Earth’s surface temperature is warming, and the Arctic region is warming even faster. Yet winters in the mid-latitudes of Asia, Europe, and the United States have been colder than average, thanks to blasts of record-breaking frigid air caused by breakdowns of the polar vortex—the winter stratospheric jet stream circulating around the polar regions. When the Arctic polar vortex is strong, the polar air remains in place over the North Pole. When the polar vortex is weaker, extremely cold air can dip south, as it did in late January 2019 in the United States. These strong and weak phases of the polar vortex are known as the Arctic Oscillation. See also: Arctic Circle; Arctic Oscillation; Extreme weather events; Middle-atmosphere dynamics; North Pole; Polar meteorology; Stratosphere; Weather

Strong and weak phases of the Arctic polar vortex. (Credit: NOAA)

One way the polar vortex is destabilized, or sometimes split, is by warm air entering the Arctic stratosphere. This phenomenon is known as a sudden stratospheric warming. Such a sudden stratospheric warming above the North Pole was responsible for the late-January 2019 big chill, causing a piece (or lobe) of the polar vortex to dip over Canada and the United States’ northern plains. [It is incorrect to refer to this cold air mass as “the polar vortex,” because it is only a part of the vortex and not the entire vortex.] The temperatures recorded on January 30, 2019, indicate that in the Chicago area temperatures plummeted to -31°C (-23°F) at O'Hare International Airport, while Norris Camp, Minnesota, was the coldest location in the country, with a low of -44°C (-48°F). By February 1, temperatures began moderating as the Arctic air retreated to the north. And on February 2, Chicago’s temperature rebounded to a high of 5°C (41°F), marking the end of the four-day Arctic blast.

Air temperatures at 2 meters above the ground at 4 a.m. Eastern Standard Time on January 29, 2019, as represented by the Goddard Earth Observing System Model. GEOS is a global atmospheric model that uses mathematical equations run through a supercomputer to represent physical processes. The figure above is not a traditional forecast, but a reanalysis of model input; that is, a representation of atmospheric conditions of that day. Measurements of temperature, moisture, wind speeds and directions, and other conditions are compiled from NASA satellites and other sources, and then added to the model to closely simulate observed reality. Note how some portions of the Arctic are close to the freezing point—significantly warmer than usual for the dark of mid-winter—while masses of cooler air plunge toward the interior of North America. (Credit: NASA)