Polar vortex

A polar vortex, more formally a circumpolar vortex, is a large region of cold, rotating air; polar vortices encircle both of Earth's polar regions. Polar vortices also exist on other rotating, low-obliquity planetary bodies. The term polar vortex can be used to describe two distinct phenomena; the stratospheric polar vortex, and the tropospheric polar vortex. The stratospheric and tropospheric polar vortices both rotate in the direction of the Earth's spin, but they are distinct phenomena that have different sizes, structures, seasonal cycles, and impacts on weather. The stratospheric polar vortex is an area of high-speed, cyclonically rotating winds around 15 to 50 kilometres (10 to 30 mi) high, poleward of 50°, and is strongest in winter. It forms during autumn when Arctic or Antarctic temperatures cool rapidly as the polar night begins. The increased temperature difference between the pole and the tropics causes strong winds, and the Coriolis effect causes the vortex to spin up. The stratospheric polar vortex breaks down during spring as the polar night ends. A sudden stratospheric warming (SSW) is an event that occurs when the stratospheric vortex breaks down during winter, and can have significant impacts on surface weather. The tropospheric polar vortex is often defined as the area poleward of the tropospheric jet stream. The equatorward edge is around 40° to 50°, and it extends from the surface up to around 10 to 15 km (6 to 9 mi). Its yearly cycle differs from the stratospheric vortex because the tropospheric vortex exists all year, but is similar to the stratospheric vortex since it is also strongest in winter when the polar regions are coldest. The tropospheric polar vortex was first described as early as 1853. The stratospheric vortex's SSWs were discovered in 1952 with radiosonde observations at altitudes higher than 20 km (12 mi). The tropospheric polar vortex was mentioned frequently in the news and weather media in the cold North American winter of 2013–2014, popularizing the term as an explanation of very cold temperatures. The tropospheric vortex increased in public visibility in 2021 as a result of extreme frigid temperatures in the central United States, with newspapers linking its effects to climate change. Ozone depletion occurs most heavily within the polar vortices – particularly over the Southern Hemisphere – reaching a maximum depletion in the spring.

Arctic and Antarctic vortices

Northern Hemisphere When the tropospheric vortex of the Arctic is strong, it has a well defined and nearly circular shape. There is a single vortex with a jet stream that is well constrained near the polar front, and the Arctic air is well contained. When this northern tropospheric vortex weakens, it breaks into two or more smaller vortices, the strongest of which are near Baffin Island, Nunavut, and the others over northeast Siberia. When it is very weak, the flow of Arctic air becomes more disorganized, and masses of cold Arctic air can push equatorward, bringing with them a rapid and sharp temperature drop. A deep freeze that gripped much of the United States and Canada in late January 2019 was blamed in some accounts on a "polar vortex". This is not the scientifically correct use of the term polar vortex, but instead is referring to outbreaks of cold Arctic air caused by a weakened polar vortex. The US National Weather Service warned that frostbite is possible within just 10 minutes of being outside in such extreme temperatures, and hundreds of schools, colleges, and universities in the affected areas were closed. Around 21 people died in US due to severe frostbite. States within the midwest region of the United States had windchills just above -50 °F (-45 °C). The polar vortex is also thought to have had effects in Europe. For example, the 2013–14 United Kingdom winter floods were blamed on the polar vortex bringing severe cold in the United States and Canada. Similarly, the severe cold in the United Kingdom in the winters of 2009–10 and 2010–11 were also blamed on the polar vortex.

Southern Hemisphere The Antarctic vortex of the Southern Hemisphere is a single low-pressure zone that is found near the edge of the Ross ice shelf, near 160 west longitude. When the polar vortex is strong, the mid-latitude Westerlies (winds at the surface level between 30° and 60° latitude from the west) increase in strength and are persistent. When the polar vortex is weak, high-pressure zones of the mid-latitudes may push poleward, moving the polar vortex, jet stream, and polar front equatorward. The jet stream is seen to "buckle" and deviate south. This rapidly brings cold dry air into contact with the warm, moist air of the mid-latitudes, resulting in a rapid and dramatic change of weather known as a "cold snap". In Australia, the polar vortex, known there as a "polar blast" or "polar plunge", is a cold front that drags air from Antarctica which brings rain showers, snow (typically inland, with blizzards occurring in the highlands), gusty icy winds, and hail in the south-eastern parts of the country, such as in Victoria, Tasmania, the southeast coast of South Australia and the southern half of New South Wales (but only on the windward side of the Great Dividing Range, whereas the leeward side will be affected by foehn winds).

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