When winds relax and water temperatures rise in the central and eastern tropical Pacific Ocean during an El Niño, more rain tends to fall over the southwestern United States and northern Mexico. Now, a new CIRES-led study finds that conditions in a different part of the Pacific Ocean can intensify or diminish the amount of El Niño rainfall and temperature change by modifying atmospheric circulation and strengthening storm tracks. The work, published in Environmental Research Letters, suggests that scientists can use atmospheric circulation and temperature patterns in the North Pacific Ocean to determine the likelihood that regions in North America will experience above- or below-average temperatures and rainfall during an El Niño or La Niña year.

“We know that two factors—one, El Niño and La Niña events and two, conditions in the North Pacific Ocean—both impact climate. But we didn’t know how they might combine to impact climate in North America,” said Nicola Maher, a CIRES visiting fellow and lead author of the paper. 

Maher and her colleagues used five climate models to evaluate the interaction between “warm” El Niño and “cool” La Niña events in the tropical Pacific Ocean (the recurring climate pattern known as El Niño Southern Oscillation, or ENSO) and the Pacific Decadal Oscillation (PDO), a warm vs. cool variation in water temperatures in the North Pacific Ocean. 

“Previous studies have tried to use observations to evaluate the influence of PDO on the climate impacts associated with El Niño and La Niña events, but the data is limited and year-to-year variations are large, making it difficult to identify significant patterns,” Maher said. “We’re the first group to look at the interactions between ENSO and PDO using multiple state-of-the-art climate models, with enough data to realistically capture the year-to-year variations.”

Using the large amount of data from the simulations, the researchers created maps of temperature and precipitation in North America for four different scenarios—El Niño and a warm eastern North Pacific Ocean, El Niño and a cool eastern North Pacific Ocean, La Niña and a warm eastern North Pacific Ocean and La Niña and a cool eastern North Pacific Ocean. To identify patterns, they compared the maps to normal conditions in the North Pacific Ocean and plotted the differences.

The team found that a warm eastern North Pacific Ocean amplifies the likelihood of El Niño temperature and precipitation impacts over North America in winter and spring—for example, intense El Niño rainfall in southern California is more likely to occur when the eastern North Pacific Ocean is warm. In contrast, a cool eastern North Pacific Ocean reduces the likelihood of severe El Niño impacts during the same months. The opposite pattern is true for La Niña—a warm eastern North Pacific Ocean reduces the likelihood of temperature and precipitation impacts in winter and spring, while a cool North Pacific Ocean amplifies the likelihood. 

In the models, the interaction between El Niño and La Niña events and warm and cool phases in the North Pacific Ocean altered atmospheric circulation, forcing a change in wintertime storm tracks and temperature over North America. 

“While the North Pacific Ocean influence doesn’t guarantee an outcome in a given year, sea-surface temperatures there can shift the chances of temperature and precipitation outcomes in some locations,” said Jennifer Kay, CIRES fellow and co-author of the paper.

The authors suggest that the results may help communities and planners better assess the likelihood of extreme temperatures and rainfall under specific conditions in the Pacific Ocean.