A Primer on the El Niño Southern Oscillation (ENSO)

This primer is excerpted from Mid-Atlantic Regional Climate Impacts Summary and Outlook: Summer 2026.

Introduction

In this primer, we define and describe how the different conditions of the El Niño Southern Oscillation (ENSO) impact weather in the Mid-Atlantic and how it is projected to change in the future.

What is ENSO?

The temperature of the Pacific Ocean along the equator and its interaction with the atmosphere, in turn impacts atmospheric circulation, temperatures, precipitation and other weather events.1 The El Niño Southern Oscillation is an ocean-atmospheric interaction that influences temperature and precipitation patterns across the planet.2,3 The temperature of the ocean in this part of the Pacific gets warmer or colder than average by about 1 to 3 degrees Celsius (C) (1.8 to 5.4 degrees F) on average every two to seven years.4 This alternating, or oscillating, warming and cooling is called the El Niño cycle, and there are three phases in this cycle: El Niño, La Niña, and ENSO-neutral, which together are referred to as ENSO.5

ENSO-neutral is when sea surface temperatures are around average and neither La Niña nor El Niño conditions are present, thus making "global seasonal conditions […] less predictable."6

Surface-level winds, that typically blow from east to west, weaken, and can even change direction along the equatorial Pacific.7 El Niño is the phase when average sea surface temperatures are warmer than normal (by at least 0.5 degrees C or 0.9 degrees F) for an extended period of time.8,9 Each El Niño is unique, but there are several predictable patterns that emerge during this phase. For example, the polar jet stream moves farther north than average while the Pacific jet stream stays along the southern United States during the winter months.10

A La Niña phase happens when sea surface temperatures are cooler than normal for at least several months.11 Surface winds blowing from east to west become stronger in La Niña conditions along the equatorial Pacific.12 In terms of the jet stream, the flow tends to be wave-like during a La Niña phase, creating an area of high pressure in the eastern North Pacific in the winter season.13 The strength of the jet stream varies, but typically brings storms along the northern continental U.S.14

Both El Niño and La Niña phases can be different strengths (weak, moderate, and strong) depending on how much warmer or colder the sea surface temperatures are than normal.15,16 Generally, the stronger the phase, the more likely the expected weather patterns are to emerge.17,18

Figure 1. Typical Winter Weather Patterns During La Niña and El Niño

Two maps of the United States comparing the typical weather systems during La Niña and El Niño. Source: NOAA
Two maps of the United States comparing the typical weather systems during La Niña and El Niño. Source: NOAA

SOURCE: NOAA

Two images comparing the weather patterns in La Niña and El Niño. In La Niña conditions, the variable polar jet stream brings colder weather to north western North America, wetter conditions in the west coast and north east, and drier conditions in southern North America. In El Niño, the extended Pacific jet stream brings wetter weather to southern North America, while conditions in the north west are warm, and the north east remains drier.

Impacts on the Mid-Atlantic

ENSO conditions can have an impact on the weather experienced in the Mid-Atlantic, particularly in the winter months. 19,20 When La Niña conditions are active during the winter, generally the Mid-Atlantic experiences less snowfall, however locations just outside the watershed that experience lake-effect snows, such as northern New York can be snowier that average.21 El Niño winters, on the other hand, have shown an increased frequency of nor'easters , which can in turn cause increased flooding, beach erosion, high winds, and snowfall and other precipitation.22 One study found that that two additional nor'easters occur during El Niño winters as compared to other winters.23 Strong El Niño winters especially see these impacts, with historically significant snowstorms such as the Blizzard of '58 and the February 2003 Presidents' Day Storm occurring during El Niño winters.24 More recently, the blizzard of January 2016, which severely impacted the region, happened during one of the strongest El Niños on record.25 In fact, in Washington, D.C., five of the ten greatest 1-day snowfalls since 1871 have occurred during El Niño winters (shaded in gray in Table 1).26

Table 1. Maximum 1-Day Total Snowfall for Washington, D.C.

1 21.0 01/28/1922
2 16.4 02/11/1983
3 15.0 12/19/2009
4 14.4 02/07/1936
5 14.0 02/19/1979
6 13.3 02/16/2003
7 13.0 01/07/1996
8 12.9 02/15/1958
9 12.0 01/29/1966
9 12.0 02/13/1899

NOTE: Period of record is 1/1/1897 to 8/18/2026. Shading indicates the snowfall occurred during an El Niño winter.

SOURCE: Northeast Regional Climate Center, 2026 (https://www.nrcc.cornell.edu). Used with permission.

Additionally, as mentioned above, El Niño can have impacts on the Atlantic hurricane season.27 El Niño can lead to greater atmospheric instability and stronger vertical wind shear and trade winds, which can reduce the number of hurricanes and named storms in the Atlantic basin.28 Whereas, when La Niña conditions are present in the Pacific, there is less wind shear and more favorable winds for hurricane development in the Atlantic.29

There are other factors beyond ENSO that affect weather conditions such as nor'easters and hurricanes, such as global snow cover patterns, and other sources of climate variability like the Arctic and North Atlantic Oscillations.30 Therefore, each ENSO is different and seasonal outlooks can vary compared with "typical" patterns.31

ENSO and Climate Change

According to a study from 2023, since 1960, scientists have seen greater variability in sea surface temperatures, and stronger and more frequent La Niña and El Niño conditions.32 The study suggests this is tied to warming from increased greenhouse gas concentrations.33 Climate models indicate that ENSO conditions will see even stronger variability by the late 21st century due to feedback cycles between the warming oceans and the atmosphere.34 This could create greater ENSO impacts such as increased severe storms, floods, heat waves, droughts, and wildfires, depending on the region. 35

Resources

Climate.us ENSO blog: https://www.climate.us/news-features/blogs/enso

NRCC webinar: https://www.nrcc.cornell.edu/workshops/webinars/2026/07/index.html

NRCC El Niño Special Report: https://elnino.nrcc.cornell.edu

NRCC La Niña Special Report: https://lanina.nrcc.cornell.edu

Footnotes

  1. https://www.noaa.gov/understanding-el-nino Return to text ⤴

  2. https://www.weather.gov/mhx/ensowhat Return to text ⤴

  3. https://www.noaa.gov/understanding-el-nino Return to text ⤴

  4. https://www.noaa.gov/understanding-el-nino Return to text ⤴

  5. https://www.weather.gov/mhx/ensowhat Return to text ⤴

  6. https://www.climate.gov/news-features/blogs/enso/april-2025-enso-update-la-nina-has-ended Return to text ⤴

  7. https://www.weather.gov/mhx/ensowhat Return to text ⤴

  8. https://elnino.nrcc.cornell.edu Return to text ⤴

  9. https://www.noaa.gov/understanding-el-nino Return to text ⤴

  10. https://elnino.nrcc.cornell.edu Return to text ⤴

  11. https://lanina.nrcc.cornell.edu Return to text ⤴

  12. https://www.weather.gov/mhx/ensowhat Return to text ⤴

  13. https://lanina.nrcc.cornell.edu Return to text ⤴

  14. https://lanina.nrcc.cornell.edu Return to text ⤴

  15. https://elnino.nrcc.cornell.edu/ Return to text ⤴

  16. https://lanina.nrcc.cornell.edu Return to text ⤴

  17. https://elnino.nrcc.cornell.edu/ Return to text ⤴

  18. https://lanina.nrcc.cornell.edu Return to text ⤴

  19. https://elnino.nrcc.cornell.edu/ Return to text ⤴

  20. https://lanina.nrcc.cornell.edu Return to text ⤴

  21. https://lanina.nrcc.cornell.edu Return to text ⤴

  22. https://lanina.nrcc.cornell.edu Return to text ⤴

  23. Hirsch, M. E., DeGaetano, A. T., and Colucci, S. J., "An East Coast Winter Storm Climatology," Journal of Climate, Vol. 14, Issue 5, March 2001, https://doi.org/10.1175/1520-0442(2001)014<0882:AECWSC>2.0.CO;2 Return to text ⤴

  24. https://elnino.nrcc.cornell.edu Return to text ⤴

  25. https://elnino.nrcc.cornell.edu Return to text ⤴

  26. https://elnino.nrcc.cornell.edu Return to text ⤴

  27. https://www.aoml.noaa.gov/how-does-el-nino-impact-atlantic-hurricane-season/ Return to text ⤴

  28. https://www.aoml.noaa.gov/how-does-el-nino-impact-atlantic-hurricane-season/ Return to text ⤴

  29. https://www.aoml.noaa.gov/how-does-el-nino-impact-atlantic-hurricane-season/ Return to text ⤴

  30. https://elnino.nrcc.cornell.edu Return to text ⤴

  31. https://elnino.nrcc.cornell.edu Return to text ⤴

  32. https://www.climate.gov/news-features/blogs/enso/has-climate-change-already-affected-enso Return to text ⤴

  33. https://www.climate.gov/news-features/blogs/enso/has-climate-change-already-affected-enso Return to text ⤴

  34. https://www.climate.gov/news-features/blogs/enso/has-climate-change-already-affected-enso Return to text ⤴

  35. https://www.climate.gov/news-features/blogs/enso/has-climate-change-already-affected-enso Return to text ⤴

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