Climate Regimes of the Arctic

Transcription

1 Climate Regimes of the Arctic

2 The climate of Greenland, recent changes and the ice sheet mass balance

3 Map of Greenland, showing elevation and the location of GC- Net automatic weather stations (+), expedition stations (x), and coastal settlements (o) [from Steffen and Box, 2001, by permission of AGU]. There are two elevation maxima, at Summit (3208 m) and near South D, about 2800 m.

4 Summit, Greenland Summit Camp consists of a handful of permanent buildings, surrounded by temporary housing in the summer months. (Photograph courtesy John Burkhart, GEOSummit.)

5 Monthly mean air temperatures for different sites in Greenland (see previous slide for locations) [from Steffen and Box, 2001, by permission of AGU]. The only station with a July mean temperature above the freezing point is JAR-1 (+0.2 deg. C).

6 Greenland, like Antarctica is home to frequent katabatic winds. The basic process is illustrated above. Air is chilled near the surface (through radiative cooling) and becomes quite dense. It wants to move outward from the slope due to a horizontal pressure gradient, but finds itself denser that its surroundings. The net effect is a wind accelerating down the slope [from Wikipedia].

7 See the YouTube piece (for Antarctica but it gives the idea)

8 Greenland tip jet The Greenland tip jet is a sporadic jet stream characterized by strong winds on the lee side of Cape Farewell. It appears to play an important role in chilling North Atlantic waters so that they sink and drive part of the thermohaline circulation. The figure at left was assembled by MIT/WHOI Joint Program graduate student Kjetil Våge using NOAA's QuikSCAT satellite. It depicts a tip-jet event for Dec. 5, Color indicates wind speeds in meters per second; arrows indicate wind direction. (Courtesy of Kjetil Våge WHOI) Tip jets form through interaction between the synoptic-scale flow and the topography of Greenland. There are both forward top jets (like the one depicted above), and reverse tip jets, extending west of the tip of Greenland

9 Storms (red tracks) causing strong tip-jet events during the winter of Composite sea level pressure during tip-jet events [from Pickart et al., 2003]

10 Annual accumulation over the Greenland Ice Sheet in mm water equivalent. The contour intervals are 200 mm, but 100 mm if smaller than 400 mm and 600 mm if larger than 1000 mm [from Chen et al., 1997, by permission of AMS]. Due to orographic precipitation, accumulation along the southeast coast of Greenland locally exceeds 2000 mm. Accumulation over the northcentral part of the ice sheet is only mm.

11 Map of estimated mean annual sublimation for the Greenland ice sheet. Sublimation is a direct change from the solid to vapor state of water. Positive values mean transfer of mass from the surface to the atmosphere [from Box and Steffen, 2001, by permission of AGU]. At the top of the ice sheet the rule is deposition (vapor to solid).

12 Part of the Greenland ice sheet sees surface melt in summer. The figure at left show surface melt extent (black areas) for the summer of 1999 based on satellite passive microwave retrievals. [adapted from Abdalati et al., 2001, by permission of AGU].

13 Seasonal melt water stream in the ablation zone Yumi Nakayama

14 Seasonal melt ponds on the ice sheet Ian Joughin / University of Washington Polar Science Center

15 Left- melt water disappearing into a moulin. Bottom cross section of a moulin. Seasonal melt ponds may drain completely to the base on the ice sheet in a few hours through crevasses or when a moulin forms. The weight of the water wedges the crevasse open. Konrad Steffen, CIRES/Univ. CO

16 Greenland is losing mass Estimated Greenland Ice Sheet mass balance changes since 1950 using three different methods (from Jiang, 2010).

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18 Greenland s contribution to sea level rise The boxes indicate the time period examined (the width) and uncertainty in the estimate (height). From Wu et al. [2010]. Total sea level rise is presently about 3.1 mm per year.

19 Over the satellite record there is a positive, albeit rather noisy positive trend in melt extent and related melt indices. There was a record melt (at the time) in Left: anomaly map of melting days for 2010 derived from passive microwave data. Hatched regions indicate where simulated meltwater production exceeds the mean by at least two standard deviations; Top Right: daily melt extent for the 2010 season, the average and the year 2007; Bottom Right: standardized melting index (the number of melting days times area subject to melting) by year from passive microwave data over the whole ice sheet and for different elevation bands [from Tedesco et al., 2011, Env. Res. Lett.]

20 Early July 2012: 97% of the ice sheet had experienced surface melt July 8, 2012 July 12,

21 The Jakobshavn glacier on the west coast and the Kangerdlugssuaq glacier on the east coast are two of the major glaciers draining the Greenland ice sheet. They have shown accelerated flow in recent years (Local ice caps and ice domes are shown in green. Ice-free areas are shown in dark gray).

22 Basal Lubrication Theory (the Zwally effect) Surface melt is increasing. This means more seasonal melt ponds. Melt ponds drain quickly to the base on the ice sheet via crevasses and moulins, which lubricates the base of the drainage glaciers, causing accelerated flow. Konrad Steffen, CIRES/Univ. CO More recent work: Back pressure effects are probably more important

23 Rapid retreat of Jacobshavn Glacier The rapidly retreating Jakobshavn Glacier in western Greenland drains the central ice sheet. This image shows the glacier in 2001, flowing from upper right to lower left. Terminus locations before 2001 were determined by surveys and more recent contours were derived from Landsat data. Thinning of the glacier leads to less back pressure, allowing for accelerated flow. Image courtesy NASA Earth Observatory

24 Back Pressure Issues Fiord walls and sills (bedrock protuberances at the glacier bed) exert a back pressure (friction) With a thinner glacier, not as much back pressure can be exerted by the walls As the glacier thins, it may float off the sills, again meaning less back pressure A thinner glacier exerts less pressure on its bed. For tidewater glaciers (glaciers that reach out into the sea, like Jacobshavn), the ocean water exerts a buoyancy effect, which becomes more important as the glacier thins. This leads to less back pressure (friction). Thinning occurs from the top (more melt) and, if the glacier is floating, from the bottom (warmer ocean waters), again yielding less back pressure. Thinning can act as a feedback thinning accelerates the flow, the acceleration leads to thinning and less back pressure. However, eventually, this can lead back to a new stable situation. This is the classic tidewater glacier cycle.

25 Polar desert Low precipitation Low annual mean temperature Generally high continentality

26 Distribution of Arctic polar desert (dark shading) and approximate southern limit of tundra (bold line) [adapted from Charlier, 1969, also see Webber, 1974, courtesy of N. Saliman, NSIDC, Boulder, CO].

27 Polar desert, Ellesmere Island Photos by C. Allen

28 Mean annual cycles of surface air temperature, precipitation, cloud cover and downwelling shortwave radiation for the polar desert site Resolute Bay, NWT, based on a number of different data sources [courtesy of M. Lavrakas, NSIDC, Boulder, CO]. Precipitation for even the wettest month (July) is less than 35 mm, and winter precipitation is very scant. February temperatures are below -30 deg. C.

29 Maritime Arctic Extensive cloudiness High relative humidity Small annual range in temperature

30 Svalbard a maritime Arctic site Photo by M. Serreze

31 Mean annual cycles of surface air temperature, precipitation, cloud cover and downwelling shortwave radiation for the maritime site Isfjord Radio (Svalbard),based on a number of different data sources [courtesy of M. Lavrakas, NSIDC, Boulder, CO]. Cloud cover is extensive year round. Precipitation is much more abundant compared to polar desert. The annual temperature range is quite small (only about 10 deg. C, compared to over 30 deg. C at Resolute)

32 Mean annual cycles of surface air temperature, precipitation, cloud cover and downwelling shortwave radiation for Barrow, AK, based on a number of different data sources [courtesy of M. Lavrakas, NSIDC, Boulder, CO]. While often categorized as a maritime site, Barrow has a large temperature range similar to polar desert sites.

33 Central Arctic Ocean Fairly large annual temperature range Summer temperatures close to freezing point Extensive low level stratus in summer Late summer/early autumn precip. maximum

34 The ice-covered central Arctic Ocean

35 Mean annual cycles of surface air temperature, precipitation, cloud cover and downwelling shortwave radiation for the central Arctic Ocean, based on data from the Russian North Pole program [courtesy of M. Lavrakas, NSIDC, Boulder, CO]. Due to the melting ice surface, the surface air temperature in July hovers near the freezing point. Cloud cover peaks in summer much of this is low-level Arctic stratus.