Atmosphere

J. Overland¹, J. Walsh², and M. Wang³

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

Summary

Autumn temperatures are at a record 5º C above normal, due to the major loss of sea ice in recent years which allows more solar heating of the ocean. Winter and springtime temperatures remain relatively warm over the entire Arctic, in contrast to the 20th century and consistent with an emerging global warming influence.

The year 2007 was the warmest on record for the Arctic, continuing a general, Arctic-wide warming trend that began in the mid-1960s (Fig. A1).

Figure A1. 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 ship observations.

The summers of 2005 through 2007 all ended with extensive areas of open water (see sea ice section). This allowed extra heat to be absorbed by the ocean from solar radiation. As a result ice freeze-up occurred later than usual in these years. Surface air temperature (SAT) remained high into the following autumns, with warm anomalies above an unprecedented +5° C during October and November across the central Arctic (Fig. A2).

Figure A2. Near surface air temperature anomaly map for October and November for recent years with a reduced sea ice cover, 2005–2007. Data are from the NCEP – NCAR reanalysis through the NOAA /Earth Systems Research Laboratory, generated online at www.cdc.noaa.gov.

The major retreat of sea ice extent during the summer of 2007 was set up by sustained winds blowing from the North Pacific across the North Pole (Fig. A3). These winds contributed to sea ice advection toward the Atlantic sector. Additionally, the advection of warm moist air into the central Arctic contributed to melting sea ice, especially in the Pacific sector. The near record minimum sea ice extent in 2008 was nearly the same as in 2007, but the causes were different. The wind pattern in 2007 was unusually constant throughout the summer, while in 2008 the winds were more variable. Summer winds in 2007 appear to be a once in a decade event, while the atmosphere had less of a contribution to sea ice loss in 2008.

Figure A3. Sea level pressure field for summer 2007. This map has the same orientation as Figure A2. Purple/Blues regions have low pressure and orange regions have high pressure. Winds tend to blow parallel to the contour lines so the flow is from north of Bering Strait across the North Pole. Data are from the NCEP – NCAR reanalysis.

Winter and spring air temperatures in 2007 and 2008 continue the nearly Arctic-wide extent of positive SAT anomalies, similar to the early years of the twenty-first century (Fig. A4). These years contrast with the twentieth century in which positive and negative SAT anomalies were more spatially distributed. This Arctic-wide background SAT anomaly of greater than +1°C is consistent with projections from IPCC climate models (Chapman and Walsh 2007). Exceptions were the Bering Sea and western Alaska, which experienced a third consecutive cold winter. The Barents Sea continues to be a hot spot, which began in 2006.

Figure A4. Near surface air temperature anomalies for January–May 2007 and 2008. Data are from the NCEP – NCAR reanalysis.

The climate of the Arctic is influenced by repeating patterns of sea level pressure (SLP) that can dominate individual months or can represent the overall atmospheric circulation flow for an individual winter and spring. The main pattern is known as the Arctic Oscillation (AO) circulation regime, widely considered the main source of Arctic climate variability during the twentieth century. A second pattern influences the Pacific sector of the Arctic, known as the Pacific North American (PNA) pattern. A positive period of the AO has lower sea level pressure over the central Arctic, brings warmer temperatures to Eurasia, and helps to export sea ice into the Atlantic. In 2007 and 2008 the AO returned to a strongly positive wintertime index value for the first time in more than a decade, but its value was still lower than the large positive values of the early 1990s (Fig. A5).

Figure A5. The extended winter (DJFM) Arctic Oscillation index, 1950 to 2008 (based on data available online at www.cpc.ncep.noaa.gov).

The actual SLP anomaly patterns for both 2007 and 2008 have minimums over western Eurasia (Fig. A6). These patterns contrast with that of the canonical positive AO, where the lowest anomalies are centered over Iceland and the central Arctic. The geostrophic wind pattern associated with the 2007 SLP anomaly field brought air flow from western Russia toward the North Pole, with a positive (warmer) SAT anomaly maximum over the northern Barents Sea (A4). 2008 actually has a "dipole" pattern of SLP anomalies with extensive high pressure over Arctic Canadian and low pressure on the Siberian side, giving decidedly non-AO regional impacts in the Pacific sector.

Figure A6. SLP anomaly pattern for (left) Jan–May 2007 and (right) Jan–May 2008. Note the extensive Arctic-wide areas of low SLP in winter and spring, which project onto a positive AO index. Data are from the NCEP – NCAR reanalysis Anomalies are relative to a 1968–96 climatological period.

References

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

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

October 6, 2008

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