There are two polar vortices in the atmosphere of planet Earth, one around
the North Pole, and one around the South Pole. Each polar vortex is a persistent,
large-scale cyclone, circling the planet's geographical poles. The bases
of the two polar vortices are located in the middle and upper troposphere and
extend into the stratosphere. They surround the polar highs and lie in the
wake of the polar front. These cold-core low-pressure areas strengthen in the
winter and weaken in the summer due to their dependence upon the temperature
differential between the equator and the poles. They usually span less than 1,000
kilometers in diameter within which the air circulates in a counter-clockwise
fashion in the Northern Hemisphere, and in a clockwise fashion in the Southern
Hemisphere. As with other cyclones, their rotation is caused by the Coriolis
effect. The Northern Hemisphere vortex often
contains two low pressure centers, one near Baffin Island, Canada and the other
over northeast Siberia. Within the Antarctic vortex in the Southern
Hemisphere a single low pressure zone tends to be located near the edge of the
Ross ice shelf near 160 west longitude. When the polar vortex is strong, the
Westerlies increase in strength. When the polar cyclone is weak, the general
flow pattern across mid-latitudes buckles and significant cold outbreaks
occur. Ozone depletion occurs within the polar vortices – particularly over the
Southern Hemisphere – reaching a maximum depletion in the spring.
History The polar vortex was first described as
early as 1853. The phenomenon's sudden stratospheric warming appears during the
winter in the Northern Hemisphere and was discovered in 1952 with radiosonde
observations at altitudes higher than 20 km.
Identification Polar cyclones are climatological
features that hover near the poles year-round. The stratospheric polar
vortex develops pole-ward and above the subtropical jet stream. Since polar
vortices exist from the stratosphere downward into the mid-troposphere, a
variety of heights/pressure levels within the atmosphere can be checked for
its existence. Within the stratosphere, strategies such as the use of the 4 mb
pressure surface, which correlates to the 1200K isentropic surface, located
midway up the stratosphere, is used to create climatologies of the feature. Due
to model data unreliability, other techniques use the 50 mb pressure
surface to identify its stratospheric location. At the level of the
tropopause, the extent of closed contours of potential temperature can be
used to determine its strength. Horizontally, most polar vortices have a
radius of less than 1,000 kilometres. Others have used levels down to the 500
hPa pressure level above sea level during the winter) to identify the polar
vortex. Duration and power
Polar vortices are weaker during summer and strongest during winter. Individual
vortices can persist for more than a month. Extratropical cyclones that
occlude and migrate into higher latitudes create cold-core lows within
the polar vortex. Volcanic eruptions in the tropics lead to a stronger polar
vortex during the winter for as long as two years afterwards. The strength and
position of the cyclone shapes the flow pattern across the hemisphere of its
influence. An index which is used in the northern hemisphere to gauge its
magnitude is the Arctic oscillation. The Arctic vortex is elongated in shape,
with two centers, one normally located over Baffin Island in Canada and the
other over northeast Siberia. Around the North Pole, the Arctic vortex spins
counterclockwise with wind speeds of 80 mph, stronger than the jet stream's
normal 70 mph winds. In rare events, when the general flow pattern is
amplified, the vortex can push farther south as a result of axis interruption,
such as during the Winter 1985 Arctic outbreak. The Antarctic polar vortex is
more pronounced and persistent than the Arctic one; this is because the
distribution of land masses at high latitudes in the Northern Hemisphere
gives rise to Rossby waves which contribute to the breakdown of the
vortex, whereas in the Southern Hemisphere the vortex remains 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 °C over a few days can occur.
The formation of the polar vortex is primarily influenced by the movement of
wind and transfer of heat in the polar region. In the autumn, the circumpolar
winds increase in speed, causing the polar vortex to spin up further into the
stratosphere and the values of potential vorticity to heighten, forming a
coherent air mass: the polar vortex. As the winter comes, the winds around the
poles decrease, and the air in the vortex core cools. The movement of the
air becomes slow, and the vortex stops growing. Once late winter and early
spring approach, heat and wind circulation return, causing the vortex
to shrink. During the final warming, or the late winter, large fragments of the
vortex air are drawn out into narrow pieces into lower latitudes. In the
bottom level of the stratosphere, strong potential vorticity gradients remain,
and the majority of air molecules remain confined into December in the Southern
Hemisphere and April in the Northern Hemisphere, well after the breakup of
the vortex in the mid-stratosphere. The breakup of the polar vortex occurs
between middle March to middle May, the average date being April 10. 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 differences
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. As
a result of increased waves into the vortex, the vortex experiences higher
amounts of heat sooner than the normal warming period, resulting in a faster
season transition from winter to summer. As for late breakups, the waves
dismantle the vortex later than normal, causing a delay in the season
transition. The early breakup years are also characterized with persistence of
remnants of the vortex, while the late breaking years have a quick
disappearance of these remnants. In the early breakup phases, only one warming
period occurs from late February to middle March, contrasting to the two
warming periods that the late breakup phases have in January and 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.
Sudden stratospheric warming events, when temperatures within the
stratosphere warm dramatically over a short time, are associated with weaker
polar vortices. This warming of stratospheric air can cause the
direction of circulation in the Arctic Polar Vortex to go from
counter-clockwise to clockwise. These changes aloft force changes below in the
troposphere. 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, causing
circulation of heat through the Atlantic ocean currents to be stopped or sped up.
Since all other oceans depend on the Atlantic ocean for the transmission of
heat and energy, climates across the planet can change dramatically. The
weakening or strengthening of the polar vortex can alter the sea circulation
more than one mile below the waves. Strengthening storm systems within the
troposphere can act to intensify the polar vortex by significantly cooling
the poles. La Niña–related climate anomalies tend to favor significant
strengthening of the polar vortex. Intensification of the polar vortex is
also associated with changes in relative humidity as downward intrusions of dry,
stratospheric air enter into 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 inside of the vortex, which places dry stratospheric
air above moist tropospheric air. 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. 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. 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 combination of these two remains small inside the
vortex compared to the 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 exist above 420 km. Below this barrier exists
the polar night jet, which is weak in the early winter, so any descending
polar air mixes with the mid-latitudes. In the late winter, air parcels do not
descend as much, causing mixing to be less frequent. After the vortex is
broken up, the ex-vortex air is dispersed into the middle latitudes
within a month. Sometimes, a piece of the polar vortex
can be broken off before the end of the final warming period. If large enough,
the piece can plunge over 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, such as the significant
northwestern push of the polar jet stream over the western part of the
United States in the winter of 2013–2014. Occasionally, the
high-pressure Greenland Block can cause the low pressure polar vortex to divert
to the south instead of sweeping across the North Atlantic.
Climate change A study in 2001 found that stratospheric
circulation can have anomalous effects on weather regimes. In the same year
researchers found a statistical correlation between weak polar vortex
and outbreaks of severe cold in the Northern Hemisphere. In more recent
years scientists identified interactions with Arctic sea ice decline, reduced
snow cover, evapotranspiration patterns, NAO anomalies or weather anomalies which
are linked to the polar vortex and jet stream configuration. However, because
the specific observations are considered short-term observations there is
considerable uncertainty in the conclusions. Climatology observations
require several decades to definitively distinguish natural variability from
climate trends. The general assumption is that reduced
snow cover and sea ice reflect less sunlight and therefore evaporation and
transpiration increases, which in turn alters the pressure and temperature
gradient of the polar vortex, causing it to weaken or collapse. This becomes
apparent when the jet stream amplitude increases over the northern hemisphere,
causing Rossby waves to propagate farther to the south or north, which in
turn transports warmer air to the north pole and polar air into lower latitudes.
The jet stream amplitude increases with a weaker polar vortex, hence increases
the chance for weather systems to become blocked. A recent blocking event emerged
when a high-pressure over Greenland steered Hurricane Sandy into the
northern Mid-Atlantic states. Ozone depletion
The chemistry of the Antarctic polar vortex has created severe ozone
depletion. The nitric acid in polar stratospheric clouds reacts with
chlorofluorocarbons to form chlorine, which catalyzes the photochemical
destruction of ozone. Chlorine concentrations build up during the polar
winter, and the consequent ozone destruction is greatest when the
sunlight returns in spring. These clouds can only form at temperatures below
about −80 °C. Since there is greater air exchange between the Arctic and the
mid-latitudes, ozone depletion at the north pole is much less severe than at
the south. Accordingly, the seasonal reduction of ozone levels over the
Arctic is usually characterized as an "ozone dent", whereas the more severe
ozone depletion over the Antarctic is considered an "ozone hole". This said,
chemical ozone destruction in the 2011 Arctic polar vortex attained, for the
first time, a level clearly identifiable as an Arctic "ozone hole".
Outside Earth Other astronomical bodies are also known
to have polar vortices, including Venus, Mars, Jupiter, Saturn, and Saturn's moon
Titan. = Hot polar vortex=
Saturn's south pole is the only known hot polar vortex in the solar system.
See also Polar amplification
Weather portal References
Further reading Nash, Eric R.; Newman, Paul A.;
Rosenfield, Joan E.; Schoeberl, Mark R.. "An objective determination of the polar
vortex using Ertel's potential vorticity". Journal of Geophysical
Research 101: 9471–8. Bibcode:1996JGR...101.9471N.
doi:10.1029/96JD00066. Butchart, Neal; Remsberg, Ellis E.. "The
Area of the Stratospheric Polar Vortex as a Diagnostic for Tracer Transport on
an Isentropic Surface". Journal of the Atmospheric Sciences 43: 1319–39.
Bibcode:1986JAtS...43.1319B. doi:10.1175/1520-0469(1986)0432.0.CO;2.
Schoeberl, Mark R.; Lait, Leslie R.; Newman, Paul A.; Rosenfield, Joan E..
"The structure of the polar vortex". Journal of Geophysical Research 97:
7859–82. Bibcode:1992JGR....97.7859S. doi:10.1029/91JD02168.
Coy, Lawrence; Nash, Eric R.; Newman, Paul A.. "Meteorology of the polar
vortex: Spring 1997". Geophysical Research Letters 24: 2693–6.
Bibcode:1997GeoRL..24.2693C. doi:10.1029/97GL52832.
Schoeberl, M. R.; Hartmann, D. L.. "The Dynamics of the Stratospheric Polar
Vortex and Its Relation to Springtime Ozone Depletions". Science 251: 46–52.
Bibcode:1991Sci...251...46S. doi:10.1126/science.251.4989.46. PMID
17778602. External links
"Archive of Northern and Southern Hemisphere Blocking Events". Global
Climate Change Group. Retrieved January 7, 2014.
"Polar vortex". Environmental Terminology and Discovery Service.
Retrieved January 6, 2014. "World temperature gradient". Integrated
Publishing: Aerographer/Meteorology. Retrieved January 29, 2005.
"The Antarctic Polar Vortex". Dynamics in the Ocean and Atmosphere. Retrieved
January 29, 2005. "The Polar Vortex and Arctic Weather
Patterns". Arctic Climatology and Meteorology. National Snow and Ice Data
Center. Retrieved January 29, 2005. "NASA Sees into the Eye of a Monster
Storm on Saturn". Cassini-Huygens: News-Press Releases-2006. Retrieved
November 12, 2006.