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

October 9, 2009

Summary

It is apparent that the heating of the ocean in areas of extreme summer sea ice loss is directly impacting surface air temperatures over the Arctic Ocean, where surface air temperature anomalies reached an unprecedented +4°C during October through December 2008. There is evidence that the effect of higher air temperatures in the lower Arctic atmosphere is contributing to changes in the atmospheric circulation in both the Arctic and northern mid-latitudes.

The annual mean Arctic temperature for the year 2008 was the fourth warmest year for land areas since 1990 (Figure A1). This continued the 21st century positive Arctic-wide surface air temperature (SAT) anomalies of greater than 1.0° C, relative to the 1961-1990 reference period. The mean annual temperature for 2008 was cooler than 2007, coinciding with cooler global and Pacific temperatures (Hansen, 2009). The outlook is for increased temperatures, because there are currently (October 2009) El Nino conditions which are expected to continue through winter 2009-2010.

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

During October through December 2008 SAT anomalies remained above an unprecedented +4° C across the central Arctic (Fig. A2(A)). This is linked to summer sea ice conditions. The summer of 2008 ended with nearly the same extreme minimum sea ice extent as in 2007, characterized by extensive areas of open water (see sea ice section). This condition allows extra heat to be absorbed by the ocean from longwave and solar radiation throughout the summer season, which is then released back to the atmosphere in the following autumn (Serreze et al., 2009). We expect similar warm fall temperatures over the Arctic in 2009, as in 2007 and 2008.

Figure A.2. Near surface air temperature anomalies for (A, top) October through December 2008 and (B, bottom) January–May 2009. Anomalies are relative to 1968-1996 mean. Data are from the NCEP – NCAR reanalysis through the NOAA /Earth Systems Research Laboratory, generated online at www.cdc.noaa.gov .

Similar to the previous years of the 21st century, in 2009 the spatial extent of positive SAT anomalies in winter and spring of greater than +1°C was nearly Arctic-wide (Figure A2 (B)), in contrast with more regional patterns in the 20th century (Chapman and Walsh, 2007). The exception was the Bering Sea/southwestern Alaska which experienced a fourth consecutive cold or average winter associated with weaker winds and colder temperatures in the North Pacific.

There is evidence that, by creating a new major surface heat source, the recent extreme loss of summer sea ice extent is having a direct feedback effect on the general atmospheric circulation into the winter season (Francis et al., 2009). Fall air temperature anomalies of greater than +1.0° C were observed well up into the atmosphere (Figure 3A), when averaged over 2003-2008 relative to a 1968-1996 base period. The higher temperatures in the lower troposphere decrease the atmospheric air density and raise the height of upper-air-constant-pressure levels over the Arctic Ocean (Figure 3B). These increased heights north of 75 °N weaken the normal north-to-south pressure gradient that drives the normal west-to-east airflow in the upper troposphere. In this sense, the effect of higher air temperatures in the lower Arctic atmosphere is contributing to changes in the atmospheric circulation in both the Arctic and northern mid-latitudes. For example, Honda et al. (2009) suggest a remote connection between loss of Arctic sea ice and colder temperatures over eastern Asia.

Figure A.3. Vertical cross section from 60° to 90° N along 180° longitude averaged for October-December 2003 through 2008 (years for which summertime sea ice extent fell to extremely low values) for (A) air temperature, and (B) geopotential height. Data are from the NCEP – NCAR reanalysis available online at www.cdc.noaa.gov.

The climate of the Arctic is influenced by repeating patterns of sea level pressure that can either dominate during individual months or represent the overall atmospheric circulation flow for an entire season. The main climate pattern for the Arctic is known as the Arctic Oscillation (AO) with anomalous winds that blow counter-clockwise around the pole when the pattern is in its positive phase. A second wind pattern has been more prevalent in the 21st century and is known as the Arctic Dipole (AD) pattern (Wu et al., 2006; Overland et al., 2008). The AD pattern has anomalous high pressure on the North American side of the Arctic and low SLP on the Eurasian side. This implies winds blowing more from south to north, compared to the AO, and increasing transport of heat into the central Arctic Ocean. The AD pattern occurred in all summer months of 2007 and helped support the major 2007 summer reduction in sea ice extent (Overland et al., 2008). Fall 2008 and winter/spring 2009 showed a return of the AO pattern, but also considerable month to month variability in the presence of these various climate patterns.

References

Chapman, W. L., and J. E. Walsh, 2007: Simulations of Arctic temperature and pressure by global coupled models. J. Climate, 20, 609–632.

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.

Hansen, J., M. Sato, R. Ruedy, and K. Lo, cited 2009: 2008 global surface temperature in GISS analysis. [Available online at www.columbia.edu/~jeh1/mailings/2009/20090113_Temperature.pdf.]

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., M. Wang, and S. Salo, 2008: The recent Arctic warm period. Tellus, 60A, 589–597.

Serreze, M. C., Barrett, A. P., Stroeve, J. C., Kindig, D. N., and Holland, M. M. 2009. The emergence of surface-based Arctic amplification, The Cryosphere, 3, 11–19.

Wu, B., J. Wang, and J. E. Walsh, 2006: Dipole anomaly in the winter Arctic atmosphere and its association with sea ice motion. J. Climate, 19, 210–225.

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