The catastrophic weather events taking place across the globe – from Brazil’s and Australia’s flooding to the Eastern United States’ heavy snowfall – have two likely explanations.
Tony Barnston, lead forecaster at Columbia University’s International Research Institute for Climate and Society, said two phenomena – La Niña and the North Atlantic Oscillation – are likely responsible for the patterns we’re seeing.
Though La Niña is different every time, it can be simply defined as a drop in water temperature in the eastern and central tropical Pacific Ocean. This particular La Niña appeared in July, Barnston said, and will last through spring.
During La Niña, there is less rainfall in the tropical Pacific and a horseshoe pattern of warm water typically forms in the North Pacific, the coast of Southeast Asia and the seas around Indonesia and Australia (check out the graphic above).
In this case, though, “the whole globe looks to be compensating,” Barnston said, noting that it’s difficult to determine if La Niña spawns individual weather events.
While Southeast Asia and Australia can typically expect warmer oceans and more rainfall during La Niña, the affected area stretches farther west this year and includes Sri Lanka and the southern tip of India, Barnston said.
There is presently a massive rescue operation under way in Sri Lanka to save victims of flooding that has already killed at least 13 people. Floods have killed another 40 people in the Philippines, and Australia has been inundated with deadly flooding reportedly estimated to cover an area the size of France and Germany combined. To the east of La Niña, South America's north coast has been slammed with rain, and hundreds have been killed in Brazil's flooding and mudslides.
La Niña usually means warmer temperatures in the United States, particularly the Southeast, but this year the North Atlantic Oscillation is trumping La Niña.
Since November, we have been in the negative phase of the North Atlantic Oscillation, Barnston said, meaning that air temperatures between the eastern U.S. and Europe will drop. This causes temperatures to rise in places like northeast Canada, Greenland and Iceland, as the mercury drops on thermometers in the eastern U.S.
Weather experts call it a see-saw pattern, and it will eventually affect water temperatures, though that’s a secondary effect, Barnston said.
So while La Niña would normally mean dryer, warmer weather for folks in the southern United States, “the North Atlantic Oscillation has superseded the La Niña” this year, bringing El Niño-like winter temperatures and snow to the Southeast, he said.
CNN meteorologists reported earlier this week that snow was present in every U.S. state but Florida.
Though La Nina typically lasts until April or May, the North Atlantic Oscillation can move from a negative to positive phase mid-winter. Barnston said the Southeast “could be warmer and dryer than normal later this year if the North Atlantic Oscillation ceases and desists.”