As students walk to class huddled beneath layers of scarves and jackets, the familiar Evanston landscape around them seems to be transformed into a scene from the Arctic.
This recent blast of cold air, which has dumped 10 to 15 inches of snow on the Chicago area, did arrive from the Arctic. An alteration in the wind pattern, which normally carries air from east to west across the United States, has swept cold air south from Canada instead of straight across the continent.
For the past week, Arctic air has been pushed further south than normal, resulting in colder temperatures throughout the Midwest.
“Air is moving from where it’s really cold to where it’s (normally) warm,” said Ed Birchfield, professor emeritus of geology.
Meteorologists call the southward thrust of Arctic winds a trough. A trough is not an uncommon occurrence, but the cause is not completely understood. A link may exist between the development of a trough and an El Nino event in the Pacific Ocean.
An El Nino event occurs when trade winds weaken and no longer have enough force to push warm equatorial waters from east to west. The warmer surface waters that normally pile up along the western edge of the Pacific remain further east.
The change in heat distribution results in an alteration of wind patterns along the equator. Although this wind change occurs south of the United States, it can still affect winds to the north and may bring frigid air down from Canada, Birchfield said.
At the end of December, climatologists noted that increasing amounts of warm water had not been pushed westward by the winds, according to the National Oceanic and Atmospheric Administration.
Chicago temperatures reached their lowest point on Jan. 17 with a high of 11 degrees and a low of 2 degrees, according to NOAA. The normal high for Jan. 17 is 29 degrees and the normal low is 14 degrees.
“Some people are saying that the (extreme) weather of this year is due to global warming,” wrote Fred Mackenzie, a visiting professor of geology, in an e-mail. “There is little evidence to document this assertion at the moment, but it could play a role.”
Although the temperatures over the past week fall below the norm, Birchfield said temperatures overall are on the rise.
“I’m pretty well convinced that if you look at the last 50 years,” he said, “the temperatures are going upward.”
Birchfield has been in the Chicago area for more than 50 years and remembers much colder spells than the one of the past week.
“When I first came to Chicago, every winter we would have at least a week when the temperatures would be below zero,” he said.
Joanna Allerhand can be reached at [email protected].
