Higher air temperatures in the lower Arctic atmosphere in fall [are] contributing to changes in the atmospheric circulation in both the Arctic and northern mid-latitudes. Winter 2009-2010 showed a new connectivity between mid-latitude extreme cold and snowy weather events and changes in the wind patterns of the Arctic; the so-called Warm Arctic-Cold Continents pattern.
Winter 2009-2010 showed a major new connectivity between Arctic climate and mid-latitude severe weather, compared to the past. Figure A7a shows normal early winter atmospheric conditions with low geopotential heights of constant pressure surfaces over the Arctic (purples). These fields indicate the tendency of wind patterns: winds tend to blow counter clockwise around the centers of lower heights, parallel to the height contours. In Figure A7a for example, winds tend to blow from west to east, thus separating cold arctic air masses from the regions further south.
In December 2009 (Fig. A7b) and February 2010 (Fig. A7c) we actually had a reversal of this climate pattern, with higher heights and pressures over the Arctic that eliminated the normal west-to-east jet stream winds. This allowed cold air from the Arctic to penetrate all the way into Europe, eastern China, and Washington DC. As a result, December 2009 and February 2010 exhibited extremes in both warm and cold temperatures with record-setting snow across lower latitudes. Northern Eurasia (north of 50° latitude to the Arctic coast) and North America (south of 55° latitude) were particularly cold (monthly anomalies of -2°C to -10°C). Arctic regions, on the other hand, had anomalies of +4°C to +12°C. This change in wind directions is called the Warm Arctic-Cold Continents climate pattern and has happened previously only three times before in the last 160 years.
While individual weather extreme events cannot be directly linked to larger scale climate changes, recent data analysis and modeling suggest a link between loss of sea ice and a shift to an increased impact from the Arctic on mid-latitude climate (Francis et al. 2009; Honda et al. 2009). Models suggest that loss of sea ice in fall favors higher geopotential heights over the Arctic. With future loss of sea ice, such conditions as winter 2009-2010 could happen more often. Thus we have a potential climate change paradox. Rather than a general warming everywhere, the loss of sea ice and a warmer Arctic can increase the impact of the Arctic on lower latitudes, bringing colder weather to southern locations.
The source article Arctic Report Card - Atmosphere - Overland, et al. was published October 19, 2010 by NOAA .