El Niño: the power of the ocean and what it means in a warming world

Every two to seven years, a huge shift takes place in the tropical Pacific Ocean. Trade winds that normally push warm surface water westwards weaken or reverse.

Cooler, nutrient-rich water that usually rises from the deep ocean is suppressed, and unusually warm water spreads across the surface. This is El Niño, a naturally occurring phase of the El Niño-Southern Oscillation (ENSO), one of the most powerful demonstrations of the ocean’s role in the world's climate.

As the ocean releases heat into the atmosphere, it alters atmospheric pressure, rainfall and winds. Ocean and atmospheric currents shift in response, redistributing heat and moisture far beyond the ocean itself. A change in the temperature of one part of the Pacific, which covers one-third of the planet, can therefore influence weather thousands of kilometres away.

El Niño’s effects vary by region, but they typically include drought and extreme heat in parts of South-east Asia and Australia, heavier rainfall across parts of the Americas, and changes to storm activity across the Pacific and Atlantic. The strongest events have been associated with some of the world's most severe floods, droughts, wildfires and coral-bleaching episodes over the past half-century.