Atmosphere

J. Overland¹, L. Bengtsson², R. Przybylak³, J. Walsh⁴

¹NOAA, Pacific Marine Environmental Laboratory, Seattle, WA
²Max-Planck Institute for Meteorology, Hamburg, Germany
³Nicolaus Copernicus University, Toruń, Poland
⁴International Arctic Research Center, Fairbanks, Alaska

Circulation regime

The annually averaged Arctic Oscillation index (AO, a measure of the strength of circumpolar winds) was slightly positive in 2006, continuing the trend of a relatively low and fluctuating index which began in the mid-1990s (Figure A1). This follows a strong, persistent positive pattern from 1989 to 1995. The current characteristics of the AO are more consistent with the characteristics of the period from the 1950s to the 1980s, when the AO switched frequently between positive and negative phases. Initial data from 2007 shows a positive AO pattern.

Figure A1. Time series of the annually-averaged Arctic Oscillation Index (AO) for the period 1950 - 2006 based on data from the website www.cpc.ncep.noaa.gov. (Courtesy of I. Rigor)

Surface Temperatures and Atmospheric Circulation

In 2006 the annual surface temperature over land areas north of 60° N was 1.0°C above the mean value for the 20th century (Figure A2). The surface temperature in this region has been consistently above the mean since the early 1990s. Figure A2 also shows warm temperatures in the 1930s and early 1940s, possibly suggesting a longer-term oscillation in climate. However, a detailed analysis shows different proximate causes for the 1930s compared to recent maxima. The early warm and cold periods are associated with internal variability in high-latitude circulation patterns, while the recent warm temperatures have an anthropogenic component (Johannessen et al. 2004; Wang et al 2007).

Figure A2. Arctic-wide and annual averaged surface air temperature anomalies (60° - 90° N) over land for 1900-2006 based on the CRU TEM2V monthly data set. Anomalies are relative to the 20th century average.

For winter and spring (Dec-May) in 2006 and 2007 there was an overall warm pattern (positive temperature anomalies) in the Arctic with a regional hot spot of +3-4°C near Svalbard, spreading north from the Barents Sea (Figure A3 Left). This pattern was slightly different than observed during 2000-2005 which also had the overall warm pattern, but the hot spot was closer to east Siberia. The pattern of 2006 and 2007 Dec-May sea level pressure (SLP) anomalies shows a dipole pattern with higher pressure over Asia and lower pressure over the North American side of the Arctic (Figure A3 Right). This current SLP dipole implies an anomalous northward (meridional) flow of air from the Barents Sea to the central Arctic which supports the 2006-2007 temperature hot spot through warm air advection. The 2006-2007 period continues the pattern set up during 2000-2005 with Arctic-wide positive temperature anomalies, and a meridional flow pattern toward the central Arctic. The recent 2000-2007 Arctic warm period contrasts with the two principal atmospheric circulation features of the 20th century: the Pacific North American Pattern, which was strong during 1977-1981, and the Arctic Oscillation/Northern Annular Mode/North Atlantic Oscillation, which was strong during 1989-1995 (Quadrelli and Wallace 2004, Overland and Wang, 2005). The positive phases of these two patterns gave positive temperature anomalies over the Arctic land masses, while the current pattern shows positive temperatures centralized over the Arctic Ocean. These contrasts illustrate that we are in a period of continuing uncertainty about the dominance of any one climate pattern over the Arctic.

Figure A3. Left: December-May temperature anomaly composites for 2006 and 2007. Right: December-May sea level pressure anomaly composite for 2006 and 2007. All of the Arctic has positive temperature anomalies with a hot spot in the central Arctic northeast of Svalbard. The SLP anomaly pattern is a dipole, suggesting anomalous northward air flow into the central Arctic from Eurasia. The figure is based on NOAA National Centers for Environmental Prediction (NCEP) reanalysis fields via the Climate Diagnostics Center, www.cdc.noaa.gov. Anomalies are relative to a 1968-1996 base period.

End of an era for the Bering Sea?

Unlike the remainder of the Arctic, as noted above, air and ocean temperatures in the Bering Sea cooled significantly in 2006 and early 2007 compared with the previous six year period of warm temperatures (Figure A4 (ocean), Figure A3 (air)). Vertically average temperatures from an oceanographic mooring on the southeastern Bering Sea continental shelf (Stabeno et al. 2002) recorded temperatures in 2000-2005 that were 2°C warmer than earlier years, with 2005 as the warmest summer. While winter 2006 was very cold (note the drop in temperature between fall 2005 and summer 2006), the spring temperatures and ice extent in 2006 were near their climatological averages because the beginning fall 2005 temperatures were warm. Temperatures in fall 2006, in contrast, started cold and the weather pattern for November-December 2006 was also cold. The six year period of sustained of warm temperatures was sufficient to restructure the ecosystem away from Arctic conditions (Grebmeier et al. 2006). Winter-spring 2007 ended by being a relatively extensive ice year in the Bering Sea region. This suggests that it took two years for the warm ocean temperature anomalies on the Bering Sea continental shelf to dissipate. Because of this dramatic shift in ocean and ice conditions, the future state of the Bering Sea ecosystem is now less certain.

Figure A4. Ocean temperatures from a mooring on the southeastern Bering Sea continental shelf.

References

Grebmeier, J.M, and co-authors (2006) A major ecosystem shift in the northern Bering Sea. Science, 311, 1461-1464.

Johannessen, O.M., and co-authors (2004) Arctic climate change: Observed and modeled temperature and sea ice variability. Tellus, 56A, 328-341.

Overland, J.E., and M. Wang (2005) The third Arctic climate pattern: 1930s and early 2000s. Geophys. Res. Lett., 32, L23808, DOI:10.1029/2005GL024254.

Quadrelli, R., and J.M. Wallace (2004) A simplified linear framework for interpreting patterns of northern hemisphere wintertime climate variability. J. Climate, 17, 3728-3744.

Stabeno, P.J., N.B. Kachel, M. Sullivan, and T.E. Whitledge (2002) Variability of physical and chemical characteristics along the 70-m isobath of the southeast Bering Sea. Deep-Sea Res. Pt. II, 49, 5931-5943.

Wang M., J.E. Overland, V. Kattsov, J.E. Walsh, X. Zhang, and T. Pavlova (2007) Intrinsic versus forced variation in coupled climate model simulations over the Arctic during the 20th century. J. Climate (in press).

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