Temperature and Clouds

J. Overland¹, U. Bhatt², J. Key³, Y. Liu⁴, J. Walsh⁵, M. Wang⁶

¹NOAA, Pacific Marine Environmental Laboratory, Seattle, WA
²Geophysical Institute, University of Alaska Fairbanks, Fairbanks, AK
³Center for Satellite Applications and Research, NOAA/NESDIS, Madison, WI
⁴Cooperative Institute of Meteorological Satellite Studies, University of Wisconsin, Madison, WI
⁵International Arctic Research Center, Fairbanks, AK
⁶Joint Institute for the Study of the Atmosphere and Ocean, University of Washington, Seattle, WA

November 10, 2011

Highlights

• Annual average temperature anomalies over the Arctic continued to be positive (warm) for October 2010 through September 2011 relative to temperatures in the 20th Century.
• Unusually strong north and south winds in fall and winter resulted in an Arctic-wide pattern of impacts, with warmer than normal temperatures of several °C over Baffin Bay/west Greenland and Bering Strait, and cooler temperatures over NW Canada and northern Europe. A new record low snow cover extent occurred over Eurasia due to persistent warm spring air temperatures. Summer also had vigorous wind patterns, with warmer than normal temperatures over NW Greenland and cooler temperatures in the Bering Sea.
• Slight decreases in cloud amount were observed in winter and slight increases were observed in summer.

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The annual mean air temperature for 2010 over Arctic land areas mirrored 2009, with slightly cooler temperatures than in recent years, although the average temperature for the last decade remained the warmest in the record beginning in 1900 (Fig. A1). The lower values in 2009 and 2010 reflect relatively cold continents in winter. In 2009 there was a coincident circum-Arctic decrease in Arctic greenness (see Fig. TE2 in the essay on Vegetation). However, the Arctic in 2011 continues to exhibit area-wide positive temperature anomalies which, together with other indicators discussed in this Report Card, suggest systematic changes since the end of the 20th Century. The past six years have been the warmest such period in the instrumental record for the region poleward of 60°N (SWIPA, 2011). There were notable local manifestations of the recent warming. Beginning on June 30, Barrow in northwesternmost Alaska had a record run of 86 consecutive summer days with minimum temperatures at or above freezing. The previous record, set in 2009, was 68 days. For the sixth consecutive year, the first freeze of autumn in Fairbanks in central Alaska occurred more than two weeks later than its long-term average.

Fig. A1. Arctic-wide annual average surface air temperature anomalies relative to the 1961-90 mean, based on land stations north of 60°N. Data are from the CRUTEM 3v dataset, available online at www.cru.uea.ac.uk/cru/ data/temperature/. Note this curve does not include marine observations.

It should be noted that this year this essay uses a new baseline interval for temperature anomaly departures using an average over the years 1981 through 2010. These include the recent warm years in the baseline average. In previous Report Cards the baseline average years were 1961 through 1990. The new baseline average is 0.77°C higher than the old baseline.

A key indicator of change is termed "Arctic Amplification". It notes that the Arctic is warming faster than more southerly latitudes and that temperature increases will be Arctic-wide. This is in contrast to more regional warming patterns typical at lower latitudes, which are associated with internal climate variability due to changes in wind patterns. This amplification effect is seen in data for 2011 (Fig. A2) and has been a prediction of climate models for 30 years (Manabe and Stouffer, 1980). Twelve-month average near-surface air temperature anomalies for October 2010 through September 2011 are above 1.5 °C for most of the Arctic Ocean area. While the cause of Arctic temperature amplification is mostly associated with current local loss of sea ice and terrestrial snow cover, Arctic amplification also results from the process of poleward movement of heat and moisture from mid-latitudes as part of the required overall global heat transport from equatorial regions to polar regions (Döscher et al., 2010, Serreze et al., 2008, Langen and Alexeev, 2007, Graversen and Wang, 2009).

Fig A2. Annual average (October 2010 through September 2011) near-surface air temperature anomalies relative to the period 1981-2010. Image provided by the NOAA/ESRL Physical Sciences Division, Boulder, CO, from its Web site at http://www.esrl.noaa.gov/psd/.

Seasonal Air Temperatures

Seasonal anomaly distributions for near-surface temperatures in late 2010 and 2011 are shown in Fig. A3. Relative hot spots in autumn and winter are seen in regions of low sea ice concentrations for the previous summer (2010), lying north of far eastern Siberia and Alaska, in the vicinity of greater Baffin Bay and far northeastern Canada, and to some extent in the Kara Sea (see Fig. SIO4 in the essay on Sea Ice). A new record low June snow cover extent (since satellite observations began in 1966) occurred over Eurasia in 2011, due to persistent warm spring air temperature anomalies over almost the entire Eurasian sector of the Arctic (see the essay on Snow). In summer, unusual warm temperature anomalies again returned to northern Canada with cool temperature anomalies in northern Siberia and the Bering Sea (see Fig. HTC7 in the essay on Glaciers and Ice Caps).

Fig. A3. Seasonal anomaly patterns for near surface air temperatures relative to the baseline period 1981-2010.

In late autumn 2010 and early winter 2011 there was a continuation of the "warm Arctic-cold Continent" climate pattern that first appeared in winter 2009-2010, when an increased linkage between Arctic climate and mid-latitude severe weather occurred. This was due to changing wind patterns, which resulted in both warmer and colder regions in the sub-Arctic. Figure A4a illustrates normal early-winter atmospheric winds. Meteorologists investigate wind fields by examining the heights of constant pressure surfaces; winds tend to blow counter-clockwise around the centers of low heights (purple), parallel to the height contours. In Fig. A4a, for example, climatological average Arctic winds in December tend to blow from west to east, thus separating cold Arctic air masses from the regions further south. In December 2009 there was a reversal of this climate pattern, with higher heights over the Arctic that eliminated the normal west-to-east winds (see the 2010 Report Card). This allowed cold air from the Arctic to penetrate into Europe, eastern China and the north-eastern USA. December 2010 also exhibited extremes in both warm and cold temperatures, with record-setting snowfall across lower latitudes as a result of changing wind patterns, as shown in the geopotential height field for that month (Fig. A4b). While the sub-Arctic temperature extremes in December 2010 tended to be similar to those in 2009, the height pattern is different. In December 2010, the normal polar height pattern separated into multiple centers (purple regions in Fig. A4b) and expanded southward over the sub-Arctic. Winds were from the south along west Greenland and in East Asia, and out of the north over eastern North America and northern Europe. What is of particular interest is that this pattern of Arctic/sub-Arctic linkage simultaneously occurs in all northern regions rather than occurring as discrete local events. The cause for the increased exchange in the last two winters is a subject for further research into whether recent changes in the Arctic are involved, whether they are the result of extreme but random events, or a combination of these and other mechanisms.

Fig. A4. (a, top) The climatological 850 mb geopotential height field for December, over the period 1981-2010. Low heights over the Arctic are representative of westerly winds centered over the Arctic. (b, centre) Observed 850 mb geopotential height field in December 2010. Note the southern locations of the multiple low height centers (purple). Air streamlines follow the height contours, bringing cold air into the eastern United States and northern Europe. (c, bottom) Observed 850 mb geopotential height field in July 2011. Data are from the NCEP-NCAR Reanalysis through the NOAA /ESRL/PSD on-line tool.

A further unusual event was the change in the wind pattern for summer 2011 (Fig. A4c for July). Unlike summer climatology, which has generally weak winds confined to the Arctic, July 2011 had a southward location of multiple low height centers not unlike December 2010. This continued the unusual warming in west Greenland (see Fig. HTC7 in the essay on Glaciers and Ice Caps and Fig. HTC10 in the essay on the Greenland Ice Sheet), the warm temperatures in Barrow mentioned in the Introduction, the cold temperatures in western Siberia, and kept warm storms from entering the Bering Sea.

The climate of the Arctic represents a unique system of interacting changes in sea ice, upper ocean temperatures, the atmosphere (including clouds), land surfaces, and linkages to lower latitudes. Many Arctic changes mentioned in the Reportcard can be related to increases in near surface atmospheric temperatures. Arctic temperature changes in turn result from a combination of gradual global warming, warm anomalies in internal climate variability in individual years, and impacts from multiple feedbacks processes. Nearly unique to the poles, however, is that once multi-year sea ice and glacial mass is gone, it is difficult to return to return to previous conditions.

Cloud Cover

Cloud optical properties have an important role in climate change as well as being significant in their own right. Clouds reflect sunlight, but also trap longwave radiation near the surface. With the decrease in sea ice cover, there is now an increased moisture source in newly opened sea ice-free ocean areas of the Arctic (see Fig. SIO4 in the essay on Sea Ice). Thus, scientists expect new interactions and modified feedbacks between sea ice loss and clouds. In 2011, Arctic cloud cover was somewhat higher than the average of the last ten years (2002-2011) in winter and lower than average in the summer, particularly over the western Arctic Ocean (Fig. A5). The magnitude and timing of the anomalies varied on the regional scale. On an annual average over the Arctic Ocean, however, cloud cover was not significantly different from recent years. The 2011 wintertime increase and summertime decrease in cloud amount resulted in greater downward energy flux and surface warming, potentially contributing to the near record low sea ice extent this year (see the essay on Sea Ice). This is in contrast to the general trend over the period 1982-2004, when a decrease in wintertime clouds and increase in springtime clouds over parts of the Arctic acted to dampen surface warming (Wang and Key, 2003,2005).

Fig. A5. Cloud cover anomalies (%) in February (left) and July (right) 2011 relative to the corresponding monthly means over the period 2002-2011 based on data from the Moderate Resolution Imaging Spectroradiometer (MODIS) on the Aqua satellite.

While clouds influence the surface energy budget (Liu et al., 2009) and thus sea ice growth and melt, they also respond to changes in the sea ice cover. This was particularly evident in September-October 2007 (not shown) at the time of the record minimum ice extent, and is also evident in 2011. Figure A6 shows the cloud cover anomaly for the first half of September 2011, with higher than normal cloud amounts in the poleward portions of the Beaufort, Chukchi, and Laptev seas. Sea ice concentration was lower than normal in these same areas (see the essay on Sea Ice). This is not coincidental, as a feedback analysis of data from 2000 to 2010 indicates that a 1% decrease in ice concentration leads to a 0.3-0.4% increase in cloud amount. There are, of course, other processes that influence cloud amount, in particular large-scale advection of heat and moisture and the frequency of synoptic scale systems (Liu et al., 2007).

Fig. A6. Cloud cover anomalies (left) and sea ice anomalies (right) in September 2011 (1-17 September) relative to the 2002-2011 September mean. Cloud cover (%) is based on Aqua MODIS data. Sea ice concentration (%) is from the Special Sensor Microwave/Imager (SSM/I) using the NASA Team algorithm.

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