El Niño and La Niña: What they mean for Iowa winters

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You may have heard the terms El Niño and La Niña in the past, but what do they mean? 

El Niño and La Niña Explained

Trade winds along the equator typically blow east to west, pushing warm water in the eastern Pacific Ocean toward Asia. However, sometimes the trade winds weaken and that warm water pushes back toward South America. This is known as El Niño. When El Niño occurs, the jet stream shifts south and intensifies. This results in more flooding across the southern United States and warmer and drier weather across the north and Canada during the winter months. 

When trade winds strengthen, the warmer water blows back into the western Pacific. Meanwhile in the eastern Pacific upwelling forces some of the deeper and colder water closer to the surface. This is known as La Niña. When La Niña occurs, the jet stream shifts north. This results in drier weather in the southern United States and cooler and wetter weather across the north and Canada during winter. 

El Niño (left) vs La Niña (right) and their impacts on winter in North America. Courtesy: NOAA.

In short, when referring to sea surface temperatures in the eastern Pacific ocean, below average temperatures point to La Niña and above average temperatures point to an El Niño event. 

Sea Surface Temperature Anomaly since August 4, 2021. Courtesy: NOAA.

What’s ahead for this winter?

The Climate Prediction Center says there’s an 87% chance that La Niña continues into winter 2021, which is when we typically see impacts to weather in North America. “For the meteorological winter months of December-January-February, there is an elevated probability of both warmer and wetter conditions in eastern Iowa with equal chances of above/below/near-average conditions for the rest of Iowa,” said Iowa’s State Climatologist, Justin Glisan. On the precipitation side, Glisan said, “Other factors that tend to be less forecastable can play a larger short-term influence in the winter outlooks.”

3 Month Outlook for Temperature and Precipitation. Valid for November, December, and January.

Last winter (2020-2021) was also during the La Niña phase. Glisan says the transition back to La Niña this winter was more likely since neutral conditions were observed by spring 2021. Looking back at what occurred during the 2020-2021 winter, Iowa was slightly cooler than average, measuring 0.9° below average, and it was also slightly drier than average with 0.49” less precipitation than average. Despite the below average moisture, it was the 12th snowiest winter in 134 years. Iowa averaged 32.2 inches of snow which was 9.4 inches above average. Glisan says that precipitation is usually highly variable during La Niña winters compared to El Niño winters.

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