J. Overland1, E. Hanna2, I. Hanssen-Bauer3, S.-J. Kim4, J. Walsh5, M. Wang6, U. S. Bhatt7
1NOAA/PMEL, Seattle, WA, USA
2Department of Geography, University of Sheffield, Sheffield, UK
3Norwegian Meteorological Institute, Blindern, Oslo, Norway
4Korea Polar Research Institute, Incheon, Republic of Korea
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
December 2, 2014
- The annual surface air temperature anomaly (+1.0°C relative to the 1981-2010 mean value) for October 2013-September 2014 continues the pattern of increasing positive anomalies since the late 20th Century.
- On a number of occasions in winter (January-March) 2014 there were strong connections between Arctic and mid-latitude weather patterns. A high amplitude (sinuous) jet stream sent warm air northward into Alaska and northern Europe, and cold air southward into eastern North America and central Russia.
- As a consequence of the sinuous jet stream in early 2014, extreme monthly temperature anomalies of +10°C were reported in Alaska, and -5°C over eastern North America and much of Russia.
- An Arctic Dipole pattern, with high pressure on the North American side of the central Arctic and low pressure on the Siberian side, contributed to low sea ice extent in summer 2014.
Arctic air temperatures are both an indicator and driver of regional and global changes. Although there are year-to-year and regional differences in air temperatures due to natural random variability, the magnitude and Arctic-wide character of the long-term temperature increase is a major indicator of global warming. Increases in Arctic temperatures cause, and are in turn influenced by, a set of feedbacks involving many parts of the Arctic environmental system: loss of sea ice and snow, changes in land ice and vegetation cover, permafrost thaw, black carbon (soot) in the atmosphere and on snow and ice surfaces, and atmospheric water vapor.
Mean Annual Surface Air Temperature
The mean annual surface air temperature anomaly (+1.0°C relative to the 1981-2010 mean value) for October 2013-September 2014 for land stations north of 60°N continues the pattern of increasing positive anomalies since the late 20th Century (Fig. 1.1). Note that 1981-2010 is the current reference period used by the World Meteorological Organization and individual national agencies such as NOAA. The 12-month period October 2013-September 2014 is the time elapsed since annual air temperature anomalies were last reported in the Arctic Report Card (Overland et al. 2013), and September 2014 is the most recent month for which data were available at the time of writing. The same applies to the next section describing seasonal air temperature variability.
The global rate of temperature increase has slowed in the last decade (Kosaka and Xie 2013), but Arctic temperatures continued to increase, such that the Arctic is warming at more than twice the rate of lower latitudes, as is evident in Fig. 1.1. The rapid warming in the Arctic is known as Arctic Amplification and is due to feedbacks involving many parts of the Arctic environment: loss of sea ice and snow cover, changes in land ice and vegetation cover, and atmospheric water vapor content (Serreze and Barry 2011).
The spatial distribution of near-surface temperatures in autumn-early winter (October-December) during recent years (2009-2014) has been warmer than the final 20 years of the 20th Century (1981-2000) in all parts of the Arctic (Fig. 1.2). These Arctic-wide positive (warm) anomalies are an indication that the early 21st Century temperature increase in the Arctic is due to global warming rather than natural regional variability (Overland 2009, Jeffries et al. 2013a).
Seasonal Surface Air Temperature Variability, October 2013 to September 2014
Seasonal air temperature variations are described for the period October 2013 to September 2014, which is divided by season into autumn 2013 (October, November, December), and winter (January, February, March), spring (April, May, June) and summer (July, August, September) of 2014 (Fig. 1.3).
Autumn 2013 was characterized by considerable month-to-month and regional variability in individual weather features that are masked by the 3-month composite (Fig. 1.3a). For example, there were anomalously high air temperatures in October over Alaska due to a strong Aleutian low pressure system, while low pressure in the Atlantic sector in November and December (similar to the winter pattern illustrated in Fig. 1.4) caused relatively warm temperatures in Siberia and cold temperatures in Greenland and Canada.
For winter 2014, each of the three months had similar regional temperature extremes (Fig. 1.3b). Extreme monthly temperature anomalies in excess of +5°C over the central Arctic spread south over Europe and Alaska. Svalbard Airport, for example, was 8°C above the 1981-2010 January-March average. Statewide, Alaska temperature anomalies were +10°C in late January 2014. Warm temperatures broke the 7-year (2007-2013) string of cold anomalies and extensive sea ice cover in the Bering Sea. Temperature anomalies were 5°C below normal in January and February over eastern North America and in January, February and March over much of Russia. Northern Siberia was relatively cool, and warm anomalies were observed in far eastern Asia. This pattern resulted from fewer storms connecting central Asia to northern Europe and was perhaps related to the greater sea ice loss that occurred in winter 2014 over the Barents and Kara seas (Kim et al. 2014).
On a number of occasions in January, February and March 2014 Arctic and mid-latitude weather patterns were strongly linked due to a high amplitude (more sinuous) "wave number 2" jet stream pattern (Fig. 1.4). This sent warm air from the south northward into Alaska and northern Europe, and cold air from the Arctic southward into eastern North America. A sinuous jet stream pattern is often associated with a negative Arctic Oscillation (AO) climate pattern, as evidenced by the higher geopotential heights north of Alaska and central Greenland (Fig. 1.4). The wave number 2 pattern had low heights over Iceland, where record low sea level pressures and warm temperatures occurred. In January, the North Atlantic Oscillation (NAO) was positive, while the AO was negative; this is unusual, as the AO and NAO often have the same sign. The wavy pattern over eastern North America and the positive NAO over the North Atlantic Ocean contributed to January flooding in the UK (Slingo et al. 2014).
Apart from low pressure over the Kara Sea causing warmer temperatures in central Siberia, which contributed to a record low April snow cover extent in Eurasia (see the essay on Snow), no major anomalies were observed in spring 2014 (Fig. 1.3c). Air temperatures were near normal during summer 2014 (Fig. 1.3d) relative to recent climatology (1981-2010), which includes a number of warm years. Summer temperatures in Greenland were above the 1981-2010 average (see the essay on the Greenland Ice Sheet), but were not unusually warm compared to the last decade. Summer 2014 was the warmest ever measured at many weather stations in Scandinavia. The Arctic Dipole (AD) (Wang et al. 2009, Overland et al. 2012) pattern dominated summer sea level pressure, with higher pressures on the North American side of the central Arctic and low pressures on the Siberian side (Fig. 1.5). In summer, this pattern tends to favor lower sea ice extent. Consistent with this observation, the minimum ice extent in September 2014 was the sixth lowest in the satellite record (see the essay on Sea Ice). Low atmospheric pressure over the eastern Aleutian Islands (Fig. 1.5) contributed to a wetter than normal summer in Interior Alaska.
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