Accuracy of El Niño simulation hones climate change estimates

1997 El Nino with warm water (red); and 1988 La Nina with cool water (blue) in the Pacific

Increased El Nino/ La Nina intensity enhances Pacific warming (L) and vice versa (R).

Correctly simulating ocean current variations hundreds of feet below the ocean surface—the so-called Pacific Equatorial Undercurrent—during El Niño events is key in reducing the uncertainty of predictions of future warming in the eastern tropical Pacific. That was revealed in a new study led by University of Hawai‘i at Mānoa researchers and published in Nature Communications.

Trade winds and temperatures in the tropical Pacific Ocean experience large changes from year-to-year due to the El Niño-Southern Oscillation, affecting weather patterns across the globe. For example, if the tropical Pacific is warmer and trade winds are weaker than usual—an El Niño event—flooding in California typically occurs and monsoons in India and East Asia are detrimental to local rice production. In contrast, during a La Niña, the global weather patterns reverse with cooler temperatures and stronger trade winds in the tropical Pacific. In Hawaiʻi, during El Niño there is usually less winter rainfall, larger surf on the north shore, and a higher chance for tropical cyclones threatening the islands. During La Niña, we typically see the reversed pattern for Hawaiʻi. These natural climate swings affect ecosystems, fisheries, agriculture and many other aspects of human society.

Computer models that are used for projecting future climates correctly predict global warming due to increasing greenhouse gas emissions, as well as short-term year-to-year natural climate variations associated with El Niño and La Niña.

“There is, however, some model discrepancy on how much the tropical Pacific will warm,” said Malte Stuecker, co-author and assistant professor in the Department of Oceanography and International Pacific Research Center at UH Mānoa’s School of Ocean and Earth Science and Technology.

Model simulations

Researchers have been working for decades to reduce the persistent model uncertainties in tropical Pacific warming projections.

Many climate models simulate El Niño and La Niña events of similar intensity. In nature, however, the warming associated with El Niño events tends to be stronger than the cooling associated with La Niña. In other words, while in most models El Niño and La Niña are symmetric, they are asymmetric in nature.

In the study, scientists analyzed observational data and numerous climate model simulations and found that when the models simulate the subsurface ocean current variations more accurately, the simulated asymmetry between El Niño and La Niña increases—becoming more like what is seen in nature.

“Identifying the models that simulate these processes associated with El Niño and La Niña correctly in the current climate can help us reduce the uncertainty of future climate projections,” said corresponding lead author Michiya Hayashi, a research associate at the National Institute for Environmental Studies, Japan, and a former postdoctoral researcher at UH Mānoa. “Only one-third of all climate models can reproduce the strength of the subsurface current and associated ocean temperature variations realistically.”

“Correctly simulating El Niño and La Niña is crucial for projecting climate change in the tropics and beyond. More research needs to be conducted to reduce the biases in the interactions between wind and ocean so that climate models can generate El Niño – La Niña asymmetry realistically,” added Fei-Fei Jin, co-author and professor in the Department of Atmospheric Sciences at UH Mānoa.

“The high uncertainty in the intensity change of El Niño and La Niña in response to greenhouse warming is another remaining issue,” said Stuecker. “A better understanding of Earth’s natural climate swings such as El Niño and La Niña will result in reducing uncertainty in future climate change in the tropics and beyond.”