During 2015 and 2016, the planet experienced one of the largest El Niño events in a century.  El Niño (Spanish for the boy) is actually the warm phase of the El Niño-Southern Oscillation and climate scientists are now suggesting that this oscillation in tropical Pacific temperature is likely tipping towards its opposite cool phase, La Niña.

Ensemble members show an increasing likelihood of La Niña conditions developing during October and November. La Niña conditions are said to develop when the sea surface temperature anomaly goes below –0.5°C.

Perhaps less well known than its larger brother, La Niña (Spanish for ‘the girl’) is an event that can trigger significant impacts.  Professor Adam Scaife, head of monthly to decadal prediction at the Met Office, said: “During El Niño, temperatures in the equatorial Pacific can warm by as much as 3°C. La Niña tends to be smaller and rarely exceeds 2°C, but its effects reach across the globe and can have just as much impact.”

Historically, La Niña events tend to occur about once every five years, but forecasts over the last few months show an increasing risk of La Niña developing this winter.  A really big La Niña like that in 2010/11 is very unlikely, but the chance of a moderate strength event is now several times the historical chance.

La Niña is associated with a shift in rainfall towards the western Pacific. Regions from Indonesia, the Philippines, Papua New Guinea, the Solomon Islands and north-east Australia will all be at risk of heavier than average rainfall if La Niña develops. At the other side of the tropical Pacific, Peru would be at risk of drought, but local fisheries would receive a boost as fish stocks can access additional nutrients rising in upwelling currents driven by La Niña.

The effects of La Niña aren’t confined to the tropical Pacific. If La Niña develops, then North-western coastal communities of North America would be at increased risk of a cold winter as La Niña increases northerly winds there. Meanwhile southern Africa would be more likely to experience a cooler and wetter summer than normal, and Brazil would have an increased risk of heavy rainfall in coming months.

Adam Scaife concluded: “As well as its regional effects, La Niña can cause a temporary cooling of global temperatures but the strength of the current event suggests that this will be no more than a tenth of the global warmth we have accrued so far due to climate change.”