Polar vortex

Japanese: 極渦 - きょくうず（英語表記）polar vortex

A low pressure system that exists in the middle to upper troposphere, almost centered on the pole. It is also called the circumpolar vortex. There is high pressure near the surface of the earth at both poles, but low pressure at around 500 hPa and above. In the upper stratosphere, the polar vortex disappears in summer and becomes a polar high. This is because the atmosphere is heated by the absorption of ultraviolet rays by ozone, and the temperature is higher at low latitudes. On the other hand, in winter, there is no solar radiation above the poles, so the ozone layer cools, especially above 25 km, and the cyclonic (westerly) circulation becomes stronger at higher altitudes in the stratosphere, and wind speeds can exceed 100 m/s at high latitudes. Such strong winds are also called the polar night jet. The polar vortex in the troposphere advances and advances repeatedly, and in winter, at altitudes of around 550 hPa or higher, a noticeable low pressure trough occurs on the east coast of the Asian continent and northeastern Canada. At such times, cold air from the Arctic flows into Japan, causing abnormally low temperatures and heavy snowfall.

Source: Encyclopaedia Britannica Concise Encyclopedia About Encyclopaedia Britannica Concise Encyclopedia Information

Japanese:

対流圏中層から上層にかけてほぼ極を中心に存在する低気圧。周極渦ともいう。両極とも地表付近では高気圧があるが，500hPa付近から上層では低気圧となっている。成層圏上部では，夏季には極渦は消滅して極高気圧となる。これは，オゾンによる紫外線の吸収で大気が加熱され低緯度側より高温化するためである。一方，冬季には極の上空に日射がないために，特にオゾン層が冷却して 25km以上の成層圏の高いところほど低気圧性（西風）循環が強くなり，高緯度上空では風速 100m/s以上になることもある。このような強風を極夜ジェットともいう。対流圏での極渦は進長を繰り返し，冬季 550hPa付近あるいはそれ以上の高度ではアジア大陸東岸とカナダ北東部に顕著な気圧の谷を生じる。このようなとき，日本上空には北極の冷たい空気が流入して，異常低温や豪雪をもたらす。

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