Last month, the Midwest experienced record-breaking cold temperatures and many are wondering how, when the climate is experiencing an unprecedented warming trend, we can still experience such frigid cold. News Bureau physical sciences editor Lois Yoksoulian asked University of Illinois atmospheric sciences professor Don Wuebbles to explain.

People often confuse weather and climate. What is the difference?

Weather is the state of the atmosphere at a given time; for example, a day might be about 40 degrees Fahrenheit and cloudy. Climate is the averages and statistics of weather over periods of 20-30 years. Mark Twain is attributed with saying, “Climate is what you expect, weather is what you get.” When we discuss climate change, we are referring to long-term trends and changes in the temperature, precipitation, severe storms and other parameters we generally associate with the weather.

What happened in late January 2019 to cause the extremely cold weather in the Midwest?

The reason for this event is complicated but involves the breakup of both the tropospheric and the stratospheric polar vortex into two pieces, and one of these pieces brought very cold air to the Midwest. The meteorological conditions were just right such that planetary atmospheric waves – driven by air flowing over mountains and other landforms – could penetrate into the stratosphere. This led to sudden, major warming of the stratosphere. This event, called sudden stratospheric warming, was one of the strongest on record. The resulting interaction of the stratospheric vortex with the tropospheric vortex led to the two blobs of extremely cold air. When combined with a weaker polar jet stream, this air was able to penetrate to the U.S. Midwest.

What is the polar vortex?

Many don’t realize that there are really two polar vortices. There is a polar vortex in the troposphere that is broader in extent and occurs throughout the year, and a polar vortex in the lower stratosphere that occurs from fall until spring. The stratospheric polar vortex is a low-pressure weather system that sits primarily in the lower stratosphere above the Arctic. The main feature of the stratospheric polar vortex is the strong west-to-east winds that usually encircle the North Pole, especially during the dark arctic winter.

Cold-air outbreaks are fundamentally tropospheric events but can be greatly affected by changes in the stratospheric vortex. While the tropospheric and stratospheric polar vortices are clearly distinct, they are able to interact. Both vortices can be influenced by sudden stratospheric warming, like the one that led to the most recent extreme cold event in the Midwest.

How does global warming affect the polar jet stream?

The polar jet stream is a year-round feature of the weather at mid-latitudes – it is like a river of high winds in the upper troposphere that circulates the globe separating arctic air from mid-latitude air. Its strength depends on the difference in temperature between the cold air over the Arctic and the milder air at mid-latitudes.

With the Arctic warming at twice the rate of the rest of the world, the temperature difference is declining between the Arctic and mid-latitudes, which leads to a weaker jet stream. The jet stream generally used to have a small s-shaped oscillation around the planet, but now we are getting a much wavier pattern than before. This can lead to situations where warm air can penetrate much further northward than previously, and where cold air can penetrate much further southward – like this past January. This can also lead to slower-moving weather systems. The combination of increased waviness in the jet stream and slower-moving fronts can worsen severe weather at a given location.

For some perspective, the included figure shows the temperature anomaly map for Jan. 28, 2019, relative to the period from 1979-2000. With the exception of the two cold blobs in blue from the divided polar vortex, major areas over the rest of the world were much warmer – shown in orange – than the past period. At the same time the Midwest was experiencing record cold, Australia had record heat waves, with temperatures at or above 110 degrees Fahrenheit in a number of areas for multiple weeks. Despite the extreme cold in the Midwest, the planet was still showing extensive warming overall.