How El Niño and La Niña Affect Rain in the United States

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El Niño and La Niña are two opposing climate patterns that occur over a large area of the Pacific Ocean. Both patterns describe alternating distributions of hot and cold water in the Pacific, which are determined by the strength of the trade winds.

El Niño and La Niña often significantly affect the weather in many large regions that border the Pacific, including the western side of the United States.

There is still a degree of uncertainty with regard to how climate change will impact on the frequency and intensity of El Niño and La Niña events, but there is more evidence in favor of these becoming stronger in nature, and having more extreme impacts on western parts of the United States. 

El Niño-Southern Oscillation

The El Niño and La Niña patterns form two out of three phases of the El Niño-Southern Oscillation (ENSO), the other being a ‘neutral’ state.

Images showing atmospheric circulation during a neutral and an El Niño phase.
With the arrival of El Niño, the atmospheric circulation over the equator, known as the Walker circulation, changes dramatically. Images: NOAA/Climate.gov.

During an El Niño phase, trade winds weaken, which in turn results in warm water being pushed into the eastern Pacific, toward the western coastal region of the Americas.

The opposite happens during a La Niña phase: trade winds strengthen, which results in warm water being pushed toward Asia and Australia. This results in lower-than-average sea surface temperatures in the central and eastern Pacific.

A ‘neutral’ state describes a phase between the two extremes, when sea surface temperatures are within 0.5°C of the average. An ENSO cycle often lasts for between three and seven years, with El Niño and La Niña events peaking during the winter months.

El Niños and La Niñas vary in strength during each cycle, and are split into ‘weak’, ‘moderate’ and ‘strong’ categories, based on how far the sea surface temperature shifts from the average. 

Effect of El Niño on Climate in the Western United States

Each phase has certain effects on the climate of the western United States. Warmer sea surface temperatures in the eastern Pacific mean that, during El Niño years, the Pacific jet stream is pushed farther south than usual. This results in south-western states of the USA often experiencing higher-than-average rainfall during these events, whereas the Pacific Northwest region will commonly see drier and warmer-than-average conditions.

Typical weather patterns during an El Niño winter.  Image: NOAA Climate.gov
Typical weather patterns during an El Niño winter. Image: NOAA Climate.gov

However, effects are not limited to coastal areas of the western United States; parts of the United States located many hundreds of miles away from the coast also experience relatively minor increases in precipitation. For example, during the extreme 1997-98 El Niño, coastal regions of California received much higher winter precipitation than average, but parts of Arizona, Nevada and Utah also recorded slightly higher rainfall amounts. In addition, lower precipitation patterns are most prominent in coastal parts of Oregon and Washington, but reductions in rainfall can spread as far east as Montana. 

However, there are occasions when rainfall patterns deviate from many typical El Niño years, including high precipitation amounts that stretch from California up into southwestern Canada, or a reduction in average rainfall in California.

Whereas the 1997-98 El Niño was centered on the state of California, higher than average rainfall amounts during the 1957-58 event were focused on northern California and the Pacific Northwest. Also, the 1963-64 El Niño saw significantly lower rainfall amounts than average in California, with only Washington state seeing slightly higher rainfall than average. This is mainly because the relationship between the strength of the El Niño is closely correlated with higher-than-average rainfall toward the southwestern United States – 1997-98 was a very strong El Niño, whereas 1963-64 is categorized as ‘moderate’. 

La Niña and Climate in the United States

Conversely, when the La Niña phase is in operation, the Pacific jet stream is diverted onto a more northerly tract, typically flowing across the Pacific Northwest. Therefore, this region often has a wetter-than-average winter; the jet stream also becomes rather wave-like, which in turn brings more cold air down into the Pacific Northwest than it would in non-La Niña years.

Typical weather patterns during an El Niño winter.  Image: NOAA Climate.gov.
Typical weather patterns during an El Niño winter. Image: NOAA Climate.gov.

By comparison, a lack of winter rainfall is pronounced across much of southern and southwestern USA, with rainfall amounts in southern and central California typically experiencing significantly less precipitation than during El Niño and ENSO neutral years. Most other states in the southern half of the western United States, including Arizona and New Mexico, also see a lack of precipitation during the winter months. 

Difference in oceanic temperatures from the average during a La Niña year in September of 2020.  Image: NASA.
Difference in oceanic temperatures from the average during a La Niña year in September of 2020. Image: NASA.

How Climate Change is Affecting El Niño and La Nina

A great amount of research has been put into understanding how climate change is affecting both El Niño and La Niña. Various scientific studies have indicated that climate change may either result in stronger or more frequent events, or possibly cause a decrease in the number of El Niños and La Niñas. This is largely due to scientists being unsure of how the drivers and components related to ENSO will change.

A 2019 study found that since the 1970s, a warmer western Pacific has caused El Niño events to form farther westward than they did before the decade. After 1978, all 11 El Niños have formed solely in the western Pacific, three of which have been rather extreme events (1982-83, 1997-98 and 2015-16). The study links increasingly warm water in the western Pacific to stronger events, ones that produce higher rates of precipitation along the western side of the United States, as was particularly the case in 1997-98. 

In 2018, scientists from the National Center for Atmospheric Research used climate models to assess the effects of impacts from El Niños and La Niñas in an increasingly warming world. Although they also acknowledged that predicting the future frequency and extremity of such events was rather difficult, climate models strongly suggested that future impacts on the weather of the western United States will likely become more extreme – that precipitation and cooler-than-average temperature extremes will occur during El Niño years in southwestern regions, and higher-than-average temperatures and dry spells will become more frequent in the same region during La Niña years.

A recent report from the National Oceanographic and Atmospheric Administration also suggests that extreme El Niños and La Niñas will likely increase to around one in every 10 years (as opposed to 20 years today) by the year 2100. It also projects rainfall events to shift eastward along the Equator during El Niños, and westward during La Niña events. 

References

Carlowicz, M., & Schollaert Uz, S. (2017, February 14). El Nino: Pacific wind and current changes bring warm, wild weather. NASA Earth Observatory. https://earthobservatory.nasa.gov/features/ElNino

Climate change is making el Niños more intense, study finds. (2019, October 23). Yale E360. https://e360.yale.edu/digest/climate-change-is-making-el-ninos-more-intense-study-finds

Di Liberto, T. (2018, September 27). Changes in ENSO impacts in a warming world. NOAA Climate.gov | science & information for a climate-smart nation. https://www.climate.gov/news-features/blogs/enso/changes-enso-impacts-warming-world

Halpert, M. (2014, June 12). United States El Nino impacts. NOAA Climate.gov | science & information for a climate-smart nation. https://www.climate.gov/news-features/blogs/enso/united-states-el-ni%C3%B1o-impacts-0

How will climate change change El Nino and La Nina? – Welcome to NOAA research. (2020, November 9). Welcome to NOAA Research. https://research.noaa.gov/article/ArtMID/587/ArticleID/2685/New-research-volume-explores-future-of-ENSO-under-influence-of-climate-change

What are El Niño and La Niña? https://oceanservice.noaa.gov/facts/NiñoNiña.html

What is El Niño-Southern Oscillation (ENSO)? https://www.weather.gov/mhx/ensowhat

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