By Zach Butler, Meteorologist Posted 2 months ago September 28, 2023
El Niño’s Effect on Atmospheric Rivers
With the upcoming El Niño for the 2023-2024 winter season, there is a lot of curiosity about how much moisture and snowfall the Western United States will receive due to the potential impact that significant Atmospheric Rivers (ARs) can bring. Let’s take a look at how El Niño affects ARs during the winter and what that means for the upcoming ski season.
El Niño and Atmospheric Rivers (ARs), Explained
The term El Niño refers to the large-scale ocean-atmosphere climate phenomenon linked to periodic warming in sea-surface temperatures (SSTs) across the central and east-central equatorial Pacific.
El Niño represents the warm phase of the ENSO cycle and means that the ocean water temperatures are warmer than average.
ARs are relatively long, narrow regions in the atmosphere – like rivers in the sky – that transport water vapor outside of the tropics. ARs affect many parts of North America but are most commonly associated with large impacts on the West Coast, coming off the Pacific Ocean.
Strong ARs are responsible for extreme rainfall events, floods, and contribute up to 30–50% of California’s water supply, as well as a major portion of the Sierra and Cascade snowpack.
How El Niño Affects ARs
Sea-surface temperatures (SSTs) along the equator impact the phase of ENSO and the strength of El Niño. ARs develop because warm SSTs (regardless of ENSO phase) are able to transport large amounts of moisture to the north.
Below is an AR affecting the Pacific Northwest (PNW) on September 25th, 2023.
The warmer colors indicate regions of high moisture. The transport of moisture (AR) is visible across the Pacific Ocean (above Hawaii) to the PNW. ARs have been called “The Pineapple Express” since the moisture source is often near Hawaii.
If SSTs are warmer than normal along the equator (El Niño), ARs can access more moisture and grow larger and stronger. It is similar to a strengthening hurricane, in that warmer SSTs allow more moisture to be transported into the atmosphere, thus allowing a storm to generate more precipitation.
Where are ARs more Frequent during El Niño?
ARs are more frequent in areas of California and the Southwestern US during El Niño. This has been observed since the National Ocean Atmospheric Association (NOAA) has kept track of ENSO since 1950.
During El Niño, the horizontal extension of the subtropical jet stream results in more zonal (west to east) moisture transport towards the West Coast of the US, especially towards California and the Southwest.
How the Strength and Location of El Niño Affect ARs
El Niño can come in different shapes and sizes. The location of the warmest SST anomalies that cause El Niño and the strength of this anomaly have been shown by past scientific studies to impact the magnitude of ARs on the West Coast of North America.
The location of the warmest SST anomalies is determined by the Niño region. The Niño region goes from 1-4 with “Niño 1+2” along the coast of South America and “Niño 4” to the east of Papua New Guinea. See the map below for the locations of each Niño region.
The location of the warmest SST anomalies in the Niño regions has been shown to affect the frequency of landfalling ARs on the West Coast.
A recent study (Kim, Zhou, & Alexander, 2015) showed that if the warmest SST anomalies are in the Eastern Pacific near Niño regions 1 and 2, this can allow the subtropical jet stream to extend to the south and the east. This can transport more moisture to the West Coast of North America, increasing the frequency of landfalling ARs.
If the warmest SST anomalies are located in the Central Pacific near Niño region 3.4, this can increase the number of ARs over the Southwest US, including those of extreme intensity.
The magnitude of the warmest SST anomalies affects how warm the ocean is. With a warmer anomaly, storms can often grow stronger and bring more precipitation. However, the research is not as clear on whether the strength of the El Niño compared to the location of the warmest SST anomalies significantly influences ARs in the US.
Bottom line: The location of the warmest SST anomalies in the Equatorial Pacific has a stronger influence on AR events as opposed to the overall strength of an El Niño event.
Where are the Warmest SST Anomalies this Winter?
The warmest SST anomalies are currently forecasted to be along the Eastern Pacific. This means ARs are more likely to impact along the West Coast rather than just the Southwest, according to past research.
This tips the odds in favor of normal to above-normal precipitation for many areas near the West Coast, while the snowfall signal for Tahoe and other West Coast ski regions is not as clear
The above-normal precipitation signal does not necessarily correlate to above-normal snowfall in the Sierra and Cascades, since:
- El Niño favors warmer surface temperatures, which can result in higher snow levels and a higher percentage of rain vs. snow events compared to normal
- The wet signal for above-normal precipitation is stronger near the Coast
2015-2016 El Niño Comparison
The upcoming winter is looking similar to the last El Niño winter in 2015-2016, which was categorized as a strong Eastern Pacific El Niño event. The number of landfalling ARs along the West Coast of North America totaled 23 events (normal 15) with most of the ARs occurring over the Pacific Northwest rather than the Southwest.
A look at the precipitation departures from December to March in 2015-2016 shows the impact the ARs had on precipitation throughout the West Coast.
Despite what past El Niños have shown, every El Niño will have its own unique impact, and the snowfall during the winter is influenced by other ocean and atmospheric circulations and patterns.
Where Else does El Niño and ARs Affect Winter precipitation?
While the warmest SST anomalies affect coastal areas through the Sierra and Cascades, as well as inland areas in the Southwest, other interior areas of the Western US can be affected by ARs.
A recent study (Ralph et al. 2019) showed that less than 20% of ARs that affect the West Coast will reach the interior areas of the Intermountain West. Interior areas in the Southwest are more likely to receive moisture from ARs than other areas.
While the connection to more or less precipitation through interior areas is not as clear as in coastal areas, some research has shown El Niño to affect precipitation in other Western US states. Below is a look at the precipitation correlation from October–March during El Niño events.
The warm colors show above-normal precipitation and the cool colors show below-normal precipitation between October–March. This map is based on El Niño events from 1949 – 2014. (Source: Colorado River Basin Climate and Hydrology State of the Science, Western Water Assessment).
Again, the above-normal precipitation signal does not necessarily correlate to above-normal snowfall due to the fact that El Niño favors warmer surface temperatures, which can result in higher snow levels and a higher percentage of rain vs snow events compared to normal.
Other areas of the US can also be affected by El Niño and ARs. Strong Nor’easters often form on the East Coast of the US during El Niño due to a storm track that favors their development.
Storms that track through California will often continue to track through the Southern US, which can allow them to develop and strengthen along the East Coast, forming into Nor’easters. Strong Nor'easters can have a tropical moisture connection and create ARs in some scenarios.
Finally, OpenSnow forecasters have published research on snowfall compared to similar El Niño events for Colorado, Utah, Tahoe, Jackson Hole, Oregon, Montana, Idaho, Washington, British Columbia, and Europe so be sure to check those out as well.
Download the free OpenSnow app for the most accurate snow forecast and snow report information and stay tuned to our weather forecasts for the latest updates.
Zach Butler, Forecaster