Air Temperature, Atmospheric Circulation and Clouds
J. Overland1, J. Key2, B.-M. Kim3, S.-J. Kim3, Y. Liu4, J. Walsh5, M. Wang6, U. Bhatt7
1Pacific Marine Environmental Laboratory, NOAA, Seattle, WA, USA
2Center for Satellite Applications and Research, NOAA/NESDIS, Madison, WI, USA
3Korea Polar Research Institute, PO Box 32, Incheon, Korea
4Cooperative Institute for Meteorological Satellite Studies, University of Wisconsin, Madison, WI, USA
5International Arctic Research Center, University of Alaska Fairbanks, Fairbanks, AK, USA
6Joint Institute for the Study of the Atmosphere and Ocean, University of Washington, Seattle, WA, USA
7Geophysical Institute, University of Alaska Fairbanks, Fairbanks, AK, USA
November 7, 2012
- October 2011 through August 2012 was a departure from typical atmospheric conditions of recent years (2003-2010), in that temperature anomalies were small over the central Arctic. Most of the notable weather activity in fall and winter occurred in the sub-Arctic due to a strong positive North Atlantic Oscillation. Summer 2012 was dominated by low sea level pressure.
- Three severe weather events included (1) unusual cold in late January to early February 2012 across Eurasia, and (2) two record storms characterized by deep central pressure and strong winds near western Alaska in November 2011 and north of Alaska in August 2012.
Mean Annual Surface Air Temperature
In contrast to the years 2003 through 2010, which had substantial positive temperature anomalies in the central Arctic, the period October 2011-August 2012 showed positive temperature anomalies in the sub-Arctic rather than over the central Arctic Ocean (Fig. 1.1).
Over a longer time interval, the annual mean surface air temperature over Arctic land areas has experienced an overall warming of about +2ºC since the mid-1960s (Fig. 1.2). In 2011, the annual mean air temperature was slightly warmer than in 2009 and 2010. The cooler temperatures in 2009 and 2010 reflected cold continents in winter, while Eurasia had warmer temperatures in spring 2011. The annual mean surface temperature for 2012 is not available, as the year was incomplete at the time of writing.
Positive temperature anomalies were seen everywhere across the central Arctic for the first decade in 21st century (2001-2011) relative to a 1971-2000 baseline period at the end of the 20th Century (Fig. 1.3). This temperature pattern is a manifestation of "Arctic Amplification", which is characterized by temperature increases 1.5°C greater than (more than double) the increases at lower latitudes (Overland et al., 2011; Stroeve et al., 2012).
Seasonal Air Temperatures
Consistent with the annual average temperatures (Fig. 1.1), each seasonal anomaly distribution for near-surface temperatures shows departures primarily in the sub-Arctic (Fig. 1.4). Fall 2011 and winter 2012 were characterized by a positive North Atlantic Oscillation (NAO). This promotes the warm temperature anomaly over the Barents and Kara Seas, which are downstream of the stronger winds and lower pressures of the Icelandic low pressure center. This is unlike the Warm Arctic/Cold Continents pattern associated with a negative Arctic Oscillation (AO) climate pattern over the central Arctic (see previous Report Cards), which dominated the previous two falls and winters (2009-10 and 2010-11).
In contrast to the positive NAO in fall 2011 and winter 2012, spring and summer 2012 had a very negative NAO, with significant consequences for snow cover duration and extent (see the Snow essay) and melting on the Greenland Ice Sheet (see the Greenland Ice Sheet essay). Spring 2012 also saw the early formation of the Arctic Dipole (AD) pattern (Fig. 1.5) with high pressure on the North American side of the Arctic and low pressure on the Siberian side. In the previous five years this has not occurred until June (Overland et al., 2012). The dipole pattern supported increased winds across the Arctic and warmer temperature anomalies over the East Siberian Sea and western Greenland (Fig. 1.4c). In summer 2012 an unusual low pressure, centered on the Pacific Arctic sector, was a new feature of central Arctic weather relative to the last decade (Fig. 1.6).
Also noteworthy in Fig. 1.6 is the high sea level pressure over Greenland, which has been a feature of early summer for the last six years. Higher pressures over Greenland and their influence on Arctic and subarctic wind patterns, a so called blocking pattern, suggests physical connections between it and reduced Arctic sea ice in the summer, loss of Greenland and Canadian Arctic glacier ice, reduced North American snow cover in May and June, and potentially extremes in mid-latitude weather (Overland et al., 2012). See the essays on Sea Ice, Glaciers and Ice Caps, Greenland Ice Sheet and Snow for further information on those topics.
The period late 2011 through summer 2012 was notable for three severe weather events.
The Bering Sea storm of November 2011, one of the most powerful extra-tropical cyclones on record to affect Alaska, caused extensive coastal flooding. Moving northeastward from its origins in the western Pacific Ocean, the storm deepened by 25 hPa in the 24 hours ending November 8, when its central pressure of 945 hPa was comparable to that of a Category 3 hurricane. The storm's forward speed exceeded 100 km/hour as it approached Alaska and turned northward, passing just offshore of Alaska's western coast, then through the Bering Strait and into the Chukchi Sea. Wind gusts of 144 km/hour and 151 km/hour were recorded on the western Seward Peninsula and Little Diomede Island, respectively.
In late January-early February 2012, a warm center occurred over the Kara and Laptev Seas and broader, severe cold anomalies occurred over the northern Eurasian sub-Arctic during a brief period of negative AO (Fig. 1.4b). North America and Eurasia exhibited a sharp contrast in surface temperature anomalies. The United States experienced its fourth warmest winter since national records began in 1895, whereas extremely low temperatures occurred across parts of the Eurasian continent during January 24th-February 14th. This was Europe's worst cold spell in at least 26 years, and >650 people died as a result of the frigid conditions in Russia, Ukraine and Poland. A significant amount of snow fell across the affected areas, resulting in the third largest February snow cover extent (Source: NOAA National Climatic Data Center, State of the Climate: Global Analysis for February 2012, published online March 2012, http://www.ncdc.noaa.gov/sotc/global/2012/2). These observations suggest that a negative AO can favor the development of cold weather over Europe and warm weather over North America.
In August 2012, a storm of exceptional intensity affected the Arctic Ocean north of Alaska. The central pressure of 965 hPa made this system one of the strongest August storms to have affected the Arctic Ocean in the past several decades. The storm likely had a significant impact on ocean mixing due to the already reduced sea ice cover, but this remains to be fully evaluated. The storm did have a significant impact on the further retreat of the pack ice, as illustrated in the Sea Ice essay (Fig. 2.5).
Unlike 2011, when Arctic cloud cover was somewhat higher than normal in winter and lower in the summer, Arctic cloud cover in 2012 was, overall, average when compared to the period 2001-2010. However, there were significant monthly anomalies that warrant closer examination, as the spatial patterns varied in important ways on the regional scale.
While clouds influence the surface energy budget, they also respond to changes in the ice cover (Liu et al., 2012). As in recent years, positive cloud cover anomalies (more cloud) over the Arctic Ocean correspond to negative sea ice anomalies (less ice). This was particularly evident in the winter months in the Barents and Kara seas region, and in the summer months from the East Siberian Sea to the Beaufort Sea. An example for February is shown in Figs. 1.7a and b. The reader is referred to the Sea Ice essay for information on Arctic-wide sea ice anomalies during the period 1979-2012.
Large-scale advection of heat and moisture and the frequency of synoptic scale systems also influence cloud cover (Liu et al., 2007). Positive cloud anomalies over northern Russia and the Kara Sea in the 2011-2012 winter months correspond to southerly flow on the western side of an anticyclonic pattern, while negative cloud anomalies over Siberia are found on the eastern side of the same pattern (Figs. 1.7 c and d). Positive cloud anomalies over the Chukchi Sea in June (not shown) also appear to be related more to changes in circulation than to changes in sea ice extent. These patterns are also seen in the surface temperature fields (Figs. 1.4b, and c).
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