How an El Nino is determined

Earlier this year, we were in the middle of a weak La Nina event, which is the cooler than normal sea-surface temperature phenomenon in the south-central Pacific Ocean. The latest observations are indicating that La Nina is weakening rapidly and now heading toward “neutral” conditions, or in-between cooler La Nina and the warmer El Nino. The neutral condition has also been referred to as a “La Nada.”

As I reported last month, long-range forecast models and sea-surface temperature forecasters are pointing to the development of a new El Nino as early as this summer and becoming very strong by late in the year.

If there is a moderate to strong El Nino later this year, there is a better chance that the 2026 tropical storm and hurricane season will be “slightly lower-than-average” in the Atlantic and Caribbean waters. According to researchers at the Colorado State University, it is predicted that approximately 13 named storms are expected, compared to an average of slightly more than 14. I’ll have more details about the upcoming season in the coming weeks.

An El Niño is a climate phenomenon, primarily along the Equatorial regions, that scientists measure using a combination of oceanic and atmospheric indicators. The most important method for measuring El Nino and La Nina events is monitoring sea surface temperatures (SSTs) in the central and eastern equatorial Pacific Ocean. Scientists compare current, weekly and monthly temperatures to long-term averages to calculate anomalies. When ocean temperatures in regions along the Equator are at least a degree Fahrenheit above average for at least several consecutive months, forecasters will determine that El Nino conditions are occurring. However, if these sea-surface temperature anomalies rise above about 2.7 degrees Fahrenheit, the event is classified as strong. This temperature-based index, often referred to as the Oceanic Nino Index (ONI), is the primary standard used worldwide.

In addition to ocean temperatures, scientists examine trade winds, prevailing easterly winds near the Equatorial. During an El Niño event, these winds will often weaken or sometimes reverse direction. This change allows warm water to move eastward toward the Americas, reinforcing the warming trend.

Another important measurement involves atmospheric pressure, known as the Southern Oscillation Index. This is measurement compares air pressure differences between Tahiti and Darwin, Australia. During El Nino, barometric air pressure tends to be lower in the eastern Pacific and higher in the western Pacific, resulting in a negative SOI value. By contrast, there is a positive value during a La Nina event.

The ocean heat content is also examined to help determine El Nino events. This refers to the amount of heat stored at the ocean’s surface and also beneath it. Before and during an El Niño, warm water often builds up below the surface and spreads across the Pacific. Instruments such as floating sensors that are anchored buoys measure temperature at different depths, which help scientists detect early signs of developing El Ninos and La Ninas, then track their progression.

Modern technology has greatly improved the accuracy of these sea-surface temperature observations. Global data coverage of ocean temperatures, winds, and sea level, are delivered by sophisticated satellites along with a network of ocean buoys, such as the Tropical Atmosphere Ocean (TAO) array. They now deliver real-time data from across the Pacific. By combining all these sources, scientists can monitor the development of El Nino and La Nina events with excellent precision.

Beginning last February, there was a change on how scientists will predict the onset of El Nino or La Nina across the Pacific Ocean. Until this year, scientists have measured ocean temperature anomalies by averaging the temperature and comparing it to the region’s past climatology. The new method will now compare the region along the Equator to average ocean temperatures across the entire global tropics. This new method is expected to help forecasters better classify El Nino and La Nina events. It should also help to improve the understanding of seasonal weather patterns across the globe and local regions as well.

In terms of our local weather, late last week was warm as high temperatures climbed into the 70s across the Coeur d’Alene region. However, showers are expected this week along with cooler temperatures. There’s also the possibility that one of the storms around the middle of the week will be cold enough to produce some snowflakes in the lower elevations. The average April snowfall in Coeur d’Alene is 0.7 inches. The long-range computer models are also indicating a chance of at least a rain and snow mix next week, so winter may not be finished.

Additional moisture is expected for the rest of April to the middle of next month. Showers are also possible in early June around the full moon cycle. As I mentioned last week, the latest spring forecast from NOAA calls for a drier than normal weather pattern across the Inland Northwest. The upcoming summer season also looks to be drier than average as well with temperatures near seasonal averages, especially if we see the formation of a new El Nino.

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Contact Randy Mann at [email protected].