Polar vortex

A polar vortex, or circumpolar vortex, is a massive region of cold, rotating air. These large systems encircle both of Earth's polar regions. They also exist on other rotating planetary bodies with low obliquity. Scientists distinguish between two different phenomena: the stratospheric polar vortex and the tropospheric polar vortex. While both rotate in the same direction as the Earth's spin, they differ in size, structure, and seasonal cycles. Understanding these systems is vital because they dictate how extreme cold moves across the globe.

The stratospheric polar vortex is a high-altitude system. It consists of high-speed, cyclonically rotating winds located between 15 km and 50 km high. This vortex forms during the autumn as Arctic or Antarctic temperatures drop rapidly during the polar night. As the temperature difference grows between the poles and the tropics, strong winds develop. The Coriolis effect then causes these winds to spin up into a coherent vortex. This system is strongest during the winter months. It eventually breaks down during the spring as the polar night ends.

A different mechanism drives the tropospheric polar vortex. This system is often defined as the area poleward of the tropospheric jet stream. It extends from the Earth's surface up to approximately 10 km to 15 km. Its equatorward edge is located around 40° to 50° latitude. Unlike the stratospheric version, the tropospheric vortex exists all year long. However, it still reaches its peak strength in winter when polar regions are coldest.

History shows how our understanding of these winds has grown. The tropospheric polar vortex was first described in a publication called "Air Maps" in 1853. Later, in 1952, scientists discovered sudden stratospheric warming (SSW) events. They used radiosonde observations at altitudes above 20 km to identify these events. More recently, the term became a household name during the cold North American winter of 2013–2014. It gained even more visibility in 2021 due to extreme frigid temperatures in the central United States.

The behavior of these vortices can lead to dramatic weather changes. A sudden stratospheric warming can cause temperatures to rise by 30–50 °C (54–90 °F) in just a few days. This event can even reverse the Arctic circulation from counter-clockwise to clockwise. In the Northern Hemisphere, a strong vortex is often elongated with two cyclone centers. One center sits over Baffin Island in Canada, while the other is over northeast Siberia. When the vortex weakens, it can break into smaller, disorganized vortices. This allows cold Arctic air to push equatorward, causing sharp temperature drops.

These shifts have significant real-world impacts. In late January 2019, a deep freeze gripped much of the United States and Canada. During this time, windchills in the U.S. Midwest reached nearly -50 °F (-45 °C). Such extreme cold can cause frostbite in just 10 minutes. In Australia, the vortex is known as a "polar blast" or "polar plunge." It can bring rain, snow, and hail to southeastern regions like Victoria and Tasmania. Additionally, the polar vortex plays a role in ozone depletion. This depletion is most heavy within the vortices, especially over the Southern Hemisphere, reaching a maximum in the spring.

Finally, the polar vortex connects to much larger global systems. Changes in the stratospheric vortex can influence the troposphere below. For example, it can affect the speed of the Atlantic Ocean circulation pattern. A specific area south of Greenland, called the "Achilles Heel of the North Atlantic," is where downwelling begins. Small changes in the vortex can trigger or delay this process. This can alter the Gulf Stream Current and other major ocean currents. Because the world's oceans depend on the movement of heat in the Atlantic, these polar winds can affect climates across the entire planet.