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The scientific community has invested considerable effort over the past 50 years in studying El Niño, ever since Jacob Bjerknes first described unusual warm events in the tropical Pacific as the consequence of coupled interactions between the ocean and the overlying atmosphere¹. El Niño and its cold counterpart La Niña represent the strongest year-to-year climate fluctuation on the planet². What has motivated so much interest in these climatic siblings (which we collectively refer to as the El Niño/Southern Oscillation, or ENSO, cycle) is not only the quest to understand how they work, but also a societal imperative to accurately predict their evolution to help anticipate impacts on lives, property, economic activity and the environment. How ENSO may change in the future owing to greenhouse gas forcing is likewise one of the most compelling problems in climate research today³.

Variations in Pacific Ocean circulation associated with ENSO can have profound effects on marine ecosystems and commercially valuable fish stocks. ENSO events also disrupt the circulation of the global atmosphere, increasing the probability of floods, droughts, heat waves and other extreme weather events in far-flung corners of the planet. Sustained and coordinated international research efforts over the past few decades have largely paid off, with scientific progress being measured in terms of new theories for the ENSO cycle, development of sophisticated computer models for seasonal forecasting and establishment of extensive ocean–atmosphere observing systems for tracking ENSO cycle variations. So it was doubly perplexing when, first, an incipient El Niño of major proportions loomed large on the horizon in early 2014, only to disappear suddenly from the radar screen; then, in early 2015, remnants of weak 2014 warming unexpectedly flared up again, instead of quietly fading into history.

What happened? During January to April 2014, a series of strong westerly wind bursts, which are abrupt relaxations of the trade winds lasting typically one to three weeks, occurred in the western equatorial Pacific^{4, 5} (see Box 1). The ocean response to the wind bursts in early 2014 (Fig. 1a) looked strikingly similar to that observed during the onset of the 1997–1998 El Niño (Fig. 1b), which is the strongest on record so far⁶. The recurrence rate of extreme El Niños such as the 1997–1998 event is once every 15–20 years, leading some experts to interpret this similarity as evidence that we were in store for a big one. Another indicator that conditions were favourable for El Niño in 2014 was the accumulation of excess heat in and above the thermocline along the Equator (Fig. 2). A build up of excess heat content has preceded, by one to three seasons, every moderate to strong El Niño in the past 35 years⁷, and the heat content anomaly in early 2014 was higher than any time since 1997. According to El Niño theory⁸, this accumulation of excess heat preconditions the ocean to El Niño onset, which can be triggered by episodic westerly wind burst forcing. Therefore, computer models used for seasonal forecasting, especially those that incorporated subsurface temperature data in their initial conditions, predicted the development of El Niño sea surface temperature (SST) warming with a high degree of confidence during the second half of 2014. For example, based on model forecasts from June 2014 initial conditions, the National Oceanic and Atmospheric Administration (NOAA) Climate Prediction Center⁹ issued an advisory in early July that indicated, “the chance of El Niño is about 70% during the Northern Hemisphere summer and is close to 80% during the fall and early winter”. Given the extraordinary observations and the general agreement among forecasts models, the scientific community and the popular press were abuzz with the possibility that a monster El Niño¹⁰ was incubating in the tropical Pacific.

Box 1: Mechanisms for El Niño development.

To understand El Niño, we first need to define what is considered normal in the tropical Pacific. The trade winds, which are a relatively steady flow of air from east to west, drive ocean currents westwards beneath them (see figure). These currents drain warm surface water heated by the sun from the eastern Pacific and pile it up in the western Pacific. In response to this zonal redistribution of upper-ocean water mass, the thermocline (a region of sharp vertical temperature gradient separating the warm, sunlit surface layer from the cold, deep interior ocean) shoals in the east and is pushed down in the west. The shallowness of the thermocline in the eastern Pacific facilitates the upward transport of cold water by the trade winds, a process that is referred to as equatorial upwelling. Upwelling results in a cold tongue of surface water that extends from the west coast of South America to near the International Date Line. The east–west surface temperature contrast reinforces the westward-flowing trade winds, because atmospheric surface pressure is higher over the cooler water in the east, which drives the trade winds westwards.

As the trade winds flow from east to west, they pick up heat and moisture from the ocean. The warm, humid air mass becomes less dense and rises over the western Pacific warm pool, where deep convection leads to towering cumulus clouds and heavy precipitation. Ascending air masses in this region of deep convection return eastwards in the upper levels of the troposphere, then sink over the cooler water of the eastern Pacific.

Schematic illustration of normal, El Niño and La Niña conditions. Arrows indicate the direction of wind and ocean currents.

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