This post was originally published on snow.news, my free Substack on snow science and the state of the snowpack.
ENSO stands for El Niño-Southern Oscillation, a climate phenomenon in which changing sea surface temperatures and atmospheric pressure patterns in the equatorial Pacific Ocean can exert significant influences on weather around the world.
ENSO has three phases:
1. El Niño: when sea-surface temperatures in the central and eastern Pacific are warmer than average. The typical trade winds, which blow from east to west, tend to weaken or even reverse into westerlies.
2. La Niña: when sea-surface temperatures in the central and eastern Pacific are cooler than average. In this case, the normal easterly trade winds tend to strengthen.
3. Neutral: when sea-surface temperatures are close to average, though as NOAA notes, “there are some instances when the ocean can look like it is in an El Niño or La Niña state, but the atmosphere is not playing along (or vice versa).”
The graphic below from NOAA shows the anomalies in sea-surface temperatures during the two opposing phases.
The maps below, from NOAA’s National Ocean Service, show the typical weather patterns during El Niño (top) and La Niña (bottom), though these aren’t set in stone.
The name “El Niño” in this context is Spanish for “the Christ child.” In the 17th century, South American fishermen noticed the pattern peaking around Christmas. During El Niño, the unusually warm waters off the west coast of Ecuador and Peru diminished their catch by disrupting the upwelling of nutrient-rich cold water.
The “Southern Oscillation” in the ENSO acronym refers to changing patterns in sea-level pressure in Indonesia and the tropical Pacific that were discovered in the early 20th century by Sir Gilbert Walker, an English physicist, mathematician, meteorologist, and namesake of the Walker Circulation.
Eventually, scientists linked the changes in sea-surface temperatures with the atmospheric patterns, but it wasn’t until the 1980s that the terms El Niño, La Niña, and ENSO took off, as shown in the chart below from Google Books Ngram Viewer, which tracks word usage.
According to the latest report from the Climate Prediction Center, we’re currently in a La Niña Watch, with a shift to neutral conditions likely in the spring. It’ll be interesting to see what’s in the next update on January 9.
“The atmosphere looks like La Niña, and has for a while, but the ocean doesn’t, at least by our traditional sea surface temperature measures,” according to a December 12 post on climate.gov’s ENSO blog. “But even if we do declare a La Niña Advisory soon, it will very likely be a weak event at most.”
What does a weak La Niña mean for snowfall in the American West?
“In the nine previous weak La Niña events, the pattern of snow was similar to that of all La Niña events with above-average snowfall observed, on-average, across the northwest and north central U.S. with below-average snowfall farther south,” according to an October 24 post on the ENSO blog, which includes the map below for January-March snowfall.
