Atmosphere

J. Overland¹, M. Wang², and J. Walsh ³

¹NOAA, Pacific Marine Environmental Laboratory, Seattle, WA
²Joint Institute for the Study of the Atmosphere and Ocean, University of Washington, Seattle, WA
³International Arctic Research Center, Fairbanks, AK

March 7 , 2011

See Warm Arctic-Cold Continents website

Summary

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.

Figure A.1. Arctic-wide annual average surface air temperature anomalies relative to the 1961–90 mean, based on land stations north of 60°N from the CRUTEM 3v dataset, available online at www.cru.uea.ac.uk/cru/ data/temperature/. Note this curve does not include marine observations.

Figure A.2. Near-surface (1000 mb) air temperature in °C anomalies for February 2009. Anomalies are relative to the 1968–96 mean, according to the NCEP–NCAR reanalysis through the NOAA /ESRL/PSD on-line tool

Figure A.3. Near-surface (1000 mb) annual air temperature in °C anomalies for 2009 over the northern hemisphere relative to 1968–96 mean according to the NCEP–NCAR reanalysis through the NOAA /ESRL/PSD on-line tool. Arctic amplification of air temperature anomalies are a factor of two or more 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).

Figure A.4. Air temperature for the first half of 2010 showing anomalies of over 4°C over northeastern Canada. (NOAA /ESRL/PSD on-line tool)

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.

Figure A.5. Map of sea level pressures (SLP) for 1-19 July 2010 showing low SLP over the central Arctic Ocean, a pattern that brought cooler and cloudier conditions and slowed the rate of sea ice loss. (NOAA /ESRL/PSD on-line tool)

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).

Figure A.6. Lower tropospheric (850 hPa) air temperature in °C anomalies for October 2009 relative to the 1968–96 mean according to the NCEP–NCAR reanalysis through the NOAA /ESRL/PSD on-line tool.

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.

Figure A.7a. The climatological 850 mb geopotential height field for December, over the period 1968-1996. Low heights over the Arctic are representative of the polar vortex of westerly winds. Data are from the NCEP–NCAR Reanalysis through the NOAA /ESRL/PSD on-line tool.		Figure A.7b. Similar to Figure 7a, but for the observed 850 mb geopotential height field in December 2009.(NOAA /ESRL/PSD on-line tool) Note the near reversal of the pattern with the maximum heights now residing over the Arctic. Air streamlines follow the height contours, showing a connection between the Beaufort Sea region and the eastern United States and that the westerly flow into Europe is strongly displaced to the south. The normal region of the Icelandic Low is at a near maximum under these conditions.		Figure A.7c. The 850 mb geopotential height field for February 2010.(NOAA /ESRL/PSD on-line tool) Again note the anomalous height maxima over the Arctic, the southward displacement of streamline into Europe and the northerly streamline into eastern Asia. Again the Icelandic Low location has a High.

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.

References

Budikova, D. (2009): Role of Arctic sea ice in global atmospheric circulation: A review. Global Planet. Change, 68(3), 149–163.

Francis, J.A., W. Chan, D.J. Leathers, J.R. Miller, and D.E. Veron (2009): Winter Northern Hemisphere weather patterns remember summer Arctic sea-ice extent. Geophys. Res. Lett., 36, L07503, doi:10.1029/2009GL037274.

Honda, M., J. Inoue, and S. Yamane (2009): Influence of low Arctic sea-ice minima on anomalously cold Eurasian winters. Geophys. Res. Lett., 36, L08707, doi:10.1029/2008GL037079.

Overland, J.E., and M. Wang (2010): Large-scale atmospheric circulation changes associated with the recent loss of Arctic sea ice. Tellus, 62A, 1–9.

Petoukhov, V., and V. Semenov (2010): A link between reduced Barents-Kara sea ice and cold winter extremes over northern continents. J. Geophys. Res.-Atmos., ISSN 0148-0227.

Seager, R., Y. Kushnir, J. Nakamura, M. Ting, and N. Naik (2010), Northern Hemisphere winter snow anomalies: ENSO, NAO and the winter of 2009/10, Geophys. Res. Lett., 37, L14703, doi:10.1029/2010GL043830.

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