This graphic from the National Oceanic and Atmospheric Administration shows the predicted wintertime El Niño pattern for this winter. Graphic from the National Oceanic and Atmospheric website:

With winter here, many Evanstonians are probably shivering at the thought of wind chills and knee-deep snow.  

But the National Weather Service (NWS) has news that is as welcome as pair of warm slippers: Thanks to a strong El Niño in the Pacific ocean, winter of 2015-16 is shaping up to be mild and manageable. 

“El Niño is the big player in the game this winter,” says Jim Angel, Illinois’ State Climatologist.

“It is expected to affect temperature and precipitation patterns across the United States, with above-average temperatures and below-median precipitation over the northern tier of the United States,” Dr. Angel adds.  

In short, the Midwest can look forward to a warmer winter with less snow – good for people who hate to shovel, and a potential disappointment to those who count on a season full of sledding, skating and snowy fun.  

El Niño occurs when sea surface temperatures in the Pacific ocean become unusually warm along the equator off the coast of Peru.  

The warmer surface water boosts the Pacific jet stream – a fast-flowing current in the upper atmosphere – and shifts it eastward toward the United States, according to NASA’s Earth Observatory. The incoming jet stream causes cooler, wetter weather in western and southern states, and warmer drier weather in the north. (Conversely, Indonesia and other parts of the western Pacific tend to be unusually dry as a result of El Niño.)

El Niño was responsible for November’s mild temperatures, rain and pre-Thanksgiving snowfall, says Dr. Angel, who works at the Illinois State Water Survey, part of the Prairie Research Institute at the University of Illinois in Champaign-Urbana.    

“Illinois just finished its third wettest and tenth warmest November on record,” Dr. Angel notes on his Climatologist Illinois blog. And despite the warm November weather, an early season storm dropped up to 17 inches of snow across northern Illinois the weekend before Thanksgiving, also an effect of El Niño, Dr. Angel says.  

“In past El Niño events, the months with the most snowfall were November and March,” he explains, “so when we have winter weather, it often hits early – like the big snow we had in November – and again on the late side of the season.”

El Niño events happen about every 3-5 years. Water temperatures begin warming in late spring in the Pacific, and we start feeling its effects many months later in late fall and throughout winter. Usually the the events are fairly moderate, says Dr. Angel, with notable exceptions being the winters of 1982-83 and 1997-98.  

“Those were two very mild winters for us,” he says, “and not just for Illinois – really the whole upper Midwest was very mild during those events.”  

Those last two Midwest El Niño effects peaked during January and February, Dr. Angel explains, so it is a bit early to tell how this year’s will turn out. Any effect El Niño is having on local weather should be over by spring.

South American fishermen dubbed the phenomenon El Niño (“the little boy” in Spanish) in honor of the Christ Child, because they noticed warmer sea waters every few years around Christmas time.  

Its opposite, La Niña (“the little girl”), occurs less frequently, and its effects are not as obvious, says Dr. Angel. “Sometimes La Niña gives us more winter weather, and sometimes drought conditions,” he explains. “It’s not as strong a pattern as we see with
El Niño.”

Although El Niño brings warmer weather, it is not a by-product of global warming but could be affected by it in the future. As far as we know, says Dr. Angel, it has been around forever.  

“We’ve been tracking El Niño probably back to the 1900s,” he says, “and earlier than that we have evidence that it’s been around for at least a few thousand years. It’s uncertain how climate change will affect El Niño going forward – whether we’ll have more, stronger, or fewer of these events. Those two will play out in the future.”

In light of the warm winter predictions, there is still a chance of some very cold weather, and possibly the dreaded polar vortex.  

“Those kind of cold air outbreaks can show up pretty much any time,” says Dr. Angel, “even in the middle of a mild winter. They are less likely to show up during an El Niño event, but we never escape the entire winter without one or two weeks of really cold weather. ”

Residents may wish to keep parkas, wool socks and shovels handy, just in case.

Meg Evans has written science stories for the Evanston RoundTable since 2015, covering topics ranging from local crayfish, coyotes and cicadas to gravitational waves, medical cannabis, invasive garden...