Greenland’s summer: The pressure is on, and off

Melting on the surface of the Greenland Ice Sheet in June and July 2014 has been well above the 1981 to 2010 average in most areas, but after a fast start in May, the southern region and the southeastern coast have seen lower-than-average melt. Mid-summer surface melting did not reach higher elevations (above 2000 meters) as often as in the reference period 1981 to 2010. Short bursts of extensive melting were related to periods of high air pressure over the ice sheet favoring sunny conditions, and promoting increased melting in darker areas of the ice sheet (wet snow, bare ice, or dirty snow).

Overview of conditions

Cumulative melt days and melt anomalies June July 2014

Figure 1. Cumulative days of surface melting (top pair) and anomalies in the number of melt days (bottom pair) for June and July, 2014 (left side and right side, respectively). Anomalies are compared to the period 1981 to 2010. Data are from the Greenland Daily Surface Melt 25km EASE-Grid 2.0 Climate Data Record. About the data

Credit: National Snow and Ice Data Center/Thomas Mote, University of Georgia
High-resolution image

Daily melt extent in June surged to nearly 40% of the ice surface area by mid-month and remained above the average extent for the 30-year reference period (1981 to 2010) for almost the entire month. Positive melt day anomalies were present in all areas except for the southeastern ice sheet. In July, melting was intense along the northwestern coast (more than 15 days above the average in some areas) and unusually low along the southeastern coast, especially near Helheim Glacier where July melt days were 3 to 8 days less than average. Overall, melting was less frequent than average in the high interior and southeastern areas of the ice sheet.

 

 

Conditions in context

surface melt and temperature graphs

Figure 2. Melt extent time-series for 2014 (top) and average air temperature anomaly at a level about 800 meters (2,500 feet, 925 hPa) above the sea level for June 1 to August 14, 2014. Melt extent data are from the Greenland Daily Surface Melt 25km EASE-Grid 2.0 Climate Data Record. Air temperature data are from the National Center for Environmental Prediction (NCEP) Reanalysis.

Top image credit: National Snow and Ice Data Center/Thomas Mote, University of Georgia
Bottom image credit: NOAA Earth System Research Laboratory, Physical Sciences Division
High-resolution image

Thus far, the maximum daily melt extent for summer 2014 was near 40% of the ice sheet surface on June 13. This was the first in a series of four warm periods followed by four periods of near-average conditions spanning the rest of June and July. Since August began, daily melt extent declined to near-average values for late summer. Summer air temperatures for June, July, and the first half of August mirrored the overall pattern of melt intensity, with cooler-than-average conditions about 1degree Celsius (1.8 degrees Fahrenheit) below normal in the high ice sheet plateau, and warmer-than-average conditions along the western coast, especially the southwestern coastal area at 1.5 degrees Celsius (2.7 degrees Fahrenheit) above average.

Models of the Greenland 2014 summer season produced by the MAR version 3.4 model of Xavier Fettweis from Liège University show that the overall snowfall-melt balance for Greenland  (the surface mass balance, or SMB) is very close to the 1981 to 2010 average. While 2013 total autumn snowfall was slightly higher than average, melting in the 2014 summer has now reduced the initial winter 2013 to 2014 snow accumulation surplus. However, despite the high total snow amount, northwestern Greenland had lower-than-normal winter snowfall accumulation. With the onset of a vigorous melt season in that area, the bright white snow cover was quickly removed, exposing darker ice below, and increasing the amount of melting as well as decreasing the ice sheet meltwater retention capacity in this area. This has produced a strong and statistically significant negative mass balance for this summer along Greenland’s northwestern coast.

Melting under pressure

graphs

Figure 3. Three six-day periods in the Greenland surface melt extent daily time-series plot outline periods of high and low melting during June and July. Below, surface air pressure anomaly plots for the same periods (A, B, and C) are shown. A and C are high-melt-extent periods, and show greater-than-average air pressure (2 to 6 millibars); the opposite is true for the low-melt period, B. The two other high-melt periods, June 15 to 20 and July 19 to 25, also show higher-than-average air pressure. Melt extent data are from the Greenland Daily Surface Melt 25km EASE-Grid 2.0 Climate Data Record. Air pressure data are from the National Center for Environmental Prediction (NCEP) Reanalysis.

Image credit: National Snow and Ice Data Center
High-resolution image

The four higher-melting periods of mid-June to late July and the intervening periods of more average melt extent appear to be associated with periods of high and low air pressure, respectively. Examining six-day intervals in air pressure reveals higher-than-average pressure dominates during the peaks in surface melt extent, and low pressure during the low-melt periods. High pressure is associated with clear skies, and therefore greater solar energy input to the surface snow, impacting mainly the low albedo zones (in particular the ablation zone). This explains why the melt extent has been abnormally high in the ablation zone and abnormally low in the higher-elevation snow accumulation zone. The melt rate in the accumulation zone is more sensitive to warm but cloudy days and the associated increase in long-wave radiation, due the high surface albedo in this zone. Moderate Resolution Imaging Spectroradiometer (MODIS) images of Greenland on June 13 and July 3 indeed show a mostly cloudless Greenland, whereas more cloud cover is present on June 30 and July 1 during the lower-melt-extent periods.

Sky selfie

ice camp photos

Figure 4. Images of the Greenland Ice Sheet near Kangerlugssuaq in west-central Greenland taken by a drone (UAV) used to evaluate the evolving albedo of the ice sheet surface during the summer melt season. At top left, Prof. Jason Box and Johnny Ryan, a Ph.D. student at Aberystwyth University, hold the drone they used. Top left, the drone takes a picture of the surface (and the operator, J. Ryan) on August 9, 2014 from low altitude, showing numerous cryoconite holes filled with black dust, grit, and soot that had accumulated in the winter snowpack, and melted out of the older ice below. Bottom, a higher-altitude image of the same area reveals sinuous melt streams and linear fractures, as well as small speckles of cryoconite holes on the ice sheet. Tents from the camp are also visible as colorful dots against the ice surface.

Credit: Photos courtesy of Johnny Ryan, Aberystwyth University, Jason Box, GEUS, and Dark Snow Project.
High-resolution image

Our colleague Jason Box of the Geological Survey of Denmark and Greenland (GEUS), and graduate student Johnny Ryan of Aberystwyth University spent much of the summer on the western ice sheet at Camp Dark Snow, near Kangerlugssuaq on the Arctic Circle (67 degrees north latitude at 1,010 meters above sea level). The team was investigating the Greenland surface albedo, climate, and surface melting, and how these evolve during summer. As part of the research, they have been using drones (Unmanned Aerial Vehicles, or UAVs) to photograph the surface from low altitude to examine the development of surface structures associated with melting. Strips of images and albedo measurements from the UAV are compared with simultaneous satellite images from the NASA MODIS sensor as an intermediate state to relate ground albedo measurements with that of the entire ice sheet. UAV photos reveal a surface riven with fractures, and drained by ephemeral rivers of melt water. The mid-summer melt surface in this area is pocked with 0.5 to 1 meter-wide (1.5 to 3 feet-wide) potholes with black grit and dust collected at the bottom. This black material is called cryoconite, and is comprised of dust and soot deposited on the surface, and melted out from the older ice exposed by melting. The dark patches are often glued together by tiny microbes.

Further reading

Fettweis, X., et al. 2014 Greenland ice sheet SMB simulated by MARv3.4 in real time.

Ryan, J.C., A.L. Hubbard, J. Todd, J.R. Carr, J.E. Box, P. Christoffersen, T.O. Holt, and N. Snooke, 2014, in review. Repeat UAV photogrammetry to assess calving front dynamics at a large outlet glacier draining the Greenland Ice Sheet. The Cryosphere Discussions 8, 2243-2275, doi:10.5194/tcd-8-2243-2014.

A warm southern welcome to spring

Surface melting on the Greenland Ice Sheet in May 2014 proceeded quickly, despite cool conditions over wide areas. We continue to explore recent evidence of lower snow reflectivity, and note its likely impact on snow melt during Greenland’s summer season.

Overview of conditions

Map of melt days

Figure 1. These images show the cumulative days of surface melting (left) and anomalies in the number of melt days (right) for May 2014 (31 days). Anomalies are compared to the period 1981 to 2010. Data are from the Greenland Daily Surface Melt 25km EASE-Grid 2.0 Climate Data Record. About the data

Credit: National Snow and Ice Data Center/Thomas Mote, University of Georgia
High-resolution image

The 2014 melt season began with a moderately fast start in the southernmost part of the Greenland Ice Sheet in May, on smaller, separate ice caps, and on snow-covered land in the far northeastern part of the island. The early pace of melting was below average for the western coast and most of the southeastern coast, but 4 to 8 days ahead of the normal pace in the far southern ice cap area near the capital of Nuuk.

Conditions in context

plot of surface melt extent

Figure 2a. The graph above shows the daily extent of melt during May 2014 on the Greenland Ice Sheet surface as a percentage. The 1981 to 2010 average is shown by a blue dashed line. The gray area around this average line shows the two standard deviation range of the data. Data are from the Greenland Daily Surface Melt 25km EASE-Grid 2.0 Climate Data Record. About the data

Credit: National Snow and Ice Data Center/Thomas Mote, University of Georgia
High-resolution image

Graph of temperature anomalies

Figure 2b. Mean air temperature for May at the 700 millibar level (about 10,000 feet or 3,200 meters altitude). Temperatures for the month were cool over the majority of Greenland.

Credit: National Snow and Ice Data Center, courtesy NOAA/ESRL Physical Sciences Division
High-resolution image

Melting extended over about 10% of Greenland during a brief mid-May warm period in the far south, and then climbed again as the month came to a close. As of this writing in mid-June 2014, melting has rapidly expanded to 30% of Greenland’s ice sheet, and at present favors the western and northern Greenland coast after little activity there in May. Greenland’s weather in May was characterized by cool conditions over the northern two-thirds of the island, but in the south, temperatures were approximately 1 degree Celsius warmer than average for the month, relative to the 1981 to 2010 reference period.

Higher temperatures and a period of light easterly and northeasterly winds in mid-May favored a rapid start to the melt season in southern Greenland. The pattern of melting on the ice sheet, and the timing of melt events, agrees well with the MAR model produced by X. Fettweis at Université de Liège.

 

 

Darkening snow

graph of albedo

Figure 3. This graph shows the mean albedo (reflectivity fraction) of the Greenland Ice Sheet for areas of the ice sheet with an elevation greater than 2,000 meters above sea level. The blue line and blue diamonds show albedo from satellite data. The black line and dots show albedo from a snow model with clean snow, and its evolution under observed summer May and June temperatures. Also shown is the difference between these estimates (red line and diamonds). Modified from Dumont et al., 2014.

Credit: M. Dumont/Météo-France–CNRS, courtesy Nature Geoscience
High-resolution image

A recent research paper by scientists at Météo-France and other French climate science institutions (Dumont et al., 2014) shows that a significant and abrupt shift in albedo took place in 2009. They hypothesize that the increase is due to a persistent increase in the amount of soot or dust in new-fallen snow over the island. This is based on satellite observations of the albedo by the Moderate-resolution Imaging Spectroradiometer (MODIS). When compared to models of the expected reflectivity of clean snow under the summer conditions, the recent satellite observations show that snow over Greenland is darker than can be explained by warm temperatures and coarser snow grains.

Samples of snow from Greenland in recent years favor the idea that dust is the main cause of the darkening. Other studies confirm that the darkening at high elevation (where the impact of a change in dry new-fallen snow would be significant) is about 2%. For clean new snow, a 2% decrease in reflectivity (for example, from 90% to 88%) represents a 15 to 20 percent increase in energy absorption (from 10% to 12%). With a somewhat darker, less pure incoming snowfall, the amount of energy from the sun that the snow absorbs increases, leading to earlier melt, more pronounced melt run-off, and later re-freezing of the ice sheet surface.

Lengthening melt

plot of melt extent

Figure 4. These graphics show trends in how the start (top image) and end (bottom image) of melt season are changing for Greenland. Trends are given in days change per decade. Positive values (red colors) mean that the event is happening later in the year; negative values (blue colors) mean that the event is happening earlier in the year. Data are from the Greenland Daily Surface Melt 25km EASE-Grid 2.0 Climate Data Record. About the data

Credit: National Snow and Ice Data Center/Thomas Mote, University of Georgia
High-resolution image

The trend identified by our colleagues in Figure 3 is underscored when we look at the trend in the date of the beginning of melt (melt onset, top image in Figure 4) and the date that the snow and ice finally re-freeze at the end of the season (freeze-up, lower image in Figure 4). Melt day data from 1978 to 2013 show a pronounced trend toward a longer melt season. Melt onset has started 4 to 12 days earlier per decade over this 35 year period (shown by the blue colors). A few areas at high elevation have shown later onset, perhaps due to increasing accumulation of snow at high elevation in Greenland. The end of the melt season, or freeze-up date, has pushed later into the autumn by 8 to 16 days per decade, particularly along the southwestern coast and east-central areas.

Further reading

Dumont, M., E. Brun, G. Picard, M. Michou, Q. Libois, J.-R. Petit, M. Geyer, S. Morin, and B. Josse. 2014. Contribution of light-absorbing impurities in snow to Greenland’s darkening since 2009. Nature Geoscience, doi:10.1038/NGEO2180.

Mote, T., and M. Anderson. 1995. Variations in snowpack melt on the Greenland ice sheet based on passive-microwave measurements. Journal of Glaciology, vol 41, pp 51-60.

2013 in review; 2014 melt begins

The Greenland Ice Sheet had a far more typical melt extent and intensity in 2013 than in 2012, when summer surface melting set a record, compared to satellite observations since 1978. After the normal winter hiatus, the 2014 melting season has now begun again along the southern Greenland coastal areas. We will review the early progress of the 2014 season in our next post, in mid-June.

Overview of conditions

Greenland melt anomaly images

Figure 1. These maps show melt anomalies, or how the number of melt days in each year compared to the average number of melt days as recorded by satellite observations from 1981 to 2010. While 2012 set records for melt extent (center), and 2011 showed strong melt anomalies along the coasts (left), 2013 melt days were within a more typical range, on average (right). Data are from the Greenland Daily Surface Melt 25km EASE-Grid 2.0 Climate Data Record. About the data

Credit: National Snow and Ice Data Center/Thomas Mote, University of Georgia
High-resolution image

Figure 1 shows the cumulative number of days that the Greenland Ice Sheet experienced surface melting during 2013 (right image), along with comparison images for 2011 and for 2012, the record year for melt days.

Overall, 2013 melt intensity, expressed as the number of melt days relative to the 1981 to 2010 average, was slightly to moderately higher than average in the southern and western Greenland Ice Sheet but unusually low along the northern and northeastern coastal areas.

In particular, surface melt did not extend to the higher-elevation interior regions in the north as much as has been typical for the 1981 to 2010 period. A narrow band along the eastern coastline showed significantly greater than average melting, but here as well the surface melt conditions did not extend inland and uphill as they have in recent years.

Conditions in context

Figure 2. The graph above shows the daily extent of melt during 2012 on the Greenland Ice Sheet surface as a percentage, compared to the average from 1981 to 2010. Data are from the Greenland Daily Surface Melt 25km EASE-Grid 2.0 Climate Data Record. About the data||Credit: National Snow and Ice Data Center/Thomas Mote, University of Georgia|High-resolution image

Figure 2. The graph above shows the daily extent of melt during 2013 on the Greenland Ice Sheet surface as a percentage. The 1981 to 2010 average is shown by a blue dashed line. The gray area around this average line shows the two standard deviation range of the data. Data are from the Greenland Daily Surface Melt 25km EASE-Grid 2.0 Climate Data Record. About the data

Credit: National Snow and Ice Data Center/Thomas Mote, University of Georgia
High-resolution image

In contrast to 2012, the extent of melt was also closer to average during 2013, compared to the period 1981 to 2010.

Extensive melting began slightly later than usual, toward the end of May, but increased rapidly to above-average levels by mid June. A brief reduction in late June and early July was followed by a late-season re-advance in melt area in late July.

 

2013 compared to previous years

Figure 3. Melt extent departure from the average for 1978 to 2013. The area represented by the bars is the sum of the daily melt extent for June, July, and August of each year, with the average subtracted. This highlights the trend in melt, and the scale of past anomalous years. ||Credit: National Snow and Ice Data Center/T. Mote, University of Georgia|  High-resolution image

Figure 3. Melt extent departure from the average for 1978 to 2013. The area represented by the bars is the sum of the daily melt extent for June, July, and August of each year, with the average subtracted. This highlights the trend in melt, and the scale of past anomalous years.

Credit: National Snow and Ice Data Center/T. Mote, University of Georgia
High-resolution image

The 2013 summer in Greenland also saw a reversal of the recent trend in summertime loss of surface snow and ice mass by run-off, as would be expected given the reduced melting. Figure 3 illustrates the relative melt area departure from the average (sum of the daily melt areas over the ice sheet for June, July, and August in each year, with the average area for 1978 to 2013 subtracted). The very large increase in 2012 is clearly shown, as is the return during 2013 to conditions typical of the late 1990s.

Climate conditions during 2013

Graph of NAO

Figure 4. This graph shows the North Atlantic Oscillation Index (NAO) for June through August, for the period 1950 to 2013 (blue dashed line) and running 5-year average (red line). 2013 saw a marked difference from recent years, with average conditions similar to the 1990s and earlier.

Credit: Xavier Fettweis, University of Liège
High-resolution image

Figure 5.

Figure 5. This plot shows air pressure anomalies (left) and air temperature anomalies (right) at the 700 mb level for June to August 2013.

Credit: Courtesy NOAA/ESRL Physical Sciences Division
High-resolution image

Figure 7.

Figure 6. This graph shows anomalies for June through August for total meltwater runoff (blue), snowfall accumulation (red), and net surface mass balance (SMB, green) over the Greenland Ice Sheet from 1960 to 2013, compared to the period 1980 to 1999. The data are from the Modèle Atmosphérique Régional (MAR model), v3.3, and are in gigatons per year. MAR was run at a resolution of 15 kilometers and forced at its lateral boundaries by the ERA-40 reanalysis over 1958-1978 and ERA-Interim afterward.

Credit: Xavier Fettweis, University of Liège
High-resolution image

Weather patterns were significantly different over Greenland during 2013 compared to 2012,  when high temperatures led to extensive melt. A dominant Arctic climate pattern, the North Atlantic Oscillation (Figure 4), was in its positive phase for the summer months (June through August) of 2013, sharply contrasting with a trend that had held for the previous six summers. As discussed in NSIDC Arctic Sea Ice News and Analysis on September 17, 2013, the positive phase of the NAO favors anticyclonic circulation over Greenland. The NAO generally produces warm and dry conditions over Europe and is associated with cooler and higher-precipitation conditions in Greenland and the central Arctic.

Consistent with this, lower-than-average air pressures (Figure 5, left) were observed over Greenland during the 2013 summer, as well as lower temperatures (Figure 5, right). Air temperatures over Greenland were 0.5 to 2 degrees Celsius (1 to 3.6 degrees Fahrenheit) below the 1981 to 2010 averages. Precipitation was also higher in summer 2013.

Figure 6 shows the pattern of total snowfall anomaly (the amount of snowfall relative to the average amount for 1980 to 1999); the meltwater runoff anomaly (the amount of mass lost to water runoff relative to the 1980 to 1999 average); and the net balance between snowfall and runoff (and evaporation of snow, a minor component of loss) called the surface mass balance (SMB), as calculated by the MAR regional climate model. Highlighted in this graphic are the major ice mass losses of the twelve years preceding 2013. Note that this accounting does not include ice that flows directly into the sea by glacier movement.

Pale by comparison

albedo image

Figure 7. This map shows average summer albedo anomaly for 2013 versus the 2000 to 2011 average, determined by satellite mapping. Data are from the NASA Terra satellite and the MODIS sensor (Moderate Resolution Imaging Spectroradiometer). The data set is the MOD10A1 collection, available from NSIDC. MODIS MOD10A1 data.

Credit: National Snow and Ice Data Center/Jason Box, Geological Survey of Denmark and Greenland (GEUS)
High-resolution image

During summer 2013, the albedo of the Greenland ice sheet surface was higher along the coastal part of the ice sheet than in recent years, indicating less wet snow and snow-free bare ice areas (ablation areas) along the ice sheet perimeter.

Reflectivity is highly variable on an ice sheet: dry snow is the brightest surface cover, above 85% reflectivity, (or, expressed on the 0 to 1 scale, albedo); wet snow has an albedo of about 0.6 to 0.8; coarse wet snow and slush is about 0.4 to 0.5; impurity rich bare ice with no snow cover has an albedo of 0.3; and clear deep water is near 0.10. As fresh snow ages, the grains become coarser and the albedo drops by as much as 10%. Melting in the snowpack has an even more dramatic effect, lowering reflectivity by up to 20%.

With less exposed bare ice near the coasts, the solar reflectance of these regions is higher than the recent average. In the interior, during 2013 summer snowfall events were widely spaced in time over the ice sheet interior, and so the snow surface tended to be older and therefore a bit darker than average.  

More to come

sample images

Figure 8. These images show the new derivative data sets for the Mote Greenland Daily Surface Melt 25km EASE-Grid 2.0 Climate Data Record data set. The lower left and diagonal panels show average annual and average monthly melt days extent maps, respectively, for the 1981 to 2010 climatology period melt season months April through September. The upper right image shows daily melt extent for 2012, including the two standard deviation range for the 1981 to 2010 period.

High-resolution image

Our next post will examine the early progress of the melt season. In the interim, new supporting data sets and analysis tools have been derived from the melt extent archives that will present a more complete picture of the melt season in 2014.

Several new data sets and graphic upgrades were generated since late September 2013. These changes are summarized in Figure 8 and include an analysis of a 30-year record of daily melt extent spanning the climatological reference period 1981 to 2010, as measured by the Mote melt algorithm (Mote, 2007). The 1981 to 2010 average is shown as a blue dashed line, and the gray area around this average line shows the two standard deviation range of the data. Annual and monthly average melt day maps were also generated, allowing an assessment of the impact of weather events and the trends of melt extent and intensity in various areas.

Resources for analysis of trends and variations for the Greenland Today website will continue to expand as funding permits. We aim to build an interactive analysis tool similar to our Sea Ice Index web pages and to make daily data available as with the Sea Ice News and Analysis web pages. We are presently working on a table of results for the melt days and extents.

References

Fettweis, X., 2007. Reconstruction of the 1979-2006 Greenland ice sheet surface mass balance using the regional climate model MAR. The Cryosphere, 1, 21-40, doi:10.5194/tc-1-21-2007.  See also: https://www.aoncadis.org/dataset/CPL_MAR.html.

Hall, D. K., V. V. Salomonson, and G. A. Riggs. 2006. MODIS/Terra Snow Cover Daily L3 Global 500m Grid. Version 5. Boulder, Colorado USA: National Snow and Ice Data Center.

Mote, T. L., 2007. Greenland surface melt trends 1973–2007: Evidence of a large increase in 2007. Geophysical Research Letters 34, L22507, doi:10.1029/ 2007GL031976.

Acknowledgements

We would like to thank Xavier Fettweis of the University of Liege, Belgium, and Jason Box of the Geological Survey of Denmark and Greenland (GEUS) for their contributions to this post.

 

 

 

Melt calibration, suspension of daily images

The Greenland melt detection algorithm is currently undergoing its annual calibration period. As a result, the daily melt extent mapping image is temporarily suspended. Calibration of the melt detection for each year requires analysis of the springtime snow conditions by a separate program. See our March 13, 2013 post for more discussion of melt calibration.

We will resume the daily image updates in April. A consistent record will be produced later that spans these winter periods retrospectively.

UPDATE, April 30, 2014: We are working to resume image updates by early May. Thank you for your patience.

An early spring re-calibration for melt detection

The algorithm for the Greenland Ice Sheet Today daily melt extent has been revised to account for unusually warm winter snow layers and residual meltwater deep in the snow. Meltwater from last summer’s intense melt season did not completely re-freeze through at least mid December. The adjusted algorithm shows greatly reduced melt extent for early 2013. This much lower extent is more consistent with available weather and climate records.

Melt extent and distribution

Figure 1. These images show cumulative melt extent before the algorithm correction (left) and after the correction (right). A few areas indicating one to two days of melting in southeast Greenland remain in the revised map. The red dot shows the location of the Danish AWS.

Credit: National Snow and Ice Data Center
High-resolution image

As shown in Figure 1, the adjustment to the algorithm resulted in fewer melt days than previously indicated. The revised image at right shows new surface melting in 2013 in a few small areas along the central southeastern Greenland coast, within the region of earlier spurious melt signals but greatly reduced.

Conditions in context

Figure 2. This image shows air temperature anomaly for Greenland for the period December 2012 to February 2013. Reds and oranges indicate higher than average air temperatures. The temperatures shown are at approximately 1,500 meters (5,000 feet) in altitude, appropriate for coastal Greenland regions. However, central Greenland is above this altitude, and values shown there do not represent the true surface conditions well.

Credit: National Snow and Ice Data Center courtesy NOAA ESRL Physical Sciences Division
High-resolution image

Temperatures in Greenland have been higher than average this winter, with air temperatures near the coast averaging 2.0 to 3.5 degrees Celsius (4 to 6 degrees Fahrenheit) warmer than the 1981 to 2010 average. This has in part been a result of the persistent circulation pattern for the Arctic this winter, characterized by a negative Arctic Oscillation (AO). The AO is a measure of the intensity of the general pattern of low pressure over the northern high latitudes. A negative AO indicates higher-than-average pressures near the North Pole, allowing more frequent southward cold air outbreaks, and more intrusions of warm air masses from higher temperature areas. Despite these anomalously high temperatures along the Greenland coast, temperatures were not high enough to result in melting.

Adjusted algorithm and melt images

Figure 3. This plot shows surface air temperature at a PROMICE on-ice Automated Weather Station (AWS) near the southeastern Greenland ice sheet edge for early 2013. Temperatures did not exceed freezing at this site. Data from PROMICE were provided by the Geological Survey of Denmark and Greenland (GEUS) and are freely available.

Credit: National Snow and Ice Data Center and J. Box, Geological Survey of Denmark and Greenland
High-resolution image

The melt extent algorithm used by Greenland Ice Sheet Today has been overestimating the melt extent, and as a result, daily images posted on this site in February and March may have indicated melt where none occurred. While the algorithm was indicating some coastal melt in February and early March, a comparison with weather data for Nuuk (the Greenland capital city, located along the southwest of the island) and data from the Programme for Monitoring of the Greenland Ice Sheet (PROMICE) suggested these might be spurious melt readings. The local Automated Weather Station (AWS) data from a glacier along the southeastern coast (the Mittivakkat glacier AWS, shown by a red dot in Figure 1; data in Figure 3) indicate that the air temperature did not rise to the melting point (0 degrees Celsius, or 32 degrees Fahrenheit) in February or early March.

Figure 4. A model of the snowpack conditions indicates residual liquid water in the deep snowpack in southeastern Greenland.

Credit: National Snow and Ice Data Center and X. Fettweis, Université de Liège, Belgium
High-resolution image

During this period, starting around mid-February in southeast Greenland, the brightness temperatures in the upper few meters of the snowpack were 2 to 10 degrees Celsius (4 to 18 degrees Fahrenheit) higher than those observed during any other year in the 34-year record (1979 to 2012). While surface melt is not unprecedented in Greenland near the coast in February and March, the totals posted prior to March 14 were a result of these unusual snow temperature conditions, and not ongoing surface melt. This winter has seen unusually warm snow at depth on the ice sheet, following the intense melting that occurred last summer.

The melt detection method, based on passive microwave emissions, is primarily sensitive to near-surface conditions, but has some input from the snowpack down several meters (10 to 20 feet). Heavy snow fell during the relatively warm winter, burying and insulating deeper snow. This contributed to anomalously high temperatures for the uppermost layers of snow this winter. Additionally, models based on snowpack properties suggested that some 2012 meltwater remained unfrozen at 5 meters depth (approximately 16 feet) in mid-December. The model results are consistent with observations from JAXA’s AMSR-2 sensor.

The algorithm was adjusted by combining the trend of observed brightness temperatures with a model of the expected microwave emission in the channels used for melt detection (the SSM/I sensor’s 37 GHz Horizontal polarization channel). This adjustment is generally performed every year in March to calibrate the melt detection thresholds. However, because of the unusual condition of the snowpack, the adjustment needed to be made much earlier than ever before.

Further information

Fettweis, X., M. Tedesco, M. van den Broeke, and J. Ettema, 2011. Melting trends over the Greenland ice sheet (1958-2009) from spaceborne microwave data and regional climate models. The Cryosphere 5, 359-375, doi: 10.5194/tc-5-359-2011.

Programme for Monitoring of the Greenland Ice sheet (PROMICE)