This year has seen the third cooling La Niña event in a row in the Pacific for the first time this century – so could it lead to an extra-cold winter in Britain?
La Niña refers to the cooling of ocean surface temperatures coupled with winds and rainfall in the Pacific, but it can have knock-on effects on weather around the world.
The Australian Bureau of Meteorology has announced that the weather phenomenon has formed for the third year in a row – for only the third time since records began.
La Niña often has the opposite impact on weather and global climate as the better-known El Nino, which is the warm phase of the so-called El Niño Southern Oscillation (ENSO).
World Meteorological Organization (WMO) secretary-general Petteri Taalas said: "It is exceptional to have three consecutive years with a La Niña event. Its cooling influence is temporarily slowing the rise in global temperatures – but it will not halt or reverse the long-term warming trend."
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Taalas said that the worsening drought in the Horn of Africa and southern South America bore the hallmarks of La Niña, as did above-average rainfall in southeast Asia and Australasia.
He said: "The new La Niña update unfortunately confirms regional climate projections that the devastating drought in the Horn of Africa will worsen and affect millions of people.”
La Niña is one of the three phases of the phenomenon known as the El Niño-Southern Oscillation.
El Niño – the warm phase, La Niña – the cool phase and lastly the neutral phase.
How does La Nina affect weather in Britain?
The Met Office says that during La Niña strong trade winds blow warm water towards the west Pacific causing an upwelling of cool water from the ocean depths in the east Pacific.
This leads to variations in global weather – and the Met Office says it can influence the Atlantic jet stream and our weather here in the UK.
Professor Adam Scaife, head of long range prediction at the Met Office, previously said: “La Niña has a profound effect on weather across the globe with us even seeing impacts that extend across the UK.
“In late autumn and early winter it historically promotes high pressure in the mid-Atlantic, which stops Atlantic weather systems from delivering mild air to the UK, and therefore can allow cold conditions to intensify.
“However, in late winter La Niña can drive a shift of the jet stream towards the Poles increasing storminess and heavy rainfall, while bringing milder conditions”.
These changes in the location of warm and cool ocean water lead to a shift in rainfall towards the western Pacific putting areas such as north-east Australia, and Indonesia at risk of heavier than normal rainfall.
Areas on the other side of the tropical Pacific, such as California, could be at risk of drought.