What is the polar vortex?

Photograph of Chicago during the 2014 polar vortex This view of Chicago was taken from Diversey Harbor on January 7, 2014, when a trough of the Northern Hemisphere polar vortex looped south across the United States, putting much of the country in a deep freeze. Credit: Edward Stojakovic (Creative Commons, http://creativecommons.org/licenses/by/2.0/)

Guest post by Mark Serreze, NSIDC Director and Professor, Department of Geography, University of Colorado Boulder Lately there has been much talk about extreme cold weather in the United States and its connection to the polar vortex. Just what is the polar vortex, and how does it affect the lower latitudes? We asked Mark Serreze, NSIDC Director and a specialist in Arctic climatology, to provide an explanation. Here is his response: A vortex is a region within a fluid where the flow is mostly a rotational motion around a given axis. The Earth’s atmosphere, while a gas, nevertheless behaves broadly as a fluid. The polar vortex is the region of the atmosphere that contains the hemisphere’s cold air, rotating from west to east. In the Northern Hemisphere, the axis of the rotation is generally located in the Arctic. There is also a polar vortex in the Southern Hemisphere, in which the axis of rotation is around the Antarctic continent. This post discusses the Northern Hemisphere polar vortex, recognizing that the same basic processes work in the Southern Hemisphere. The air within the Northern Hemisphere polar vortex is referred to as polar air or Arctic air, depending on temperature, with the Arctic air being the coldest. South of the polar vortex, the atmosphere is much warmer, representing sub-tropical air, and, further south, tropical air. The boundary between the cold air within the vortex and the warmer air to the south is a region of sharp horizontal gradients in temperature, with temperatures decreasing to the north. These sharp temperature gradients give rise to the polar front jet stream, a fairly narrow ribbon of especially fast-moving air, flowing broadly from west to east. Hence, the southern boundary of the polar vortex essentially corresponds to the location of the polar front jet stream.
Data images showing average polar vortexes compared to the 2014 polar vortex This pair of images compares the boundaries of the polar vortex. The left image plots the height of the 500 millibar surface, about halfway up in the atmosphere, averaged for all Januarys over the period 1981-2010. The right image maps the 500 millibar height surface averaged for 1-5 January 2014. The shape of the polar vortex is quite different. The polar vortex is defined by the blue and purple colors poleward of the yellow colors, separating the polar and Arctic air and from warmer air to the south. The boundary of the polar vortex is where the yellow transitions to the green, and the tightly packed lines indicate a strong horizontal temperature gradient and define the jet stream. The air within in the jet stream, and within the vortex as a whole, rotates from west to east like a big whirlpool. Note the meanders in the vortex—these are the longwave ridges and troughs. This period saw a very strong Arctic outbreak over the central United States linked to a strong longwave trough moving into the area. Credit: M. Serreze, NSIDC

However, the polar vortex is typically irregularly shaped with a number of large meanders in the flow known as longwaves. Regions where the cold air dips southward are known as longwave troughs. Regions where the warm air extends northward are known as longwave ridges. The location and strength of the longwave ridges and troughs vary from week to week and the polar vortex as a whole can also extend toward the equator or toward the pole. These variations in the polar vortex are expressed as changes in the weather, which can be quite pronounced. For example, if one is in a location where the polar vortex is dipping strongly to the south, meaning a longwave trough is moving in, there is a sharp drop in temperature. This drop is known as a polar, or Arctic, outbreak depending on the temperature of the invading air. Storms tend to form along the jet stream that defines the boundary of the polar vortex. Hence, polar or Arctic outbreaks are often accompanied by stormy weather. These storms tend to be associated with shortwaves, which are smaller meanders embedded with the longwaves. The polar vortex and its associated longwave ridges, troughs, embedded shortwaves and polar jet stream tend to be more pronounced in winter than during summer. The character of the polar vortex is linked to the phase of the Arctic Oscillation, which you can read more about on the NSIDC All About Arctic Climatology and Meteorology site. When the Arctic Oscillation switches to a positive phase, the polar vortex tends to extend toward the pole. When the Arctic Oscillation switches to a negative phase, it tends to expand further to the south and become more wavy, meaning the longwave ridges and troughs are more pronounced. The polar outbreak over the central United States during January 1-5, 2014, may be related to a shift towards the negative phase of the Arctic Oscillation. Some scientists  link changes in the vortex to strong warming in the Arctic linked to the ongoing loss of its sea ice cover. This connection is controversial and the subject of considerable scientific research. References Francis, J.A., S. J. and Vavrus. 2012. Evidence linking Arctic amplification to extreme weather in mid-latitudes. Geophyical Research Letters 39, L06801, doi:10.1029/2012GL051000. Overland, J.E., and M. Wang. 2010. Large-scale atmospheric circulation changes are associated with the recent loss of Arctic sea ice. Tellus 62(1): 1-9, doi: 10.1111/j.1600-0870.2009.00421.x. Related Icelights posts The Arctic Oscillation, winter storms, and sea ice Arctic sea ice and U.S. weather Related information All About Arctic Climatology and Meteorology