Explainer: what is ENSO?

Due to the Coriolis Effect, the Trade Winds blow east-to-west across the Pacific. These surface winds push warm tropical surface water west, where it builds up above northern Australia and Indonesia, raising sea levels 40-60cm higher than the eastern Pacific. Warm water leads to more evaporation, resulting in monsoon rainfall and tropical cyclones (Fig. 1). On the eastern side of the Pacific, cool water from the deep is drawn to the surface, bringing vast quantities of fish.

In the 1600s South American fishermen noticed a periodic warming of these (eastern) coastal Pacific waters, and with it, fewer fish. As it occurred around Christmas they named it El Niño; Spanish for ‘Little Boy’ or the ‘Christ Child’. Others noticed that the Trade Winds had reversed, which explained why warm water from around Indonesia was pushed back towards South America (Fig. 2).

The reversal of the Trade Winds changes the Walker Circulation. This generally inhibits the formation of cyclones and typhoons in the Pacific, and hurricanes in the Atlantic Oceans. However, fed by so much heat in the ocean, those that do form are amongst the most powerful and destructive.

The peaks in global temperatures generally occur the year after El Niños commence. These are often followed by the cooling effects of La Niñas (Fig. 3). While La Niña wind patterns are similar to ENSO neutral conditions (Fig. 1), they are more intense, leading to more powerful cyclones, typhoons, hurricanes, and stronger monsoon rains.

By 1969 the ENSO phenomena was broadly understood. But exactly what triggers it remains unclear. Still, by examining past weather records the overall pattern was consistent—until the late 1970s when each El Niño seemed to permanently ratchet up global temperatures (Fig. 4).