El Niño and La Niña: How Are They Different, And What Effects Do They Have On The U.S. Climate?

A simple graphic showing the difference between El Niño and La Niña, with blues representing cooler waters and darker reds and yellows representing warmer waters.

As winter approaches, El Niño and La Niña are terms that weather channels often throw around, but what exactly is the difference between them? Well, El Niño and La Niña—Spanish words for “The Boy” and “The Girl,” respectively—stand as two opposite parts of a climate cycle in the Pacific Ocean that scientists name the El Niño-Southern Oscillation (ENSO). By changing sea temperatures and wind patterns, this cycle shifts weather across the world for months or even years at a time. El Niño is when the central and eastern Pacific is at least 0.5°C above normal, all while trade winds slow down or reverse and push warm surface water eastward from its usual spot near Indonesia. La Niña, on the other hand, cools the same ocean area by at least 0.5°C as stronger trade winds pile warm water in the west and pull cold deep water up along the coast of South America. There are times when these ocean temperatures are neither above nor below normal, and when that happens, meteorologists label the pattern as neutral.

These ENSO phases (La Niña and El Niño) don’t just stay confined to the equatorial Pacific, as their effects ripple out through the atmosphere, influencing jet streams, storm tracks, and precipitation patterns thousands of miles away. In the United States, the impacts of El Niño or La Niña are particularly pronounced during the winter months, when the effects peak. These two ENSO phases are arguably the biggest driving forces of the climate in the U.S. They end up affecting everything from snowfall in the Rockies to hurricane activity in the Atlantic Ocean. 

El Niño Effects on the United States

El Niño winters tend to bring a split personality to U.S. weather. The weather is usually wetter and cooler in the south, while it’s drier and milder in the north. The weakened trade winds allow warm water to slosh eastward, which strengthens the subtropical jet stream across the southern tier of the country. This enhanced jet draws in moisture from the Pacific Ocean and feeds it to the Gulf Coast and Southeast, leading to above-average precipitation.

In the Southwest, including California, Arizona, and New Mexico, El Niño often delivers a bounty of rain and mountain snow. Historical strong events, like the 1997-98 El Niño, dumped record rainfall on Southern California. Though there were numerous mudslides, the rainfall also replenished reservoirs parched from prior droughts. When there’s El Niño, the Sierra Nevada snowpack can swell to 150% of normal. Given that agriculture is a crucial sector in the surrounding areas, the spring thaw can supply much needed water. Further east, Texas, the Gulf Coast, and Florida see increased storminess, with heavier rains raising flood risks in low-lying areas like Houston or New Orleans.

Meanwhile, the northern U.S. bears the drier side of El Niño. The Pacific Northwest, which consists of states like Washington, Oregon, and Idaho, experiences reduced precipitation as storm tracks shift south. Snowfall in the Cascades and Olympics also drop, thus impacting ski resorts and hydropower generation. The Upper Midwest and Great Lakes region often see milder temperatures and less snow, and thus not only disrupts winter recreation and reduces the need for road salt, but it also stresses cold-dependent ecosystems.

Looking at temperatures, El Niño suppresses cold air outbreaks in the North. Alaska frequently turns unusually warm, with reduced sea ice aiding marine shipping but harming indigenous hunting traditions. The Ohio Valley and Northeast sometimes encounter a rollercoaster, where even though there are fewer Arctic blasts, there is more potential for dangerous ice storms if southern moisture meets lingering cold.

El Niño also modulates the Atlantic hurricane season, which usually runs from June to November. Warmer Pacific waters increase wind shear over the Atlantic, often suppressing tropical storm formation. The 2015-16 El Niño, for example, contributed to a below-average season with only 11 named storms, providing a lot of relief for the Southeast coast that year.

La Niña Effects on the United States

La Niña flips the script, where it amplifies the polar jet stream and steers storms northward while drying out the south. Stronger trade winds pile warm water in the western Pacific, cooling the east and invigorating the atmospheric circulation that favors a “negative Pacific-North American (PNA) pattern.” In layman’s terms, this setup often results in colder, wetter conditions in the north and warmer, drier ones in the south. 

The Pacific Northwest thrives under La Niña, with copious rain and snow. Seattle and Portland can see 20-30% above-normal precipitation, boosting salmon runs. However, this also heightens landslide risks on saturated slopes. The Cascade and Olympic mountains accumulate deep snowpacks, benefiting irrigation and spring runoff for the Columbia River Basin’s dams.

Moving inland, the Northern Rockies and Plains embrace their classic winters, with bitter cold and heavy snow. Montana, the Dakotas, and Minnesota often endure prolonged sub-zero spells, with blizzards piling up feet of snow. The 2020-21 La Niña brought record cold to the Midwest, freezing the Great Lakes earlier and disrupting shipping on the St. Lawrence Seaway. Ski areas in Colorado and Utah often get a ton of snow, drawing tourists and supporting local economies.

The southern U.S., however, faces drought exacerbation. California enters La Niña winters with heightened fire risk if preceding falls were dry. Fewer storms mean a paltry Sierra snowpack, which can sometimes be just 50-70% of average. This forces water rationing in Central Valley farms and in cities like Los Angeles. Texas and the Southeast swing toward aridity, and crops like cotton and citrus suffer yield losses. The 2011-12 La Niña intensified the Texas drought, leading to billions in agricultural damages and widespread wildfires.

Temperature anomalies reinforce the divide, and as the South bakes with milder winters, the North shivers! La Niña also diminishes Atlantic wind shear, priming the basin for busier hurricane seasons. The 2020 La Niña coincided with an especially hyperactive hurricane season. With 30 named storms, including devastating hits on Louisiana, it cost $47 billion in damages.