Climate change may make El Niño and La Niña less predictable

Climate change may make it harder to predict the most severe of the El Niño and La Niña weather disturbances in the Pacific Ocean. That’s because these events will become less connected with what happens halfway around the world in the Atlantic, researchers report online August 21 in Science Advances.

In today’s climate, cooling in the waters of the equatorial Atlantic, called an Atlantic Niña, can lead to especially warm water in the equatorial Pacific, or El Niño (SN: 5/28/16, p. 13). Meanwhile, warmer Atlantic Niño waters tend to give rise to the cooler waters of La Niña in the Pacific. That call-and-response relationship, which involves air being swept into the atmosphere from over the Atlantic and settling down over the Pacific, can give forecasters an edge in anticipating destructive El Niño and La Niña events.

But as the atmosphere gets warmer, that gas exchange is expected to become more sluggish, weakening the Atlantic’s sway over the Pacific. Future El Niños and La Niñas may not follow the Atlantic events as reliably as in the past, new simulations suggest. That could make it harder to prepare for especially disruptive El Niño and La Niña episodes, which can incite flooding in some regions while drying up others or making hurricanes stronger (SN Online: 11/10/16).

Researchers gauged just how reliably Atlantic Niñas and Niños have been bellwethers for El Niños and La Niñas in the past by comparing the patterns of these events over the last several decades. The team found that the strongest El Niños and La Niñas — including the 1998 La Niña that caused floods in China and hurricanes in the Caribbean, killing tens of thousands of people — were almost always preceded by Atlantic events.

Computer simulations revealed how the Atlantic-Pacific relationship might change if climate-warming carbon emissions continue to rise throughout the 21st century. The simulations suggest that, while future extreme El Niño and La Niña events are expected to happen more frequently, only about half of these events will be foreshadowed by Atlantic events (SN Online: 1/26/15).

So “it’s going to be harder to predict the Pacific extreme El Niño and extreme La Niña,” says study coauthor Wenju Cai, a climate scientist at the Commonwealth Scientific and Industrial Research Organization in Aspendale, Australia.

These findings, while plausible, “must be interpreted with some caution” because computer simulations don’t perfectly emulate ocean-atmosphere interactions, says climate scientist Maria Belen Rodriguez-Fonseca of the Complutense University of Madrid. If future Atlantic Niños and Niñas aren’t as useful for predicting Pacific activity, “it’s not going to be catastrophic,” she says. The Atlantic Ocean is but one element among many environmental factors, such as conditions in the Pacific and Indian oceans, that feed into forecasts for these meteorological juggernauts.