Meteorology explains science of polar vortices

Credit: National Science Foundation via Wikimedia Credit: National Science Foundation via Wikimedia

Sometimes it is best to immediately address the elephant in the room: It’s cold outside.

Some public and private schools in Pittsburgh were closed on Friday not because of the potential snow accumulation, but because of the bitter cold. A rush of icy arctic air caused temperatures from the Eastern Seaboard through the Midwest to drop to record lows. Many U.S. cities experienced temperatures in the negative teens, including Pittsburgh. The frigid conditions have had many negative impacts, affecting everything from air travel to our desire to venture outside.

Meteorologists explain the most recent drop in temperatures this past week through the displacement of the polar vortex, a pocket of cold air that sits above the polar region.

The Earth contains two polar vortices, one at each pole, that are are defined as persistent circulations of cold, low-pressure air that strengthen in the winter and weaken in the summer. They generally exist in the middle to upper troposphere, the lowest atmospheric layer, and extend into the lower stratosphere, the second lowest atmospheric layer.

Meteorologists defined this phenomenon in the early 1850s, but the term “polar vortex” has only recently garnered significant attention. Its newfound familiarity, however, has led to some using the term incorrectly to describe all severe drops in temperature in the United States. Understanding a polar vortex can help explain how a polar vortex may contribute to some of the lower temperatures such as the drop we all recently experienced.

The northern and southern polar vortices are slightly different; the air in the north generally circulates in a counter-clockwise fashion, while the air in the south rotates clockwise.

In addition, the southern polar vortex is larger and more powerful than its northern counterpart. The presence of more landmasses and a larger land-sea contrast in the north results in large-scale waves that move upward from the troposphere to the stratosphere, erode the vortex, and increase its internal temperature.

The northern polar vortex is circled by a strong west to east jet stream referred to as the polar night jet. This jet stream separates the polar region from the midlatitudes, acting as a barrier that keeps the polar vortex from shifting downwards. Occasionally, unstable planetary waves in the early winter can cause the polar air to break the barrier and mix with the midlatitudes, causing cooling.

Conversely, the current cooling phenomenon is due to the natural warming cycle of the polar vortex that leads to its eventual breakup. As the earth begins its transition from winter to spring, the first warming period occurs between late February to mid-March.

This warming causes a piece of the polar vortex to break-off, displace the polar night jet, and plunge into Canada and a large portion of the United States.

This, however, is not a very common event for several reasons. First, it is not very likely for cold air to push past a powerful jet stream. Second, the chuck of the vortex that splits off must be large enough to cover a wide portion of land in order for us to see effects in the Northeastern United States. The occurrence of both of these factors is considered an anomaly of stratospheric circulation.

Research is currently ongoing to explore whether global warming may play a role in an increased frequency of polar bursts into the midlatitudes. A rise in the earth’s temperature is already known to produce anomalies such as the distortion of the jet stream, which allows the polar vortex to escape. Current scientific observations regarding this phenomenon are considered short-term, starting only 13 years ago. In order to make confident conclusions, scientists need several decades of data to distinguish between natural variability and climate trends.

The polar vortex is an intriguing concept for climatologists, not due to its potential to deliver us cold air, but because its presence can have a negative effect on the ozone layer. The ozone layer is found in the stratosphere and plays a large role in shielding us from potentially harmful ultraviolet rays from the sun. As temperatures drop in the polar vortex, polar stratospheric clouds form. Chemical reactions on these clouds due to the presence of of chlorofluorocarbons (CFCs) release chlorine that can rapidly destroy ozone.

Over the past several years, the polar vortex has made itself more relevant and moved beyond purely scientific importance by sneaking into the midlatitudes and impacting the public’s way of life in the Northeastern United States.