Peoria, Ill. (WMBD) — La Niña continues and it appears we’ll be in for a rare triple dip La Niña this winter, something that’s happened only two other times since El Niño Southern Oscillation records began in 1950 (1975-76 and 2000-01). While it’s not the only big player, the impacts of the El Niño Southern Oscillation have the greatest impacts on weather during Northern Hemisphere winters, so forecasters often pay attention to it.

What is the El Niño Southern Oscillation (ENSO)?

ENSO consist of three phases, El Niño (positive phase), Neutral, and La Niña (negative phase). Each phase is determined by sea surface temperatures and how they compare to the long term average along with stronger atmospheric circulations over the equatorial Pacific. We are considered to be in a La Niña when sea-surface temperatures (SST) in the east-central Pacific, a region called Niño-3.4, is at least 0.5 °C cooler than the long term average. In July, Niño 3.4 was 0.7 °C cooler than average.

Based on data from NOAA we have been in La Niña in 21 of the last 24 months as we experienced a double-dip La Niña last winter. This chart from NOAA shows the history of SST anomalies over the past two years (purple line) and the three year plots of every double dip La Niña since 1950 (gray lines). Only two of those La Niñas dipped for a third straight winter.

Source: NOAA

How long will La Niña stick around?

While SSTs have show some warming in the past few months, wind patterns across the equatorial Pacific will allow colder water sitting several hundred feet beneath the surface to rise, likely sending SSTs back down for the upcoming fall and winter. The latest model data and forecasts from the Climate Prediction Center show that La Niña is likely continue through the winter season (December through February). By early Spring, forecasters give even chances (47%) either La Niña or a neutral phase of ENSO with Neutral conditions favored by the start of summer 2023.

What does La Niña mean for Illinois?

Sea Surface Temperatures across the equatorial Pacific ultimately influence the strength and positions of the jet stream in the Northern Hemisphere, and specifically impact North America. La Niña patterns result in variable polar and pacific jet streams which typically bring cooler and wetter conditions to the northwest U.S and warmer and drier conditions to the southern U.S.

In Illinois, La Niña has historically resulted in warmer and drier summers with fall turning cooler and wetter, at least in the northern part of the state. By winter, La Niña typically resulted in warmer and wetter than average conditions along with above average snowfall and more winter storms. It’s also worth noting that in recent years we’ve seen some pretty big severe weather events as we head into the cold season. Its not clear if this is actually tied to La Niña but its an interesting trend.

However, it’s important to understand that La Niñas tend to be highly variable and can vary significantly from one La Niña winter to the next. Differences in strength and interactions with other teleconnections such as the Arctic Oscillation, North Atlantic Oscillation and Eastern Pacific Oscillations often determine how our winter seasons pan out.