J. Overland1, M. Wang2, and J. Walsh 3
1NOAA, Pacific Marine Environmental Laboratory, Seattle, WA
See Warm Arctic-Cold Continents website
While 2009 showed a slowdown in the rate of annual air temperature increases in the Arctic, the first half of 2010 shows a near record pace with monthly anomalies of over 4°C in northern Canada. There continues to be significant excess heat storage in the Arctic Ocean at the end of summer due to continued near-record sea ice loss. There is evidence that the effect of higher air temperatures in the lower Arctic atmosphere in fall is 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.
The annual mean air temperature for 2009 over Arctic land areas was cooler than in recent years, although the average temperature for the last decade remained the warmest in the record beginning in 1900 (Fig. A.1). The 2009 average was dominated by very cold temperatures in Eurasia in February (the coldest of the decade) and December, while the remainder of the Arctic remained warm (Fig. A.2). The spatial distribution of annual temperature anomalies for 2009 has a pattern with values greater than 2.0°C throughout the Arctic, relative to a 1968–96 reference period (Fig. A.3). These anomalies show the major feature of current Arctic conditions, where there is a factor of two (or more) amplification of air temperature relative to lower latitudes.
The first 7 months of 2010 achieved a record high level of global mean air temperature, but this could moderate for the rest of the year due to La Niña influences. The warmest temperature anomalies for the Arctic in the first half of 2010 were over northeastern Canada (Fig. A4).
While 2009 and 2010 did not meet or exceed the record minimum sea ice extent set in 2007, the summer sea ice cover remains relatively small. As noted in the Arctic sea ice section, September 2009 had the third lowest minimum sea ice extent relative to the period when observations began in 1979. The minimum sea ice extent in 2010 is similar to 2008, and lower than 2009. Nearly the same atmospheric conditions have existed in the summers of 2008, 2009 and 2010, helping to drive the characteristics of the summer sea ice season.
Summer 2007 atmospheric conditions were extraordinary and helped lead to the record low ice extent in September 2007. This involved the development of a new climatic wind pattern, the Arctic Dipole Anomaly (DA) which has southerly wind flow from the Bering Strait across the North Pole and persisted throughout the summer of 2007. In May and June of 2009 and 2010, the DA was present again, helping to initiate rapid summer ice loss. Although ice extent at the end of June 2010 was in fact slightly lower than that observed at the same time in 2007, in July the sea level pressure (SLP) pattern shifted back to a more typical low pressure region over the central Arctic Ocean (Figure A5). This shift created conditions that significantly slowed the rate of ice loss during mid-summer 2010. As a result of the increased within-summer atmospheric variability in 2009 and 2010, we did not meet or exceed the record minimum sea ice cover extent set in 2007. However, these conditions have still resulted in a four year sequence of extremely low sea ice extent years.
Although 2009 did not have a record sea ice minimum in September, there were still extensive regions of open water in the Chukchi, East Siberian Laptev, and Kara Seas (see sea ice section, Fig. I2), which allowed extra solar and longwave radiation to be absorbed by the ocean (see ocean section, Fig. O1). The heat accumulated in the ocean can be released back to the atmosphere the following autumn, impacting temperatures in the lower troposphere (Fig. A6) and creating consequences for regional and far field wind patterns through large scale atmospheric teleconnections patterns (Overland and Wang 2010).
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. The most extreme winter (December, January, February) Arctic high-pressure event in 145 years of the historical record occurred in Winter 2009-2010.
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.
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