What Is ENSO?
Seasonal weather forecasts often include predictions such as 'warmer-than-normal winters' or 'wetter-than-usual seasons.' These variations are frequently influenced by a natural climate phenomenon known as the El Niño-Southern Oscillation (ENSO). ENSO is a natural climate pattern that involves periodic changes in sea surface temperatures and atmospheric pressure in the tropical Pacific Ocean, which in turn influence global weather and climate. ENSO is a climate system marked by periodic changes in sea surface temperatures (SSTs) and atmospheric pressure across the equatorial Pacific Ocean called Niño 3.4 (Figure 1). These variations influence global weather and climate patterns and are expressed in three distinct phases:
El Niño: El Niño occurs when sea surface temperatures in the Niño-3.4 region are at least 0.5°C warmer than average for 5 consecutive 3-month periods, accompanied by typical atmospheric changes.
- La Niña: The opposite of El Niño, La Niña occurs when sea surface temperatures in the Niño-3.4 region are at least 0.5°C cooler than average for 5 consecutive 3-month periods.
- Neutral Phase: This phase represents normal conditions between the extremes of El Niño and La Niña. However, the absence of an extreme ENSO event does not mean the weather will remain stable, as other climate factors can still drive variability.
Detection and Monitoring of ENSO
NOAA's Climate Prediction Center and International Research Institute for Climate and Society provide monthly updates on the status of ENSO. The ENSO updates can be assessed at https://www.cpc.ncep.noaa.gov/. The alert system includes different categories:
The onset of El Niño or La Niña in the tropical Pacific Ocean sets off significant changes in tropical rainfall and wind patterns, disrupting normal weather globally, often leading to heavy rainfall in some regions, drought in others, and shifts toward drier or cooler conditions in certain areas with increased precipitation elsewhere.
ENSO in the USA
Impacts on precipitation and temperature patterns
El Niño and La Niña significantly impact U.S. weather, primarily by altering the path of the mid-latitude jet streams, which influence temperature and precipitation patterns. During La Niña, the Pacific jet stream shifts north, bringing cooler, wetter conditions to the Pacific Northwest and Alaska (Figure 2A), while the southern U.S. becomes warmer and drier. In contrast, El Niño tends to reverse these patterns, often resulting in wetter conditions across the southern U.S. and drier, warmer conditions in the north (Figure 2B).
These effects are most pronounced in winter, though no two events are exactly alike, making the impacts probabilistic rather than certain.
Impacts on hurricane season
El Niño and La Niña influence hurricane activity in both the Atlantic and Pacific basins. El Niño tends to suppress hurricanes in the Atlantic by increasing wind shear and atmospheric stability, while enhancing hurricane activity in the central and eastern Pacific due to reduced wind shear. In contrast, La Niña
strengthens hurricane activity in the Atlantic by lowering wind shear and promoting unstable atmospheric conditions, while suppressing storms in the Pacific. The overall impact of ENSO on hurricane seasons is further influenced by long-term climate patterns, such as the Atlantic Multi-Decadal Oscillation (AMO), with each ENSO phase interacting differently depending on whether the Atlantic or Pacific is in a high- or low-activity era.
ENSO impacts in California
In California, the impacts of El Niño and La Niña are particularly significant due to the state's reliance on winter precipitation for water supply and agriculture. During El Niño events, California often experiences wetter-than-normal winters, especially in Southern California, as the subtropical jet stream shifts, bringing more storms and rainfall. This can lead to flooding, mudslides, and a boost to the state's water reservoirs.
Conversely, La Niña typically brings drier and warmer conditions, particularly in Southern California, heightening the risk of drought. The northern part of the state may still receive normal or slightly below-average rainfall, but La Niña years often result in reduced snowpack in the Sierra Nevada, which is critical for water supply during the dry season.
While the general impacts of El Niño and La Niña are well understood, the relationship between ENSO phases and California's weather is not always straightforward. Recent research by Guirguis et al. 2024 reveals that although ENSO influences precipitation patterns, it does not directly control the frequency of key wet weather events, such as atmospheric rivers. Instead, ENSO may modulate the characteristics of these storms—making them wetter during El Niño or drier during La Niña—but it does not always guarantee the predicted outcomes. For example, La Niña years like 2017 and 2023 brought significant precipitation and snow, defying typical expectations. This underscores that ENSO's effects are probabilistic rather than deterministic, meaning variability exists in how predictably these patterns play out.
Current Lookout
La Niña conditions are currently observed and are projected to persist through the February-April 2025 period, with a 59% probability. A transition to ENSO-neutral conditions is anticipated to occur during the March-May 2025 timeframe. These statements are updated monthly on the second Thursday of each month in conjunction with the ENSO Diagnostics Discussion, which is accessible here.
In Northern California, while rainfall during La Niña is typically near normal or slightly below average, snowpack in the Sierra Nevada becomes a crucial factor for water supply. Preparation: Focus on careful monitoring of snowpack levels and maintaining reservoir capacities to ensure downstream needs are met during the dry season.
In Central California, particularly the southern Central Valley, La Niña often results in drier conditions and reduced snowpack, impacting agriculture. Preparation: Invest in groundwater recharge, enhance water storage infrastructure, and support drought-resistant farming practices.
In Southern California, historically drier winters during La Niña heighten drought risks. Preparation: Strengthen water conservation measures, invest in drought-resilient water infrastructure, and prepare for stricter water restrictions, especially in urban areas dependent on imported water.
References
Guirguis, K., Hatchett, B., Gershunov, A.,DeFlorio, M., Clemesha, R., Brandt, W. T.,et al. (2024). Reinterpreting ENSO's role inmodulating impactful precipitation eventsin California. Geophysical ResearchLetters, 51, e2024GL110326. https://doi.org/10.1029/2024GL110326