, the winter-spring westerly wind bursts in 2015 were followed by a series of state-dependent westerly bursts as part of a strong positive Bjerknes feedback. Analogue simulations with a coupled GCM wherein we superimpose the observed sequences of westerly and easterly wind bursts support these conclusions, stressing the role of the failed 2014 event in preconditioning the ocean-atmosphere system for the development of an extreme El Nino. In our simulations the probability of an extreme event following early-year westerly wind bursts increases from 14% to nearly 60% due to this preconditioning. Thus, the interplay between westerly and easterly wind bursts shapes El Nino development and diversity.