The Role of Westerly and Easterly Wind Bursts in El Niño Diversity: Contrasting Warm Events of 2014 and 2015

At the beginning of 2014 and again in 2015, the scientific community anticipated that an El Niño event could develop by the year-end. Such projections were related to the pronounced westerly wind bursts that occurred during winter-spring, generating strong downwelling Kelvin waves indicative of an emerging El Niño. However, in 2014 the event progression quickly stalled and eventually the event barely qualified as an El Niño, while in 2015 the development actively continued. At the time of writing (September 2015) there is a strong possibility that the 2015 event can become an extreme El Niño. In this study, we compare and contrast the evolution of the climate system during these two years and also explore their connections with global climate. We find that the key difference between the event developments in these two years is the fate of the positive Bjerknes feedback. During the first year this feedback was interrupted in mid-year, whereas during the second year a strong Bjekness feedback persisted through the summer. Critical for this interruption in 2014 was an exceptionally strong easterly wind burst in June of 2014, most likely induced outside of the equatorial band. Nevertheless, the coupled system evolution in 2014 left ocean heat content recharged, and with slightly warmer SSTs and more easterly position of the warm pool edge, creating favorable conditions for El Niño development in 2015, which included a series of westerly wind bursts throughout the year maintaining the Bjerknes feedback. Ensemble simulations with coupled GCMs wherein we superimpose such wind bursts support these conclusions. We discuss these results in the context of El Niño diversity and extreme events, and further explore their implications for decadal climate variability and global warming hiatus.