Warm Arctic - Cold Continents
The Polar Vortex
Cold air is normally trapped in the Arctic in winter by strong Polar Vortex winds, which circle the North Pole from west to east and the strong pressure field that is shown in purple/blue colors in Figure 1a, below left. This pattern broke down in December 2009, and in February 2010, (Figures 1b and 1c, below middle and right). North-south winds increased, allowing cold Arctic air to spill southwards.
Warm Arctic - Cold Continents
This creates the Warm Arctic-Cold Continent Pattern, shown in Figure 2 below for December 2009 and 2010. Red colors indicate areas where the Arctic was 9°F or 5°C warmer than normal. Purple colors indicate areas where the continents were 9°F or 5°C cooler than normal. Warmer than normal Arctic temperatures were seen especially in regions that were sea-ice-free in summer: north of Alaska and in the Barents Sea. The cold continents are seen where Arctic air penetrated southward. Some warm air penetrates northward near Bering Strait and east Greenland.
The Polar Vortex was weak in other years as well. In late autumn and early winter 2005, 2008, 2010, but especially 2009, a weak Polar Vortex and associated increase in southward flowing winds coming out of the Arctic brought record cold and snow conditions to northern Europe, eastern Asia and eastern North America. In autumn 2009, Northern Eurasia (north of 50° latitude to the Arctic coast) and North America (south of 55° latitude) were particularly cold, 3 -18° F cooler than the normal monthly average, and Arctic regions were more than 7°F warmer than average.
The North Atlantic Oscillation (NAO) climate index had it's lowest value in 145 years for Winter 2009/2010
One indicator of a weak Polar Vortex is the North Atlantic Oscillation (NAO) index. Winter 2009/2010, which saw two major winter cold continent events, had the lowest NAO value in 145 years of historical record. In other years, Winter 2005-2006 had a primary influence in Eurasia and December 2008 had a more local influence in northeastern Canada.
Influences on sub-Arctic weather
Meteorological attribution to these sub-Arctic events is difficult. The last five years have been the warmest recorded period in the Arctic and climate conditions over the Arctic cannot be ruled out as influencing weather in some sub-Arctic regions, making it relative colder for part of the winter.
Certainly many factors, including random chaos in the development of weather patterns, can produce such extreme winter events. For example, the US experienced severe winter weather in early February 2011, but the NAO and Arctic pressure fields were not necessarily strong. This serves to show that not all severe weather events can be directly attributed to changes in the Arctic.
Sub-Arctic weather can also be influenced by changes in the Arctic stratosphere, snow cover, and other climate forcing such as El Nino. More combined observational and modeling studies to understand causes and latitudinal extent of this recent Warm Arctic – Cold Continent pattern are a high priority in Arctic research. In summary, the most we can now say is that loss of sea ice pushes in the right direction to weaken the Polar Vortex and increase the chance for sub-Arctic impacts.
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