Editing Polar vortex (section)

==Duration and strength==
[[File:Polarvortexwinter.jpg|thumb|upright=1.75|Polar vortex and weather impacts due to {{clarification needed span|text=stratospheric warming|reason=What is "stratospheric warming"?  Please provide a wikilink to appropriate content here, or a brief explanation with a reliable citation|date=February 2025}}]]

Polar vortices are weakest during summer and strongest during winter. [[Extratropical cyclone]]s that migrate into higher latitudes when the polar vortex is weak can disrupt the single vortex creating smaller vortices ([[cold-core low]]s) within the polar air mass.<ref>{{cite book|url=https://books.google.com/books?id=-tBa1DWYoDIC&pg=PA167|page=174|title=Polar lows: mesoscale weather systems in the polar regions|author=Erik A. Rasmussen and John Turner|year=2003|publisher=Cambridge University Press|isbn=978-0-521-62430-5}}</ref> Those individual vortices can persist for more than a month.<ref name="pause"/>

[[Volcano|Volcanic]] eruptions in the [[tropics]] can lead to a stronger polar vortex during winter for as long as two years afterwards.<ref>{{cite journal |bibcode=2000RvGeo..38..191R |title=Volcanic eruptions and climate |last1=Robock |first1=Alan |volume=38 |year=2000 |pages=191–219 |journal=Reviews of Geophysics |doi=10.1029/1998RG000054 |issue=2|s2cid=1299888 |url=https://pdfs.semanticscholar.org/ce8e/392a97dbd25c7f20855547ec8be444416c4e.pdf |archive-url=https://web.archive.org/web/20200219140441/https://pdfs.semanticscholar.org/ce8e/392a97dbd25c7f20855547ec8be444416c4e.pdf |url-status=dead |archive-date=2020-02-19 }}</ref> 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]].<ref>Todd Mitchell (2004). [http://jisao.washington.edu/ao/ Arctic Oscillation (AO) time series, 1899 – June 2002] {{Webarchive|url=https://web.archive.org/web/20031212174712/http://jisao.washington.edu/ao/ |date=2003-12-12 }}. [[University of Washington]]. Retrieved on 2009-03-02.</ref>

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 [[Siberian High|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]].<ref name="roanoke">Kevin Myatt (2005-01-17). [http://www.roanoke.com/weather/wb/16914 Cold enough for snow, and more's on the way] {{webarchive|url=https://archive.today/20130201105159/http://www.roanoke.com/weather/wb/16914 |date=2013-02-01 }}. ''[[Roanoke Times]]''. Retrieved on 2012-02-24.</ref> 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 wave]]s 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&nbsp;°C (54–90&nbsp;°F){{clarify|date=February 2019}} 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 [[wikt:Special:Search/circumpolar|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.<ref>{{cite journal |last1=Nash |first1=E |last2=Newman |first2=P |last3=Rosenfield |first3=J |last4=Schoeberl |first4=M |year=2012 |title=An objective determination of the polar vortex using Ertel's potential vorticity |journal=Journal of Geophysical Research |volume=101 |issue=D5 |pages=9471–9478 |doi=10.1029/96JD00066 |bibcode = 1996JGR...101.9471N |url=https://zenodo.org/record/1231378 }}</ref>

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.{{clarify|date=April 2016}} 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{{Clarify|reason = by what?|date=November 2018}} 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.<ref>{{cite journal |last1=Li |first1=L |last2=Li |first2=C |last3=Pan |first3=Y |date=2012 |title=On the differences and climate impacts of early and late stratospheric polar vortex breakup |journal=Advances in Atmospheric Sciences |volume=29 |issue=5 |pages=1119–1128 |doi=10.1007/s00376-012-1012-4|bibcode = 2012AdAtS..29.1119L |s2cid=123846176 }}</ref><ref>{{cite journal |last1=Wei |first1=K |last2=Chen |first2=W |last3=Huang |first3=R |date=2007 |title=Dynamical diagnosis of the breakup of the stratospheric polar vortex in the northern hemisphere| journal=Science in China Series D: Earth Sciences |volume=50 |issue=9 |pages=1369–1379 |doi=10.1007/s11430-007-0100-2 |bibcode=2007ScChD..50.1369W |s2cid=195309667 }}</ref>{{clarify|date=April 2016}}
[[File:Polarvortexjan211985.jpg|thumb|250px|Low pressure area over [[Quebec]], [[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.<ref>{{cite journal |last1=Reichler| first=Tom| last2=Kim| first2=J |last3=Manzini |first3=E |last4=Kroger |first4=J|year=2012 |title=A stratospheric connection to Atlantic climate variability |journal=Nature Geoscience |volume=5 | issue=11|pages=783–787 |doi= 10.1038/ngeo1586|bibcode = 2012NatGe...5..783R }}</ref> These changes aloft force changes in the troposphere below.<ref>{{cite journal |last=Ripesi |first=Patrizio |year=2012 |title=The February 2010 Artcic Oscillation Index and its stratospheric connection|journal=Quarterly Journal of the Royal Meteorological Society|volume=138|issue=669 |pages=1961–1969|url=http://clima.meteoam.it/Articoli/Art14.pdf |doi=10.1002/qj.1935|bibcode = 2012QJRMS.138.1961R |s2cid=122729063 |display-authors=etal}}</ref> 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 [[North Atlantic Current|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.<ref>{{cite journal |last1=Reichler |first1=Tom |last2=Kim |first2=J |last3=Manzini |first3=E |last4=Kroger |first4=J |year=2012 |title=A stratospheric connection to Atlantic climate variability |journal=Nature Geoscience |volume=5 |issue=11 |pages=783–787 |doi=10.1038/ngeo1586 |bibcode = 2012NatGe...5..783R }}</ref> Strengthening storm systems within the troposphere that cool the poles, intensify the polar vortex. [[La Niña]]–related climate anomalies significantly strengthen the polar vortex.<ref>{{cite journal |last1=Limpasuvan |first1=Varavut |last2=Hartmann |first2=Dennis L. |last3=Thompson |first3=David W.J. |last4=Jeev |first4=Kumar |last5=Yung |first5=Yuk L. |year=2005 |title=Stratosphere-troposphere evolution during polar vortex intensification |journal=Journal of Geophysical Research |volume=110 |issue=D24 |page=27 |doi=10.1029/2005JD006302 |url=http://yly-mac.gps.caltech.edu/ReprintsYLY/n173limpasuvan_2005.pdf |bibcode=2005JGRD..11024101L |citeseerx=10.1.1.526.9159 |access-date=2014-01-06 |archive-date=2017-08-12 |archive-url=https://web.archive.org/web/20170812193924/http://yly-mac.gps.caltech.edu/ReprintsYLY/n173limpasuvan_2005.pdf |url-status=dead }}</ref>  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.<ref>{{cite journal |last1=Cavallo |first1=S |last2=Hakim |first2=G.J. |year=2013 |title=Physical mechanisms of tropopause polar vortex intensity change |journal=Journal of the Atmospheric Sciences |volume=70 |issue=11 |pages=3359–3373 |doi=10.1175/JAS-D-13-088.1 |bibcode=2013JAtS...70.3359C |doi-access=free }}</ref> 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.<ref>{{cite journal |last1=Hartmann |first1=D |last2=Schoeberl |first2=M |year=1991 |title=The dynamics of the stratospheric polar vortex and its relation to springtime ozone depletions |journal=Science |volume=251 |issue=4989 |pages=46–52 | doi = 10.1126/science.251.4989.46 |pmid=17778602|bibcode = 1991Sci...251...46S |s2cid=24664477 |url=http://pdfs.semanticscholar.org/b06e/6b492b632e72ea08f30a681500e8859bb4c7.pdf |archive-url=https://web.archive.org/web/20190302053349/http://pdfs.semanticscholar.org/b06e/6b492b632e72ea08f30a681500e8859bb4c7.pdf |url-status=dead |archive-date=2019-03-02 }}</ref> 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.<ref>{{cite journal |last1=Widnall |first1=S |last2=Sullivan |first2=J |year=1973 |title=On the stability of vortex rings |journal=Proceedings of the Royal Society of London. Series A, Mathematical and Physical Sciences |volume=332 |issue=1590 |pages=335–353 |doi=10.1098/rspa.1973.0029 |bibcode = 1973RSPSA.332..335W |s2cid=119959924 }}</ref>

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&nbsp;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.<ref>{{cite journal |last1=Manney |first1=G |last2=Zurek |first2=R |last3=O'Neill |first3=A |last4=Swinbank |first4=R |year=1994 |title=On the motion of air through the stratospheric polar vortex |journal=Journal of the Atmospheric Sciences |doi=10.1175/1520-0469(1994)051<2973:otmoat>2.0.co;2 |volume =  51 |issue =20 | pages= 2973–2994 |bibcode = 1994JAtS...51.2973M |doi-access=free }}</ref> After the vortex is broken up, the ex-vortex air is dispersed into the middle latitudes within a month.<ref name=JGR2012/>

Sometimes, a mass of the polar vortex breaks off before the end of the final warming period. If large enough, the piece can move into Canada and the Midwestern, Central, Southern, and Northeastern United States. This diversion of the polar vortex can occur due to the displacement of the polar jet stream; for example, the significant northwestward direction of the polar jet stream in the western part of the United States during the winters of 2013–2014, and 2014–2015. This caused warm, dry conditions in the west, and cold, snowy conditions in the north-central and northeast.<ref>{{cite web|url=http://climatenexus.org/learn/planetary-systems/warm-west-cool-east-us-temperature-divide |title=The Warm West, Cool East U.S. Temperature Divide &#124; Climate Nexus |access-date=2015-11-26 |url-status=dead |archive-url=https://web.archive.org/web/20151207233627/http://climatenexus.org/learn/planetary-systems/warm-west-cool-east-us-temperature-divide |archive-date=2015-12-07 }}</ref> 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.<ref>{{cite web| last=Erdman |first=Jon |date=2014 |title=What's a Polar Vortex?: The Science Behind Arctic Outbreaks |website=wunderground |url=http://www.wunderground.com/news/polar-vortex-plunge-science-behind-arctic-cold-outbreaks-20140106 |
access-date= 25 February 2014}}</ref>