A polar vortex is an upper level low-pressure area lying near the Earth’s pole. There are two polar vortices in the Earth’s atmosphere, which overlie the North, and South Poles. Each polar vortex is a persistent, large-scale, low- pressure zone that rotates counter-clockwise at the North Pole (called a cyclone), and clockwise at the South Pole.
A polar vortex strengthens in the winter and weakens in the summer due to its dependence on the temperature difference between the equator and the poles.
The vortices span less than 1,000 km in diameter within which they rotate counter-clockwise in the Northern Hemisphere, and in a clockwise fashion in the Southern Hemisphere. As with other cyclones, their rotation is driven by the Coriolis Effect.
Normally, when the vortex is strong and healthy, it helps keep a current of air known as the jet stream traveling around the globe in a pretty circular path. This current keeps the cold air up north and the warm air down south. But without that strong low-pressure system, the jet stream doesn’t have much to keep it in line. It becomes wavy and rambling. Put a couple of areas of high-pressure systems in its way, and all of a sudden you have a river of cold air being pushed down south along with the rest of the polar vortex system.
Fronogenesis of cyclone
Polar vortex (circumpolar vortex) is a polar cyclone. It is a large pocket of very cold air in the Northern Hemisphere, which sits over the polar region during the winter season.
Polar vortex is closely associated with jet streams. It surrounds polar highs and lie within the polar front (boundary separating the temperate and polar air masses).
Polar front is the boundary between the polar cell and the Ferrel cell around the 60° latitude in each hemisphere. At this boundary a sharp gradient in temperature occurs between these two air masses, each at very different temperatures.
The polar front arises as a result of cold polar air meeting warm tropical air. It is a stationary front as the air masses are not moving against each other. Off the coast of eastern North America, especially in winter, there is a sharp temperature gradient between the snow-covered land and the warm offshore currents.
The polar front theory says that mid-latitude cyclones form on boundaries between warm and cold air. In winter, 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 equator ward.
Movement of polar front further south leads to formation of Polar vortex and bomb cyclone since it brings into picture such low temperature in temperate region required for formation of bomb cyclone and vortex in these region.
Impact of polar vortex on global climate
- In early January, the polar vortex weakened and broke down, allowing fragments of cold air to slosh out of the bowl into mid-latitudes.
- The high pressure building up in the Arctic slowed down the jet stream, which caused it to buckle into deep folds and flow farther south than usual, introducing cold Arctic air into the central and eastern U.S.
- In recent years, climate scientists have noticed that the jet stream has taken on a more wavy shape instead of the more typical oval around the North Pole, leading to outbreaks of colder weather down in the mid-latitudes and milder temperatures in the Arctic, a so-called “warm Arctic-cold continents” pattern.
- But less sea ice and snow cover in the Arctic and relatively warmer Arctic air temperatures at the end of autumn suggests more wavy jet stream pattern and more variability between the straight and wavy pattern.