An El Niño weather system is forming over the Pacific Ocean this summer, and scientists are watching closely to see how strong the event might be.
The answer could help determine how high global temperatures will rise this year, how severe a drought could afflict Australia and Asia, and whether or not rain-starved California will get the precipitation it desperately needs. All of this depends on how the waves in the Pacific respond to the winds overhead, and vice versa.
To understand how an El Niño works, it helps to know what happens during “normal” conditions.
In the Pacific Ocean, the water closest to the equator is warmed by the sun. That warm surface water helps to create more clouds, which in turn dump rain over wherever they happen to be.
Most of the time, the wind over the ocean blows from east to west due to the spinning of the Earth. Those gusts—called prevailing winds—continuously skim off the warm water like cream and push it toward Asia and Australia. The water near the Americas, as a result, is usually cooler by comparison.
During a non-El Nino year, warm water in the Pacific ocean moves west to east, and rain clouds hover closer to Asia. U.S. National Oceanic and Atmospheric Administration
The warm water builds and builds in the east, “until at some point, the system says, ‘Whoa, too much! I’m going to get rid of it!’” Kevin Trenberth, a climate scientist at the U.S. National Center for Atmospheric Research in Colorado, explained.
This is when an El Niño event has the potential to kick in.
It usually happens every three to seven years, starting slowly in the summer, peaking in the winter and dying off quickly in the New Year. Peruvian fishermen are credited with naming the event after they noticed that every few years around Christmas, virtually no fish could be found in the unusually warm waters. (“El Niño” is Spanish for “The Christ Child”).
El Niño events are aided by oceanic Kelvin waves. These undulations only travel west to east, so they effectively shed the warm layers in the Asia-Australia region like a dog would its winter fur coat and send the warmth east. Strong westerly bursts can propel the warm water even farther, pushing greater amounts closer to the Americas and leaving cooler waters behind in the western Pacific.
This shift in warmth — and thus change in clouds — increases the potential of heavy rains, flooding and disrupted fish habitats near the Americas and creates drought conditions in the western Pacific. It also boosts the number of tropical storms and hurricanes in the eastern Pacific while decreasing activity in the Atlantic Ocean, Gulf of Mexico and the Caribbean Sea.
During an El Nino event, warm water moves west to east, pushing rain clouds toward the Americas. U.S. National Oceanic and Atmospheric Administration
As the warm water moves west to east, the winds slosh it all around in the ocean. This increases the overall sea surface temperature, and that in turn heats up the atmosphere. Trenberth called this phenomenon a “mini” global warming event. It’s largely why 2010, the year of the most recent El Niño event, was also the hottest year on record, according to U.S. scientists.
This feedback between the waves and winds is ultimately what decides whether an El Niño event will form—and if it will be weak, moderate or strong, Tim Stockdale, a principal scientist at the European Center for Medium-Range Weather Forecasts near London, said.
“Things in the ocean move pretty slowly and they’ve got a lot more inertia, whereas the atmosphere flips around at random,” he said. “The eventual outcome [of El Niño] will depend on the signal in the ocean, but also on the random element of what the atmosphere might do.”
Normal wind and temperature patterns in the Pacific Ocean are shown left. Patterns during the 1982-83 El Nino event appear right. U.S. National Oceanic and Atmospheric Administration
Officially, an event is deemed a weak El Niño event if the sea surface temperatures increases by more than 0.5 degrees Celsius (0.9 degrees Fahrenheit) above normal. At 1.0 degree Celsius above normal, an event is moderate, while anything above 1.0 degrees is considered strong.
