Computer model simulation of a lobe of the polar vortex over the Great Lakes on Jan. 30. (PivotalWeather.com/PivotalWeather.com)

The author, Richard Rood, is a professor of atmospheric science at the University of Michigan.

A few years ago, the “polar vortex” surged into the public language of weather and climate change. Now, in 2019, when I ask my students to describe a cold wave in Michigan, they immediately talk about the vortex coming down from the pole. The story continues: The polar vortex is surrounded by the jet stream; the jet stream is getting more wavy; it is more wavy because of climate change, and likely, the loss of Arctic sea ice.

This is a concise narrative, reasonably extracted from news reports and knowledgeable scientists. As with most concise narratives of complex situations, there is a core of truths, surrounded by a quagmire of confused details.

There is, in fact, an official definition of cold wave, namely, a drop of temperature within a 24-hour period that requires increased protection of our enterprise. The cold wave of late January 2019 in the central eastern United States and Canada was characterized by temperatures dropping rapidly to values that are dangerous to human life and damaging to buildings and infrastructure.

In Ann Arbor, Mich., where I teach, the low temperature on Jan. 31 was minus-17. Farther to the west, in places where the temperature is less influenced by the Great Lakes, it was much colder. For example, in Lacrosse, Wis., it was minus-31 on Jan. 30. The record or near-record cold temperatures came after record or near-record warm temperatures earlier in the month.

The cold wave was predicted days in advance, and people and institutions took steps to protect themselves. As predictable as the weather was the emergence of the political messaging that the reality of a cold wave showed the falseness of global warming. This political messaging is so predictable that scientists, like myself, reposted their explainers and prepared for calls from reporters.

Potential record-cold temperatures and threats to life and property are compelling events, as are the messages of such events. Plus, on its surface, it is, perhaps, paradoxical that we see record-cold temperatures on a planet that is, on average, definitively warming. It is, therefore, worth disentangling the confusion that surrounds the core of truths in my students’ ready explanation of the polar vortex coming to Michigan.


Temperature differences from normal around the globe for the planet averaged over the past five years (2014-2018). (NASA)

Because Earth’s rotation influences atmospheric motion, many weather systems can be described as either a vortex or a wave. A vortex in the atmosphere is, simply, air in a circular flow. Most are familiar with the circular motion of hurricanes or tornadoes; both are vortices.

The vortex of our wintertime cold waves is often called the polar vortex. It is, approximately, a circular flow that forms around the wintertime pole. This flow has a narrow high-speed wind field, a jet stream, at its edge. The jet stream isolates the air inside the polar vortex. That air is cold — very cold, because it is at the wintertime pole, and the sun is not present at the winter pole. Because the darkness at the wintertime pole is related to the tilt of Earth, relative to the sun, it gets cold at the pole whether or not Earth’s carbon dioxide is increasing and the planet is warming.

Therefore, even in a warming world, we have the physical processes that form cold air. There are important details about how the cold air is changing. For example, the area of very cold air should get smaller from year to year — a phenomenon that is already observed. The coldness, that is the absolute minimum temperatures that we might expect, would likely stay about the same as in the past, but air getting to those temperatures would be less common. Indeed, we would expect very cold air to become more rare.

In our warming world, there is more heat to transport and more energy to displace the cold polar air. We have the situation where a smaller vortex gets shoved around more than in the past. If that air gets shoved to different places than in the past, as it touches those places, we set records. New cold records are, however, increasingly rare.

The increased heat at the poles is going to melt sea ice and permafrost. The decrease of sea ice and Arctic warming might feed back to make the displacement of the wintertime polar vortex more common and its wanderings more ranging. However, it remains a matter of nuance, controversy and research.

There are other characteristics of the late-January weather pattern that are more telling than the small area of cold air transiting the eastern United States. At the same time, it was warmer in parts of Norway than in North Florida. Anchorage was above freezing for several days. Warm air is moving toward the poles, and from decade to decade that air is getting warmer, and its areal extent is increasing.

And, back in Michigan, less than a week after that minus-17, the temperature shot up to 51 degrees.

We seem to want to amplify our experience of coldness — to declare that we are cold, but we know the planet is warming up. What we are seeing is that the Arctic is relentlessly warming, and the little Arctic cold that remains is being unceremoniously shoved out. We are in a time when the climate is changing, and we should both psychologically and actually expect the unexpected.

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