Polar vortex is a circular wind pattern that centers on the North and South poles . The term is generally used to describe either of two separate, large-scale wind patterns. The tropospheric polar vortex is a major flow of air in the troposphere , the layer of the atmosphere closest to Earth’s surface. Thestratospheric polar vortex is the primary flow of air in the stratosphere , the layer of the atmosphere that lies above the troposphere. Both are large vortices (plural ofvortex) that can contribute to strong changes in surface weather . In the Northern and Southern hemispheres, these changes include extreme cold air outbreaks, winter storms, severe weather, dry spells, and flooding.
The tropospheric polar vortex
is the vortex that most influences weather. It is bounded by the polar jet stream, a narrow band of strong high-altitude winds. The polar jet stream is strongest at the tropopause, the uppermost limit of the troposphere, about 6 miles (10 kilometers) above the ground. The winds here can exceed 200 miles per hour (320 kilometers per hour). The polar jet streams flow in a wavelike pattern around the Northern and Southern hemispheres. They change position constantly and move or split vertically and horizontally.
The polar jet stream sits atop a vertical boundary called the polar front. The polar front is the leading edge of an advancing cold air mass. It separates the cold polar air mass from a warmer, subtropical air mass. As shifts in the polar jet stream cause dips and bulges, the cold air of the tropospheric polar vortex may extend along the polar front to around 45° to 65° latitude north or south. On average, such outbreaks of the tropospheric polar vortex reach the lowest latitudes over continents during the winter, particularly in North America and Siberia. The size and shape of the polar vortex can vary from day to day, or they can persist for a long time. Frequent and extreme changes in latitude and shape are not unusual. At times, this polar vortex affects weather over the entire continental United States or most of Europe, often with extreme cold and winter storms.
The strength and size of the polar vortex is strongest during the winter, when the temperature difference between the equator and pole is the greatest. Although the polar vortex is smaller during the summer, similar changes in latitude and shape remain common. With nearly 24 hours of heating from the sun and less sea ice during the summer, the air inside the polar vortex warms, and outbreaks are much weaker. But such outbreaks can influence the development of thunderstorms, dry spells, flooding, and hurricanes.
Many factors can affect the shape and location of the tropospheric polar vortex, including interactions with other tropospheric jet streams, mountains, hurricanes , tropopause polar vortices, and variations in sea surface temperature and sea ice. Interactions with hurricanes are most frequent in the autumn when the polar vortex expands in size, coming closer to the latitudes where hurricanes form and move.
The stratospheric polar vortex
extends upward in the stratosphere from a height of around 6 miles (10 kilometers) and from the pole to around 40° to 60° latitude on average. The stratospheric polar vortex exists because of strong warming from large amounts of ozone in a layer of the atmosphere 12 to 19 miles (20 to 30 kilometers) above Earth’s surface. Because warming from ozone can only occur when there is sunlight, the temperature difference between the equator and the pole in the stratosphere reverses between the summer and winter. This temperature change is accompanied by a reversal in wind direction from west in the winter to east in the summer.
Occasionally, rapid reversals in wind direction from west to east calledsudden stratospheric warmings occur in the winter vortex. These can affect tropospheric jet streams. For example, a polar jet stream over the Atlantic Ocean could weaken and wiggle such that winds blow from the north or south instead of from west to east. These bulges in the jet stream prevent air from flowing across the ocean, and instead allow cold air to move toward the equator over North America or Europe.
Interactions.
Although the stratospheric and tropospheric polar vortices are unique features, they may occasionally influence each other. Their interactions lead to temporary but major fluctuations in surface weather patterns. These are mainly noticed as changes in the locations of storms and cold air.
