Summary of observed variability and change 1121 Surface air temperature

If we restrict ourselves to zonal means, SAT variability over northern high latitudes can be estimated over the past century. Figure 11.1 shows SAT records for 19002004 for the 55-85° N zonal band expressed as anomalies with respect to 1950-81 means. The plots are based primarily on land stations. Data coverage during the early part of the record is sparse, in particular for high Arctic latitudes. Nevertheless, some useful conclusions can be drawn. Over the twentieth century, zonally averaged annual temperatures have shown an overall increase of about 1 ° C, but with large variability. The major features that stand out are: (1) a general warming from 1900 through about 1940, best expressed from 1920 to 1940; (2) general cooling from 1940 through 1970; (3) general warming from 1970 onward. This basic structure is observed for all seasons but with the strongest variability during winter.

A great deal of attention has been paid to the pronounced recent winter warming, which, in being larger than for the Northern Hemisphere as a whole, is often viewed as evidence for high-latitude amplification of climate change. From simply looking at the running means in Figure 11.1, from about 1970 onward there has been an overall

Figure 11.1 Annual and seasonal temperature anomalies (55-85° N) for 1900-2004 evaluated with respect to 1951-80 means. The smoothed line represents results from a nine-point low-pass filter (courtesy of J. Eischeid, Climate Diagnostics Center, Boulder CO).

winter warming in the 55-85° N zonal band of about 2 °C, which is larger than for the other seasons. Spring has also warmed rather strongly. However, for any season and for annual means, it was not until about 1980 that departures turned positive. Put differently, part of the recent warming represents a recovery from cold conditions of the 1960s and 1970s. The spatial pattern of winter SAT change over the period 19812002 is illustrated in Figure 11.2 as anomalies from 1951-80 means. This analysis, which does not include the Arctic Ocean, due to insufficient data for the earlier years, points to strong warming over both northwest North America and northern Eurasia,

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SPLAnom (hPa)

Figure 11.2 Anomalies in winter (December-March) surface air temperature (oC) for the period 1981-2002 with respect to 1950-81 means.Temperature anomalies of >0.5 °C are indicated by dark shading, and those < -0.10 °C are indicated by light shading. The contour increment is 0.10 °C for negative anomalies and the 0.25, 0.50, 1.0, 1.5 and 2.0 °C contours are plotted for positive anomalies. The Arctic Ocean and other regions with insufficient temperature data are not contoured (from Hurrell et al., 2003, by permission of AGU).

SPLAnom (hPa)

Figure 11.2 Anomalies in winter (December-March) surface air temperature (oC) for the period 1981-2002 with respect to 1950-81 means.Temperature anomalies of >0.5 °C are indicated by dark shading, and those < -0.10 °C are indicated by light shading. The contour increment is 0.10 °C for negative anomalies and the 0.25, 0.50, 1.0, 1.5 and 2.0 °C contours are plotted for positive anomalies. The Arctic Ocean and other regions with insufficient temperature data are not contoured (from Hurrell et al., 2003, by permission of AGU).

but weak cooling over the northern North Atlantic, eastern Canada, and part of the North Pacific.

Rigor et al. (2000) assessed trends for the period 1979-97 north of 60° N by combining land station records with data from the Russian NP stations and IABP. The IABP data allow for the construction of gridded fields over the Arctic Ocean. There is considerable warming over Eurasia in winter, especially for spring and to a lesser extent for autumn. Positive trends are most widespread during spring and cover most of the Arctic Ocean, strongest along the Siberian side. An analysis of just the scattered NP records (1961-90) by Martin et al. (1997) showed a significant increase

Plate 8 Seasonal trends in surface air temperature (1981-2003) based on AVHRR clear-sky retrievals (adapted from Comiso, 2003, by permission of AMS). See color plates section.

in Arctic Ocean SATs during May and June. Broadly similar conclusions are drawn from surface temperature trends for clear skies based on AVHRR data. These seasonal maps for the period 1981-2003 (Plate 8) are updates from the study of Comiso (2003) that incorporate improvements to the original retrieval algorithms. The Arctic Ocean has experienced warming during winter, spring and autumn, largest in spring and autumn. Especially large warming (> 2° C per decade) is observed over portions of the Beaufort Sea (spring/autumn) and Baffin Bay (winter/spring), which relates in part to observed sea ice retreat (addressed shortly). The lack of summer warming over the central Arctic Ocean reflects the fixed temperature of the melting sea ice surface. Over land, warming is most widespread in spring and summer. But some areas, especially large parts of northern Eurasia in winter and autumn, cooled. The implication is that recent Arctic changes are sensitive to the period chosen for analysis, the data source, and the way in which the data are analyzed. Over the Arctic Ocean, a spring warming over recent decades, implying a longer sea ice melt season (see Belchansky et al., 2004 for a detailed assessment), emerges as a fairly consistent theme.

Returning to the century-long time series of Figure 11.1, Polyakov et al. (2002) show that while high-latitude SAT trends are often amplified relative to Northern Hemisphere trends, periods can be identified when the high-latitude SAT trends were smaller than or of opposite sign to those for the Northern Hemisphere. An important point with respect to the high-latitude time series is that the magnitude of the earlier warming from about 1920 through 1940 is entirely comparable to that for the post 1970 period. According to Polyakov et al. (2002), if one computes a high-latitude (north of 62° N) trend from 1920 onwards from annual means, one actually gets a small cooling trend. As evaluated for the period 1901-97, the difference in the Northern Hemisphere warming trend and that for the Arctic is statistically insignificant. Amplified high-latitude warming hence does not seem to hold when recent decades are placed in a longer-term context. Przybylak (2000) further articulates some of these issues. A related issue emerging from the studies of Polyakov et al. (2002), Alley et al. (2003), and Johannessen etal. (2004) is that while the early warming was largely restricted to northern high-latitudes, the recent (post 1970) warming is essentially global (albeit more strongly expressed over sub-Arctic lands). As shown by Overland et al. (2004), the earlier warm period had complex seasonal and spatial expressions.

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