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Many times, meteorologists use terminology during weather discussions. Some of the acronyms used can understandably come off as a form of "Alphabet soup". Perhaps one of the most widely used terms in the industry is “El Niño” or its counterpart “La Niña.” While you may have heard of these, what do they actually mean and how can they impact our weather patterns? Let's investigate...
El Niño and La Niña are the two primary phases of a larger climate cycle known as the El Niño Southern Oscillation (ENSO).
ENSO is a naturally occurring interaction between the tropical Pacific Ocean and the atmosphere. While it begins over the equatorial Pacific, it's influence extends across much of the globe. At the heart of ENSO, however is a circulation pattern that is responsible for its formation...known as the Walker Circulation.
Under normal conditions (neutral ENSO), trade winds across the tropics blow in a counterclockwise pattern and result in easterly winds across equatorial regions.
This circulation has two main impacts:
This results in a circulation pattern that promotes increased ocean temperatures, cloud cover and precipitation across the Western Pacific Ocean and most of Indonesia. Conversely, cooler ocean temperatures, associated with high pressure and drier weather, are typically found across the Eastern Pacific.
During La Niña, this pattern is enhanced, resulting in stronger easterly trade winds, more upwelling (of cooler water) off the coast of South America and a pronounced increase in sea surface temperatures across the Western Pacific Ocean. During El Niño, the opposite occurs, and the trade wind pattern either weakens considerably or is even reversed. As a result, warmer than average water temperatures are usually found off the South American Coast and across the Eastern Pacific in general. Similarly, warmer water is transported away from the West Pacific, which results in cooler ocean temperatures across Indonesia (see bottom part of previous diagram).
While both weak and strong El Nino events feature an active southern jet stream, because ocean temperature departures are the main impact of this circulation, the strength of ENSO depends upon the sea surface temperature anomalies (departures from average) for various regions across the Pacific Ocean. The attached diagram below shows the four main regions for which temperature anomalies are calculated. Although all of these regions are important to some degree, years of research has shown that departures measured across region 3.4 (outlined in the black box) are most important when calculating the phase and strength of ENSO.
Generally speaking, La Niña conditions correspond to at least five consecutive months averaging temperatures less than -0.5°C. Conversely, El Niño conditions correspond to readings averaging at or greater than +0.5° over the same time period. Note: the further removed the sea surface temperature is from its average position will result in an increasingly strong El Niño or La Niña. For instance, a departure of just +0.6° would indicate a weak El Niño, whereas a departure of +1.8° would correspond to a strong El Niño. Similarly, a departure of -0.6° would be indicate of a weak La Niña, whereas an anomaly of -1.8° would correspond to a strong La Niña.
So why is the strength of an El Niño so important? The reason is (with all other things being equal) a strong El Niño promotes a different weather pattern than a weaker or even neutral one. Overall, El Niños (regardless of strength) are often marked by an enhance split flow pattern and a stronger than average southern jet stream. This will favor wetter and cooler conditions across a good portion of the Southern US. Conversely, the Pacific Northwest will generally experience milder than normal temperatures during an El Niño. The images below portray ideal patterns and their associated temperature anomalies
A strong El Niño often allows a ridge of high pressure to develop across the western portion of North America, while the polar jet stream is displaces further north, keeping more of a broad trough over eastern Canada. This allows milder, Pacific air to spread across much of the northern United States, with an active storm track over the southern tier. This was the dominate pattern during the winters of 1982-83 and 1997-98, the two strongest events on record (so far).
While winters in the Midwest usually end up milder and drier than usual, it is important note that the correlation between strong El Niño becomes weaker as you move into the Northeast and Mid-Atlantic. While snowfall totals average out below normal overall during strong El Niños, they have been quite variable from event to event. Also we stress that there are other factors to consider when preparing a seasonal forecast, such as the sea surface temperatures across the Gulf of Alaska. In addition, teleconnections such as the Arctic Oscillation have a profound impact on how much snow we get in the Eastern US.
A weaker El Niño also has a strong split flow and a ridge of high pressure that settles across the western portion of North America. However, the ridge is much shaper and the eastern trough can dig deeper into the United States (vs locked up in Canada). This setup generlaly favors cooler tempeartures across the eastern half of the US, more opportunities for arctic air outbreaks, and increased snow probabilities.
