Midwest in January Cold temperatures due to the polar vortex are not rare in the United States. But, it is important to remember that not all cold weather comes from the polar vortex. Although cold air from the polar vortex can be pushed south, it typically remains parked in polar regions. It takes pretty unusual conditions for the vortex to weaken or migrate far south. Other weather conditions can cause cold arctic weather to travel south, too. Cold air from a polar vortex caused this fountain in Greenville, South Carolina, to freeze in What we see in the record is this very interesting period in the s, when there were no sudden stratospheric warming events observed in the Arctic.
In other words, the vortex was strong and stable. But then they started back up again in the late s, and over the next decade there was one almost every year. So there was a window of time in the early s where it seemed like there might be a trend toward weaker, more disrupted or shifted states of the Arctic polar vortex. The disruption of the Arctic polar vortex in January Globes show show millibar geopotential heights—an indicator of air pressure—for left December 26, as the polar vortex began to weaken, middle on January 5, when the stratospheric winds reversed sign, and right on January 15, during the sudden stratospheric warming event.
One researcher did a historical reconstruction by correlating the overlapping portions of the North Atlantic Oscillation index—which goes back much farther—and the polar vortex record, and then extrapolating the polar vortex record farther back in time using the NAO index.
It showed no long-term trend, and no big differences in recent decades compared to previous decades. Some climate model experiments do predict that continued warming will lead to a weakening of the polar vortex. And some studies combining models and observations have shown a connection between low sea ice extent in the Barents and Kara Seas of the eastern Arctic, sudden stratospheric warming events, and cold winters in North America.
At the same time, other model simulations predict that warming and sea ice loss will lead to a stronger polar vortex. The sensitivity to the timing and location of sea ice loss is only part of the complexity, however.
There also appears to be a tug-of-war between climate change processes that could strengthen the Arctic polar vortex and processes that could weaken it. Touching on this topic in a recent post for Climate. For example, the tropical upper troposphere is predicted to become warmer, which will likely enhance the equator-to-pole temperature gradient across the tropopause the atmospheric layer that separates the troposphere from the stratosphere , which would speed up the polar vortex in both hemispheres.
However, enhanced warming of the Arctic surface relative to the middle latitudes reduces the surface temperature gradient and may act on the Northern Hemisphere polar vortex in the opposite direction.
Other climate factors being equal, a weaker vortex, with more frequent disruptions, could slow the rate of winter warming in the mid-latitudes while accelerating it in the Arctic. A stronger polar vortex, with few disruptions might be expected to slow Arctic warming at the expense of more rapid winter warming in the mid-latitudes. To Arctic climate expert Jim Overland, those two points are familiar arguments against the broader hypothesis that rapid Arctic warming could be affecting mid-latitude winter weather in various ways.
As an example, he points to a study that described how an atmospheric river event combined with historically low November sea ice extent in the Bering and Chukchi Seas in to intensify a ridge of high pressure over the Pacific Arctic.
Both up and downstream of the amplified ridge, the polar jet stream developed deep troughs, which allowed cold polar air to spill south into both East Asia and North America in early winter. The winds that blow through our continental Antarctic stations are so strong and consistent in their direction, that they affect station design.
Station buildings are lined up parallel to the axis of the wind. The Antarctic continent slopes upward from the coast, where our stations are built. The shape of this icy terrain guides the cold air rolling down off the continent towards the coast. The air close to the icy surface cools as it radiates away its heat. This makes it heavier and forces it to flow downhill. The accumulation of this flow down to the coast makes the katabatic.
Southern hemisphere total ozone, potential vorticity on the K potential temperature surface, and temperature on the 50 hPa pressure surface for 22 August We illustrate the southern polar vortex with three images for 22 August similar to the northern polar vortex above.
The southern vortex is much stronger and larger than the northern vortex middle image. The streamlines show wind speeds near 60 m s -1 , much greater than would be observed at the comparable time in the northern vortex. Temperatures are also much colder right image , with values below K over a wide region.
In contrast, northern temperatures only sporadically fall below K over small regions. Ozone levels left image are also much different, with low values during the mid-winter period. The difference between the southern and northern polar vortices is caused by the planetary wave effects discussed above. Large-amplitude planetary-scale wave events that cause warmings are quite frequent in the northern hemisphere.
In contrast to the northern hemisphere, the southern hemisphere does not have major mountain ranges the Andes are tall, but very narrow in longitude and is mainly a water covered hemisphere. Hence, southern hemisphere forcing of planetary-scale wave events in the troposphere is very weak, and there is an absence of wave events to warm the polar region and erode the polar vortex. Southern hemisphere potential vorticity on the K potential temperature surface for 24 and 31 August and 7 September The PV images above illustrate the lack of large-scale waves in the southern hemisphere during the mid-winter period.
Some undulations of PV can be seen along the rim of the vortex. However, the vortex is nearly always centered upon the South Pole.
This is in remarkable contrast to the northern hemisphere see images of NH vortex in the section above during January In addition, note that the southern hemisphere vortex images have much higher PV values deeper red color and are larger than their northern counterparts.
This is because southern wave events are not present to degrade the strength and area coverage of the southern polar vortex during the mid-winter period. The lack of planetary-scale wave events in the southern hemisphere leads to a very symmetric polar vortex that is quite strong. Further, because of the lack of erosion of the vortex, it is very persistent.
Towards the end of the southern winter season, planetary-scale wave events do begin to form and propagate upward into the stratosphere. These waves erode the vortex, decelerate the jet stream, warm the polar region, and increase ozone levels. While the northern polar vortex usually persists to March or April, the southern vortex persists an additional 1—2 months November or December. In addition, temperatures remain quite cold below K in the southern vortex to early October.
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