Martin Sharp, Professor at the Department of Earth and Atmospheric Sciences University of Alberta, Canada, discusses drivers of Arctic ice cap change and his thoughts on linking climate and weather
It is now widely recognised that the Arctic is one of the most rapidly warming regions on Earth and that this warming is driving significant changes in the Arctic cryosphere – the seasonal snow cover, permafrost, sea ice and glaciers that cover much of the Arctic. In general, this warming is more rapid than that occurring at lower latitudes, a phenomenon referred to as “Polar Amplification”. However, it is spatially and temporally non-uniform and this has important implications for how it affects the Arctic ice caps. This issue has attracted the interest of glaciologists since the early 1960s. Here I discuss it with particular reference to work that has been conducted on the ice caps in Arctic Canada.
Bea Alt from the Geological Survey of Canada was one of the first to explore the nature of the specific weather systems that generate snowfall and summer melting over ice caps in the Arctic through field studies on the Devon Ice Cap and Meighen Island Ice Cap in the Canadian Arctic (Alt, 1978, 1979, 1983). Through a 14-year (1961-74) study of the Devon Island Ice Cap, she investigated how synoptic weather conditions control variations in the mass balance of the ice cap. She noted that the occurrence of a strong cyclone in Baffin Bay during the summer tended to suppress melt on the ice cap and was often associated with summer snowfall that had the same effect by increasing its surface albedo. Tracking cyclones from elsewhere might add to accumulation on the ice cap, although surface melting might occur if the warm sector of these systems intruded north of Devon Island. By contrast, anticyclonic conditions would result in melting over the outlet glaciers draining the ice cap, while warm air advection associated with anticyclonic blocking would trigger periods of intense melting and mass loss.
The important inference from Alt’s studies is that changes in the relative frequency of cyclonic and anticyclonic modes of atmospheric circulation are likely major influences on rates of both snowfall and surface melting on these ice caps – and hence on their mass balance. This idea was picked up in a 1987 paper that explored the nature of the synoptic weather conditions associated with extreme mass balance years in the Canadian Arctic Islands. Three recurring sets of conditions were identified:
Years with high summer melt, when there was a high-pressure ridge across the islands at all levels in the troposphere and no trough over North America;
Years with suppressed summer melt, when there was a deep cold trough over Ellesmere Island and down Baffin Bay that encouraged flow into the islands from the Arctic Ocean that suppressed melting and resulted in positive surface mass balance on the ice caps and;
Years with high accumulation of snow when low-pressure systems tracked south and southeast across the Arctic islands from the Arctic Ocean, producing summer snowfall and positive glacier mass balance.
This was one of the earliest attempts to connect glacier change in the Arctic to variations in synoptic meteorology and climate.
In a 2007 paper, Alex Gardner returned to the theme of Alt’s work by linking the record of variability in glacier mass balance in the Canadian Arctic Islands to the history of changes in the position of the summer Circumpolar Vortex in the Arctic. This work was triggered by the recognition of a regime shift in the surface mass balance of glaciers in the Canadian Arctic Islands that took place between 1986 and 1987 and that was associated with a positive shift in summer air temperatures over the region. Gardner showed that annual glacier mass balances tended to be positive in years when the Vortex in July was located in the western hemisphere and negative in years when the Vortex was either weak or strong but not elongated over the Canadian High Arctic Islands and when its centre was located in the eastern hemisphere. Gardner tracked changes in the atmospheric circulation configuration over time, showing that the occurrence of westerly centred vortices decreased by ~40% after 1987. After that date, atmospheric ridging over the Canadian High Arctic Islands in summer became more common, surface air temperatures increased and regional glacier mass balances became more negative.
By the late 2000s, it was becoming clear that further reductions in glacier mass were occurring in Arctic Canada. Gabrielle Gascon (2013) took up the challenge of understanding how these related to the changing climate. She found that the summer melt season on the Devon Ice Cap became longer between 2004 and 2010 by 3.4 days/year at 1800m elevation, 6.1 days/year at 1300m and 8.8 days/year at 1000m (where the surface melt rate rose from 74 to 133 cm yr-1 from 2007-2010).
These changes were linked to two major drivers: (a) strengthening of the high-pressure ridge over the Arctic in June and July, which resulted in increased advection of warm air into the region, coupled with clear skies over the ice cap, which increased the energy available for melt by 4-24% and (b) increased frequency of southwesterly low- pressure systems in August after 2004 which advected warm air into the Arctic and increased the available melt energy by 12-38% relative to the 2007-10 daily mean, whilst at the same time reducing the shortwave radiation flux and increasing the longwave radiation flux. This had the effect of extending the end of the melt season by an average of ~5.5 days per year at the three monitored elevations on the ice cap. The important message to take from this is that the timing and intensity of melting on Arctic ice caps can vary significantly from year-to-year in ways that are connected to the changing configuration of the regional atmospheric circulation and the nature of the weather systems that it spawns. These influences are, therefore, apparent in records of the mass balance of these glaciers.
In a follow-up study, Peter Bezeau (2015) investigated how glacier mass balance in the Canadian Arctic was affected by variability in the frequency of occurrence of anticyclonic circulation over the region in summer for the period 2007-2012. This was a period when positive summer geopotential height anomalies over the Canadian Arctic Islands were 2.5 times more common than the climatological mean and when glacier mass balances were unusually negative. Various explanations have been proposed for the increased frequency and duration of anticyclonic circulation over the Canadian Arctic between 2007 and 2012, these include increased meridional heat advection and decreasing sea ice extent, sea ice volume and snow-covered area in spring. All of these could potentially contribute to a feedback that would result in an increase in the frequency of positive geopotential height anomalies and anticyclonic circulation in the region. However, the extent to which these changes can be attributed to either natural climate variability or anthropogenic forcing on the climate system has yet to be quantified. Other important points to make are that the timing and rates of climate warming vary across the Arctic so that glaciers across the region will likely respond to warming at different times, at different rates and to varying degrees. It is important to recognise this when trying to imagine how future climate change will impact the Arctic ice caps and to determine how and when the changes that do occur will affect global sea levels and regional scale water resources and water quality.
References
Alt, B.T., 1978. Synoptic Climate Controls of Mass-Balance Variations on Devon Island Ice Cap. Arctic and Alpine Research 10, 61-80.
Alt, B.T., 1983. Synoptic Analogues: A technique for studying climatic change in the Canadian High Arctic. Climatic Change in Canada 3, C.R. Harington (Ed), Syllogeus 33, 70-107.
Alt, B.T., 1987. Developing Synoptic Analogues for Extreme Mass Balance Conditions on Queen Elizabeth Island Ice Caps. Journal of Climate and Applied Meteorology 26, 1605-1623.
Gardner, A.S. and Sharp, M. 2007. Influence of the Arctic Circumpolar Vortex on the Mass Balance of Canadian High Arctic Glaciers. Journal of Climate 20, 4586-4598.
Bezeau, P., Sharp, M., and Gascon, G. 2015. Variability in summer anticyclonic circulation over the Canadian Arctic archipelago and west Greenland in the late 20th/early21st centuries and its effect on glacier mass balance. International Journal of Climatology 35, 540-557.
Gascon, G., Sharp, M., and Bush, A.B.G. 2013. Changes in Melt Season Characteristics on Devon Ice Cap, Arctic Canada and their association with the Arctic atmospheric circulation. Annals of Glaciology 53. 101-110.
Martin Sharp
Professor
Department of Earth and
Atmospheric Sciences
University of Alberta, Canada
Tel: +1 780 492 8546
www.ualberta.ca/science/about-us/contact-us/faculty-directory/martin-sharp
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