El Niño or La Niña? What They Mean and Why They Matter

This winter, northern California and Nevada are experiencing moderate La Niña conditions - graphic: National Atmospheric and Oceanic Administration

In the world of oceanography and climatology, the names El Niño and La Niña carry a particularly important meaning that can be complex and difficult to understand for the average person. Defined by the National Oceanic and Atmospheric Administration (NOAA) as “complex weather patterns resulting from variations in ocean temperatures in the Equatorial Pacific,” El Niño and La Niña’s weather implications for the Western United States, and the state of Nevada in particular, are equally, if not more complex. 

“These are names that we give to describe certain points in the atmosphere-ocean relationships,” Clair Ketchum, Warning Coordination Meteorologist with the National Weather Service in Elko, Nevada said. “The Pacific Ocean is such a large body of water that it has a huge impact in how our atmosphere behaves. So for the most part, even though it’s way out thousands of miles from Northern and Central Nevada, we can still get affected by it.”

El Niño and La Niña are known to be opposite phases of the El Niño-Southern Oscillation (ENSO) cycle. The El Niño phase represents the warm phase of the atmosphere-ocean relationship, during which the trade winds along the Equatorial Pacific weaken. With less friction between the wind and water, the warmer water that is traditionally pushed to the Western Pacific by those trade winds then oscillates, or swings back, toward Central and South America.

To demonstrate the idea of the ocean’s oscillation and how it affects weather systems in the United States, Ketchum likens it to a hand forcefully pushing water across a full bathtub.

“When you’re in a bathtub full of water and you put your hand in and push it towards the edge, you can kind of see a wave go across and once it hits the side, it comes back towards you,” Ketchum said. “But for that brief moment, you actually have more water on one side of your bathtub than on the other side. So this is pretty much what happens with El Niño and the weakening of those equatorial winds across the Pacific Ocean. When you get that warmer water moving back [to the Eastern Pacific], you get those warmer ocean temperatures across the western portions of South and Central America.” 

On the other hand, La Niña occurs when the trade winds of the Equatorial Pacific strengthen.

“That means those stronger winds push water towards East Asia,” Ketchum said. “So you get more water in the East and upwelling [of cold water] along the western shores of Central and South America. So colder water is usually what we find in the Eastern Pacific off the coast of Central and South America.”

Upwelling is the term used to describe the process of the colder, more nutrient-rich ocean water rising to replace the warmer, often nutrient-depleted ocean water at the surface level.

When the trade winds strengthen to push the warmer, surface-level water toward the Western Pacific and East Asia, La Niña occurs with cold-water upwelling off the western coast of Central and South America. But when the trade winds weaken and the warmer, surface-level water shifts back toward the Eastern Pacific and western coast of Central and South America, El Niño occurs. 

Each of these phases occur in periodic episodes with varying lengths of time. 

“The episodes usually come in cycles,” Ketchum said. “Normally, we’ll see an El Niño or La Niña that lasts a couple months to maybe one to two years. So we’ll have periods of El Niño, followed by periods of neutral conditions, followed by periods of La Niña. This oscillation occurs through time so we’re not always in one or the other, it will go back and forth.”

These atmosphere-ocean relationships can have major implications for weather patterns across the globe, particularly in the Western United States. During El Niño conditions, when warmer ocean temperatures are concentrated off the western coast of Central and South America, the atmospheric jet stream is pushed southward. This generates conditions that can create cooler and wetter conditions over the Southwest United States and warmer, drier conditions over the Pacific Northwest.

Conversely, during La Niña conditions, when warm ocean temperatures shift to the Western Pacific and East Asia and an upwelling of colder water is concentrated off the western coast of South and Central America, the atmospheric jet stream is pushed northward. This generates conditions that can create cooler, wetter conditions over the Pacific Northwest and warmer, drier conditions over the Southwest United States.

But the presence of El Niño or La Niña does not guarantee these types of conditions, or the extreme weather events that can follow.

“I want to emphasize that it’s not a guarantee that if you have El Niño conditions or La Niña conditions, that you’ll get the sort of ‘most likely outcome’ in the West, as there are lots of variability,” Mike DeFlorio, a researcher at the Center for Western Weather and Water Extremes, part of Scripps Institution of Oceanography at UC San Diego, said. “Although it’s in the Southwest [United States] and the Pacific Northwest that you see the strongest relationship, the presence of an El Niño or La Niña event only makes it more likely or less likely that you will get drier or wetter than average conditions; it’s not a guarantee.”

Although the state of Nevada is typically unaffected by these ENSO phases, that doesn’t mean everyday Nevadans can’t be affected indirectly. 

“When we talk about El Niño or La Niña in the relationship between the ocean and the atmosphere, this can increase the chances of bad weather happening, especially when it’s an El Niño,” Ketchum said. “One of the challenges is if we get a lot of rain in the central part of our nation, a lot of farmland gets flooded. So crop disasters occur that could affect the food supply and that is an issue that is really important and can impact all of us.”

Nevadans, however, from time to time may still be affected directly by the presence of El Niño or La Niña. Aside from risks to the average resident that come with increased chances of flooding or lengthy droughts, the livelihoods of many Nevadans can be compromised.

In particular, Ketchum points to those whose livelihoods depend on cattle ranching. Although cattle are relatively robust in their ability to handle extreme weather events, they can still become stressed if they have to sustain themselves through extreme temperatures over a lengthy period of time. 

“With respect to Central and Northern Nevada itself, we have a lot of cattle ranching,” Ketchum said. “But if the temperatures are really warm over a long period of time, it causes stress to the cattle which can have issues for the ranchers because the cattle may not be eating as much or the ranchers have to provide additional water so that their cattle can survive. Going into the winter when calving occurs across the area, the calves can be stressed if we have too cold and too wet of weather because that draws on the newborn baby’s fat reserves and that causes them a lot of stress and they won’t grow up healthy.”

That’s why it’s important for agencies like NOAA and The National Weather Service to track and monitor the Pacific Ocean’s water temperatures, which can help them forecast the presence of El Niño or La Niña conditions.

“We know that these conditions are going to occur by looking at the sea surface temperatures using data collection methods with satellites and buoys,” Ketchum said. “By anticipating the intensity of the La Niña or El Niño episode, we can help to mitigate the possible impacts by notifying our partners at the federal, state and local level.”

According to Ketchum, the NOAA uses what’s called an Oceanic Niño Index (ONI) to forecast the presence of El Niño or La Niña. The ONI follows an average anomaly, or abnormality, in sea surface temperature within a designated region of the Equatorial Pacific. 

El Niño conditions are defined as five consecutive three-month periods at or above the 0.5 degree Celsius, while La Niña conditions are defined along the same timeframe but at or below the -0.5 degree Celsius. The ONI also follows a categorical scale of weak, moderate and strong conditions. 

“Weak [conditions] are considered 0.5 to 0.9. degree sea surface temperature. Moderate is considered 1 to 1.4 degrees. Strong is considered 1.5 to 1.9 degrees and then very strong events are those that are two degrees or higher, and that’s an anomaly,” Ketchum said. “The last update NOAA gave was in November of 2020 and I think we’re going to do another update in mid January.”

Graphic: National Oceanic and Atmospheric Administration

According to the NOAA’s November update, the October sea surface temperatures in the measured region of the Pacific was recorded at -1.3 degrees Celsius, cooler than average. Since it takes time for those conditions to manifest from the Equatorial Pacific to the Western United States, we are only just now beginning to experience moderate La Niña conditions in our region. 

“El Niño and La Niña can help to drive the weather, but it doesn’t always happen,” Ketchum said. “We hope that [extreme weather events] don’t happen and that’s why we do what we do. El Niño and La Niña are really large players in the world in regards to the globe’s weather, but there’s a lot of other complicated atmosphere ocean interactions, as well. So it’s super complicated and it’s just one of those things that we’re learning new things about everyday.”


Scott King writes about science and the environment for the Sierra Nevada Ally. Support his work.