Polar vortex

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). ==Identification==

The bases of the two polar vortices are located in the middle and upper troposphere and extend into the stratosphere. Beneath that lies a large mass of cold, dense Arctic air. The interface between the cold dry air mass of the pole and the warm moist air mass farther south defines the location of the polar front. The polar front is centered roughly at 60° latitude. A polar vortex strengthens in the winter and weakens in the summer because of its dependence on the temperature difference between the equator and the poles. Polar cyclones are low-pressure zones embedded within the polar air masses, and exist year-round. The stratospheric polar vortex develops at latitudes above the subtropical jet stream. Horizontally, most polar vortices have a radius of less than . At the level of the tropopause, the extent of closed contours of potential temperature can be used to determine its strength. Others have used levels down to the 500 hPa pressure level (about above sea level during the winter) to identify the polar vortex. ==Duration and strength==

Polar vortices are weakest during summer and strongest during winter. Extratropical cyclones that migrate into higher latitudes when the polar vortex is weak can disrupt the single vortex creating smaller vortices (cold-core lows) within the polar air mass. Those individual vortices can persist for more than a month. The strength and position of the polar vortex shapes the flow pattern in a broad area about it. An index which is used in the Northern Hemisphere to gauge its magnitude is the Arctic oscillation. When the Arctic vortex is at its strongest, there is a single vortex, but normally, the Arctic vortex is elongated in shape, with two cyclone centers, one over Baffin Island in Canada and the other over northeast Siberia. When the Arctic pattern is at its weakest, subtropic air masses can intrude poleward causing the Arctic air masses to move equatorward, as during the Winter 1985 Arctic outbreak. The Antarctic polar vortex is more pronounced and persistent than the Arctic one. In the Arctic the distribution of land masses at high latitudes in the Northern Hemisphere gives rise to Rossby waves which contribute to the breakdown of the polar vortex, whereas in the Southern Hemisphere the vortex is less disturbed. The breakdown of the polar vortex is an extreme event known as a sudden stratospheric warming, here the vortex completely breaks down and an associated warming of 30–50 °C (54–90 °F) over a few days can occur. The waxing and waning of the polar vortex is driven by the movement of mass and the transfer of heat in the polar region. In the autumn, the circumpolar winds increase in speed and the polar vortex rises into the stratosphere. The result is that the polar air forms a coherent rotating air mass: the polar vortex. As winter approaches, the vortex core cools, the winds decrease, and the vortex energy declines. Once late winter and early spring approach the vortex is at its weakest. As a result, during late winter, large fragments of the vortex air can be diverted into lower latitudes by stronger weather systems intruding from those latitudes. In the lowest level of the stratosphere, strong potential vorticity gradients remain, and the majority of that air remains confined within the polar air mass into December in the Southern Hemisphere and April in the Northern Hemisphere, well after the breakup of the vortex in the mid-stratosphere. The breakup of the northern polar vortex occurs between mid March to mid May. This event signifies the transition from winter to spring, and has impacts on the hydrological cycle, growing seasons of vegetation, and overall ecosystem productivity. The timing of the transition also influences changes in sea ice, ozone, air temperature, and cloudiness. Early and late polar breakup episodes have occurred, due to variations in the stratospheric flow structure and upward spreading of planetary waves from the troposphere. As a result of increased waves into the vortex, the vortex experiences more rapid warming than normal, resulting in an earlier breakup and spring. When the breakup comes early, it is characterized by with persistent of remnants of the vortex. When the breakup is late, the remnants dissipate rapidly. When the breakup is early, there is one warming period from late February to middle March. When the breakup is late, there are two warming periods, one January, and one in March. Zonal mean temperature, wind, and geopotential height exert varying deviations from their normal values before and after early breakups, while the deviations remain constant before and after late breakups. Scientists are connecting a delay in the Arctic vortex breakup with a reduction of planetary wave activities, few stratospheric sudden warming events, and depletion of ozone. , Maine, and New Brunswick, part of the northern polar vortex weakening, on the record-setting cold morning of January 21, 1985 Sudden stratospheric warming events are associated with weaker polar vortices. This warming of stratospheric air can reverse the circulation in the Arctic Polar Vortex from counter-clockwise to clockwise. These changes aloft force changes in the troposphere below. An example of an effect on the troposphere is the change in speed of the Atlantic Ocean circulation pattern. A soft spot just south of Greenland is where the initial step of downwelling occurs, nicknamed the "Achilles Heel of the North Atlantic". Small amounts of heating or cooling traveling from the polar vortex can trigger or delay downwelling, altering the Gulf Stream Current of the Atlantic, and the speed of other ocean currents. Since all other oceans depend on the Atlantic Ocean's movement of heat energy, climates across the planet can be dramatically affected. The weakening or strengthening of the polar vortex can alter the sea circulation more than a mile beneath the waves. Strengthening storm systems within the troposphere that cool the poles, intensify the polar vortex. La Niña–related climate anomalies significantly strengthen the polar vortex. Intensification of the polar vortex produces changes in relative humidity as downward intrusions of dry, stratospheric air enter the vortex core. With a strengthening of the vortex comes a longwave cooling due to a decrease in water vapor concentration near the vortex. The decreased water content is a result of a lower tropopause within the vortex, which places dry stratospheric air above moist tropospheric air. Instability is caused when the vortex tube, the line of concentrated vorticity, is displaced. When this occurs, the vortex rings become more unstable and prone to shifting by planetary waves. The planetary wave activity in both hemispheres varies year-to-year, producing a corresponding response in the strength and temperature of the polar vortex. The number of waves around the perimeter of the vortex are related to the core size; as the vortex core decreases, the number of waves increase. The degree of the mixing of polar and mid-latitude air depends on the evolution and position of the polar night jet. In general, the mixing is less inside the vortex than outside. Mixing occurs with unstable planetary waves that are characteristic of the middle and upper stratosphere in winter. Prior to vortex breakdown, there is little transport of air out of the Arctic Polar Vortex due to strong barriers above 420 km (261 miles). The polar night jet which exists below this, is weak in the early winter. As a result, it does not deviate any descending polar air, which then mixes with air in the mid-latitudes. In the late winter, air parcels do not descend as much, reducing mixing. After the vortex is broken up, the ex-vortex air is dispersed into the middle latitudes within a month. Occasionally, the high-pressure air mass, called the Greenland Block, can cause the polar vortex to divert to the south, rather than follow its normal path over the North Atlantic. ==Extreme weather==

A study in 2001 found that stratospheric circulation can have anomalous effects on weather regimes. Climate change ==Ozone depletion==

The chemistry of the Antarctic polar vortex has created severe ozone depletion, although the effect has been weakening since the 2000s. It is expected to return to 1980 levels in about 2075. The nitric acid in polar stratospheric clouds reacts with chlorofluorocarbons to form chlorine, which catalyzes the photochemical destruction of ozone. Chlorine concentrations build up during the polar winter, and the consequent ozone destruction is greatest when the sunlight returns in spring. These clouds can only form at temperatures below about . Since there is greater air exchange between the Arctic and the mid-latitudes, ozone depletion at the north pole is much less severe than at the south. Accordingly, the seasonal reduction of ozone levels over the Arctic is usually characterized as an "ozone dent", whereas the more severe ozone depletion over the Antarctic is considered an "ozone hole". That said, chemical ozone destruction in the 2011 Arctic polar vortex attained, for the first time, a level clearly identifiable as an Arctic "ozone hole". ==Outside Earth==

view of the colossal polar cloud on Mars Other astronomical bodies are also known to have polar vortices, including Venus (double vortex – that is, two polar vortices at a pole), Mars, Jupiter, Saturn, and Saturn's moon Titan. Saturn's south pole is the only known hot polar vortex in the Solar System. ==See also==