Scientists officially declared that La Niña is over. That’s newsworthy because we just experienced a rare “triple-dip“ La Niña. According to the World Meteorological Organization (WMO), this means the event spanned three consecutive northern hemisphere winters (or southern hemisphere summers). How do climatologists know this one is over – for now anyhow?
First, let’s refresh your memory on La Niña. According to the WMO website, “La Niña refers to the large-scale cooling of the ocean surface temperatures in the central and eastern equatorial Pacific Ocean, coupled with changes in the tropical atmospheric circulation, namely winds, pressure and rainfall.” El Niño, arguably the more famous of the two climate “siblings,” is the warm phase of the natural cycle called the El Niño Southern Oscillation (ENSO).
In the United States, La Niña can impact weather across the entire country. According the the National Oceanic and Atmospheric Administration (NOAA) website, “During the winter, La Niña typically brings above-average precipitation and colder-than-average temperatures along the northern tier of the U.S., along with below-average precipitation and above-average temperatures across the South.” La Niña is also typically associated with more active Atlantic hurricane seasons. However, these are average expectations. Things can certainly vary. For example, Robin Meadows does an excellent job in Scientific American explaining how the ENSO cycle has been a bit “Jekyll and Hyde” with recent Atmospheric Rivers lashing the U.S. West.
Ok, let’s get back to the question posed in the title. How do scientists know La Niña is over for now? On March 9th, NOAA’s Climate Prediction Center (not a groundhog) announced that ENSO-neutral conditions were expected into early Summer. They base this information on sea surface temperatures (SSTs) and various indices. NOAA wrote, “During February 2023, below-average sea surface temperatures (SSTs) weakened and currently persist only in the central Pacific Ocean (map above).” They went on to say that SSTs in the eastern Pacific Ocean were above average. They also noted that wind conditions and precipitation patterns in the Pacific Ocean were consistent with ENSO-neutral conditions. The graphic below provides insight on the complex interactions involving the ocean and atmosphere within the ENSO.
One thing is guaranteed. La Niña will return as ENSO is a cycle that oscillates roughly every 2 to 7 years. I will be keeping my eyes out to see if there is a transition to El Niño later this summer. For a comprehensive “101” on El Niño and La Niña, I highly recommend the NOAA resources at this link.