Atmosphere Assessments

Aaron Letterly
Cooperative Institute for Meteorological Satellite Studies, University of Wisconsin, Madison, WI, USA
[5 June 2015]

Mean Annual Surface Air Temperature

Similar to 2012, the period October 2013 - August 2014 showed the most significant positive temperature anomalies in the Kara Sea. Widespread, but weaker, positive temperature anomalies spanned from the Chukchi and Eastern Siberian Seas into the Central Arctic, reminiscent of temperature anomaly patterns in the years 2003-2010 (Figure 1). The Canadian Archipelago and northern Europe also experienced above-average surface temperatures in 2014, in contrast with the colder annual temperatures in the North American continent.

Figure 1: Annual Average (October 2013 through August 2014) near-surface air temperature anomalies releative to the period 1981-2010. Data are from NOAA/ESRL, Boulder, CO.

Increased surface temperatures in 2014 across much of the Arctic are a demonstration of “Arctic amplification”, which is defined by greater temperature increases at high latitudes relative to mid-latitudes. The temperature increases in the Arctic over the 21st century are more than double the 1.5° C increase observed over the Northern Hemisphere (Figure 2) (Overland et al., 2011; Stroeve et al. 2012). Positive temperature anomalies relative to the 1971-2001 baseline period were seen throughout the Central Arctic over the five-year period 2009-2014 (Figure 3). These surface temperature increases were greater than the positive anomalies in the 2001-2011 period.

Figure 2: Northern Hemispheric annual average surface air temperature (SAT) anomalies for the period 1900-2011 relative to the 1961-1990 mean value, based on land and sea stations north of 2.5°N. Data are from the CRUTEM4v dataset at www.cru.uea.ac.uk/cru/data/temperature/. Note: this plot does not include data for 2014, as the year was incomplete at the time this was written.

Figure 3: Annual Average near-surface air temperature anomalies for the first decade of the 21stcentury (2001-11) relative to the baseline period of 1971-2001. Data are from NOAA/ESRL, Boulder, CO.

Seasonal Air Temperature

Seasonal distributions for near-surface temperature anomalies (Figure 4) show greatest deviations in the sub-Arctic and in the Chukchi-Beaufort Sea region, which is consistent with annual average temperatures (Figure 1). A strongly positive North Atlantic Oscillation (NAO) in fall 2013 weakened into winter 2014, supporting increased temperatures over the Barents and Kara Seas, which are downstream of the strong low-pressure center present over Iceland from October 2013-March 2014 (not shown). The Warm Arctic/Cold Continents pattern seen over the 2014 winter is associated with a positive Arctic Oscillation (AO) in the Central Arctic, similar to 2012.

Figure 4: Seasonal anomaly patterns for the near-surface air temperatures in 2014 relatie to the baseline period 1981-2010. Fall 2013 (a), winter 2014 (b), spring 2014 (c), and summer 2014 (d). Data are from NOAA/ESRL, Boulder, CO.

Spring and summer 2014 did not show a distinct NAO pattern. Spring saw strong low pressure in the Siberian Arctic and moderate high pressure anomalies in the American Arctic, resulting in an early Arctic Dipole (AD) formation (Figure 5). The circulation did little increased southerly winds and promoted only positive temperature anomalies over the Beaufort Sea and Central Arctic during summer. Low-pressure anomalies were centered over the Laptev Sea (Figure 6), which was similar to the system there in the summer of 2012. The high sea level pressure over Greenland has become a recurring feature of early summer during the last 8 years. These higher sea level pressures influence Arctic and sub-Arctic wind patterns, referred to as a “blocking pattern” which can divert cold air to Central Europe and prolong heat waves in North America.

Figure 5: Sea level pressure field for April through June 2014 showing the Arctic Dipole (AD) pattern with high pressure on the North American side of the Arctic and low pressure on the Siberian side. Data are from NOAA/ESRL, Boulder, CO.

Figure 6: In summer 2014, an expansive low sea level pressure anomaly was concentrated over the Kara Sea while high pressure anomalies remained over Greeenland and the Barents Sea.

Severe Weather

Late 2013 through summer 2014 saw few notable severe weather events, in stark contrast to the 2011-12 season which saw multiple powerful extratropical cyclones. High sea level pressures over Greenland were above the 1981-2010 average for much of the summer, which was not conducive to cyclone formation in the Atlantic side of the Arctic (Figure 6). Apparent in Figure 6, the deep low over the Laptev Sea created a strong pressure gradient, bringing southerly winds and aided in northward sea ice drift in the region and open water off of Siberia. This sea ice anomaly is further explored below.

Cloud Cover

Unlike 2013, when decreased winter cloud and increased summer cloud encouraged a significant sea ice rebound, cloud cover in 2014 did not greatly deviate from mean conditions. One instances, however, warrants closer examination as a region of significant positive cloud cover anomalies (less cloud) coincided negative sea ice anomaly (less ice) on a regional scale. Clouds influence the surface energy budget as well as respond to changes in the ice cover (Liu et al., 2012). In the wintertime, clouds act to absorb and emit longwave radiation and impose a net warming effect on the surface (Schweiger and Key, 1994). Many other factors also force changes in sea ice, and there is not always an immediate, first-order relationship between cloud cover anomalies and ice concentration (Liu and Key, 2014).

An example of the complex relationship between clouds and sea ice for in the late fall of 2014 is shown in Figures 7a and 7b. Positive cloud anomalies in the Laptev Sea during October 2014 correspond to southerly flow on the southwestern side of an anticyclonic pattern over the Central Arctic (Figures 7c and 7d). The increased cloud in these regions inhibits radiative cooling of open water and may have slowed the refreezing of ice. These patterns are supported by temperature fields, as positive surface temperature anomalies in the Laptev Sea during this same time period (not shown) may have also contributed to the decreased sea ice concentration.

Figure 7: Cloud cover (a) and sea ice concentration (b) anomalies (%) in October 2014 relative to the corresponding monthly means for the period 2002-2010. Data are from the Moderate Resolution Spectroradiometer (MODIS) on the Terra satellite. Corresponding 500 mb geopotential height (c) and 500 mb meridional wind field (d) anomalies in October 2014 are also shown. Data are from NOAA/ESRL, Boulder, CO.

References

Liu, Y., J. R. Key, Z. Liu, X. Wang and S. J. Vavrus. 2012. A cloudier Arctic expected with diminishing sea ice. Geophys. Res. Lett., 39, L05705, doi:10.1029/2012GL051251.

Liu Y. and J.R. Key, 2014, Less winter cloud aids summer 2013 Arctic sea ice return from 2012 minimum, Environ. Res. Lett., 9, 044002.

Overland, J. E., J. A. Francis, E. Hanna and M. Wang. 2012. The recent shift in early summer arctic atmospheric circulation. Geophys. Res. Lett., doi: 10.1029/2012GL053268.

Schweiger, A.J. and J. Key, 1994. Arctic Ocean radiation fluxes and cloud forcing based on the ISCCP C2 cloud data set, 1983-90. J. Appl. Meteorol., 33(8), 948-963.

Stroeve, J. C., M. C. Serreze, M. M. Holland, J. E. Kay, J. Maslanik and A. P. Barrett. 2012. The Arctic's rapidly shrinking sea ice cover: a research synthesis. Climatic Change, doi 10.1007/s10584-011-0101-1.