Jason E. Box

Mass balance of the Greenland ice sheet from 1958 to 2007

We combine estimates of the surface mass balance, SMB, of the Greenland ice sheet for years 1958 to 2007 with measurements of the temporal variability in ice discharge, D, to deduce the total ice sheet mass balance. During that time period, we find a robust correlation (R2 = 0.83) between anomalies in SMB and in D, which we use to reconstruct a continuous series of total ice sheet mass balance. We. find that the ice sheet was losing 110 ± 70 Gt/yr in the 1960s, 30 ± 50 Gt/yr or near balance in the 1970s-1980s, and 97 ± 47 Gt/yr in 1996 increasing rapidly to 267 ± 38 Gt/yr in 2007. Multi-year variations in ice discharge, themselves related to variations in SMB, cause 60 ± 20% more variation in total mass balance SMB, and therefore dominate the ice sheet mass budget. Copyright 2008 by the American Geophysical Union.

Surface climatology of the Greenland Ice Sheet: Greenland Climate Network 1995–1999

The Greenland climate network has currently 18 automatic weather stations (AWS) distributed in most climate regions of the ice sheet. The present network captures well the regional climates and their differences in the accumulation region of the ice sheet. An annual mean latitudinal temperature gradient of −0.78°C/1° latitude was derived from the AWS data for the western slope of the ice sheet, and an annual mean latitudinal temperature gradient of −0.82°C/1° latitude was derived for the eastern slope. The mean annual lapse rate along the surface slope is 0.71°C/100 m, with monthly mean lapse rates varying between 0.4°C/100 m in summer and 1.0°C/100 m in winter. The annual range of monthly mean temperatures is between 23.5°C and 30.3°C for the western slope of the ice sheet, with increasing ranges from south to north and with increase in elevation. The annual mean air temperature was found to be 2°C warmer for the central part of Greenland for the time period 1995–1999, as compared to the standard decade 1951–1960. This annual mean temperature change decreased to approximately 1°C for the elevation 1000–2000 m, whereas at lower elevations, no AWS data are available with sufficient spatial and temporal coverage to verify any temperature trend. Firn temperatures (10-m depth) at high-elevation sites were found to be colder than the mean annual air temperature of the preceding year for the central part and northern Greenland by as much as 2.5°C. In the percolation zone and at the equilibrium line altitude the firn and ice temperatures at 10 m were consistently warmer than the annual mean air temperature because of percolation of meltwater and the isolation effect of the snow cover. The wind speed and direction are affected by the katabatic outflow of the cold air along the slope of the ice sheet, whereas at higher elevations the large-scale synoptic condition is the dominant factor that governs the wind field. The surface height change at high elevations (accumulation minus sublimation) can be approximated with a linear model over an annual cycle using AWS data, whereas in the ablation region and along the equilibrium line altitude the surface height change shows a strong annual cycle.

Greenland Ice Sheet Surface Mass Balance Variability (1988–2004) from Calibrated Polar MM5 Output*

Regional climate model runs using the fifth-generation Pennsylvania State University–National Center for Atmospheric Research Mesocale Model modified for use in polar regions (Polar MM5), calibrated by independent in situ observations, demonstrate coherent regional patterns of Greenland ice sheet surface mass balance (SMB) change over a 17-yr period characterized by warming (1988–2004). Both accumulation and melt rates increased, partly counteracting each other for an overall negligible SMB trend. However, a 30% increase in meltwater runoff over this period suggests that the overall ice sheet mass balance has been increasingly negative, given observed meltwater-induced flow acceleration. SMB temporal variability of the whole ice sheet is best represented by ablation zone variability, suggesting that increased melting dominates over increased accumulation in a warming scenario. The melt season grew in duration over nearly the entire ablation zone by up to 40 days, 10 days on average. Accumulation area ratio decreased by 3%. Albedo reductions are apparent in five years of the Moderate Resolution Imaging Spectroradiometer (MODIS) derived data (2000–04). The Advanced Very High Resolution Radiometer (AVHRR)-derived albedo changes (1988–99) were less consistent spatially. A conservative assumption as to glacier discharge and basal melting suggests an ice sheet mass loss over this period greater than 100 km 3 yr 1 , framing the Greenland ice sheet as the largest single glacial contributor to recent global sea level rise. Surface mass balance uncertainty, quantified from residual random error between model and independent observations, suggests two things: 1) changes smaller than approximately 200 km 3 yr 1 would not satisfy conservative statistical significance thresholds (i.e., two standard deviations) and 2) although natural variability and model uncertainty were separated in this analysis, the magnitude of each were roughly equivalent. Therefore, improvements in model accuracy and analysis of longer periods (assuming larger changes) are both needed for definitive mass balance change assessments.

Characteristics of Arctic synoptic activity, 1952–1989

SummarySynoptic activity for the Arctic is examined for the period 1952–1989 using the National Meteorological Center sea level pressure data set. Winter cyclone activity is most common near Iceland, between Svalbard and Scandinavia, the Norwegian and Kara seas, Baffin Bay and the eastern Canadian Arctic Archipelago; the strongest systems are found in the Iceland and Norwegian seas. Mean cyclone tracks, prepared for 1975–1989, confirm that winter cyclones most frequently enter the Arctic from the Norwegian and Barents seas. Winter anticyclones are most frequent and strongest over Siberia and Alaska/Yukon, with additional frequency maxima of weaker systems found over the central Arctic Ocean and Greenland.During summer, cyclonic activity remains common in the same regions as observed for winter, but increases over Siberia, the Canadian Arctic Archipelago and the Central Aretic, related to cyclogenesis over northern parts of Eurasia and North America. Eurasian cyclones tend to enter the Aretic Ocean from the Laptev Sea eastward to the Chukchi Sea, augmenting the influx of systems from the Norwegian and Barents seas. The Siberian and Alaska/Yukon anticyclone centers disappear, with anticyclone maxima forming over the Kara, Laptev, East Siberian and Beaufort seas, and southeastward across Canada. Summer cyclones and anticyclones exhibit little regional variability in mean central pressure, and are typically 5–10 mb weaker than their winter counterparts.North of 65°N, cyclone and anticyclone activity peaks curing summer, and is at a minimum during winter. Trends in cyclone and anticyclone activity north of 65°N are examined through least squares regression. Since 1952, significant positive trends are found for cyclone numbers during winter, spring and summer, and for anticyclone numbers during spring, summer and autumn.

Mesoscale Modeling of Katabatic Winds over Greenland with the Polar MM5

Abstract Verification of two months, April and May 1997, of 48-h mesoscale model simulations of the atmospheric state around Greenland are presented. The simulations are performed with a modified version of The Pennsylvania State University–National Center for Atmospheric Research fifth-generation Mesoscale Model (MM5), referred to as the Polar MM5. Global atmospheric analyses as well as automatic weather station and instrumented aircraft observations from Greenland are used to verify the forecast atmospheric state. The model is found to reproduce the observed atmospheric state with a high degree of realism. Monthly mean values of the near-surface temperature and wind speed predicted by the Polar MM5 differ from the observations by less than 1 K and 1 m s−1, respectively, at most sites considered. In addition, the model is able to simulate a realistic diurnal cycle for the surface variables, as well as capturing the large-scale, synoptically forced changes in these variables. Comparisons of modeled profil...

The role of albedo and accumulation in the 2010 melting record in Greenland

Analyses of remote sensing data, surface observations and output from a regional atmosphere model point to new records in 2010 for surface melt and albedo, runoff, the number of days when bare ice is exposed and surface mass balance of the Greenland ice sheet, especially over its west and southwest regions. Early melt onset in spring, triggered by above-normal near-surface air temperatures, contributed to accelerated snowpack metamorphism and premature bare ice exposure, rapidly reducing the surface albedo. Warm conditions persisted through summer, with the positive albedo feedback mechanism being a major contributor to large negative surface mass balance anomalies. Summer snowfall was below average. This helped to maintain low albedo through the 2010 melting season, which also lasted longer than usual.

Evaluation of Polar MM5 simulations of Greenland's atmospheric circulation

A complete annual cycle over the Greenland ice sheet is simulated with the Polar MM5, a mesoscale model optimized for use over extensive ice sheets. These simulations are compiled from a series of short duration (48 hour), forecast mode, simulations. The model output is compared to observations primarily from the Greenland Climate Network automatic weather station (AWS) array. The model simulations show a high degree of skill for all variables evaluated with the AWS data (pressure, temperature, water vapor mixing ratio, wind speed and direction, downwelling shortwave radiation, and net radiation) for all seasons, although the use of a fixed albedo in the Polar MM5 leads to large errors in the simulated net radiation budget over melting ice surfaces during the summer months. The modeled precipitation distribution agrees with available observations in the interior of the ice sheet but is excessive along the steep margins of the island. A discussion of possible future applications of the Polar MM5 is presented.

Seasonal variability in the dynamics of marine-terminating outlet glaciers in Greenland

Recent studies indicate that the dynamics of fast-flowing, marine-terminating outlet glaciers of the Greenland ice sheet may be sensitive to climate and ocean forcing on sub-annual timescales. Observations of seasonal behavior of these glaciers at such high temporal resolution, however, are currently few. Here we present observations of front position, flow speed, near-surface air temperature and ocean conditions for six large marine-terminating glaciers in the Uummannaq region of West Greenland, to investigate controls on short-term glacier dynamics. As proposed by other studies, we find that seasonal front advance and retreat correlates with the formation and disappearance of an ice melange. Our data suggest that high sea-surface temperature, anomalously low sea-ice concentration and reduced melange formation in early 2003 have triggered multi-year retreat of several glaciers in the study area, which is consistent with other regions in Greenland. Of the stable glaciers, only Rink Isbrae exhibits a seasonal speed variation that correlates with variations in front position, with the others undergoing mid-summer deceleration that indicates the effects of subglacial meltwater discharge and drainage system evolution. Drainage of supraglacial lakes and water-filled crevasses results in substantial decreases in speed (40-60%) on fast-flowing glaciers. Our results demonstrate that attempts to model ice-sheet evolution must take into account short-timescale flow dynamics resulting from drainage events and oceanographic conditions.

Greenland ice sheet surface air temperature variability: 1840-2007.

Meteorological station records and regional climate model output are combined to develop a continuous 168-yr (1840-2007) spatial reconstruction of monthly, seasonal, and annual mean Greenland ice sheet near-surface air temperatures. Independent observations are used to assess and compensate for systematic errors in the model output. Uncertainty is quantified using residual nonsystematic error. Spatial and temporal temperature variability is investigated on seasonal and annual time scales. It is found that volcanic cooling episodes are concentrated in winter and along the western ice sheet slope. Interdecadal warming trends coincide with an absence of major volcanic eruptions. Year 2003 was the only year of 1840-2007 with a warm anomaly that exceeds three standard deviations from the 1951-80 base period. The annual whole ice sheet 1919-32 warming trend is 33% greater in magnitude than the 1994-2007 warming. The recent warming was, however, stronger along western Greenland in autumn and southern Greenland in winter. Spring trends marked the 1920s warming onset, while autumn leads the 1994-2007 warming. In contrast to the 1920s warming, the 1994-2007 warming has not surpassed the Northern Hemisphere anomaly. An additional 1.08-1.5 degrees C of annual mean warming would be needed for Greenland to be in phase with the Northern Hemispheric pattern. Thus, it is expected that the ice sheet melt rates and mass deficit will continue to grow in the early twenty-first century as Greenlands climate catches up with the Northern Hemisphere warming trend and the Arctic climate warms according to global climate model predictions.

Remote sounding of Greenland supraglacial melt lakes: implications for subglacial hydraulics

A supraglacial lake-depth retrieval function is developed, based on the correspondence between moderate-resolution imaging spectroradiometer (MODIS) reflectance and water depth measured during raft surveys. Individual lake depth, area and volume statistics, including short-term temporal changes for Greenlands southwestern ablation region, were compiled for 2000-05. The maximum area of an individual lake was found to be 8.9 km 2 , the maximum volume 53.0 � 10 6 m 3 and the maximum depth 12.2 m, sampling over 0.0625 km 2 pixel areas. The total lake volume reaches >1 km 3 in this region by July each year. The importance of melt lake reservoirs to Greenland ice-sheet flow may be a feedback between abrupt lake drainage events and ice dynamics. Lake-outburst volumes up to 31.5 � 10 6 m 3 d -1 are capable of providing sufficient water via moulins to hydraulically pressurize the subglacial environment. Since the overburden pressure at the base of a flooded moulin is greater than that provided by ice, lake-outburst events seem capable of exerting sufficient upward force to lift the ice sheet locally, if water flow in the subglacial environment is constrained laterally. Considering a moulin with a 10 m 2 cross-sectional area, basal pressurization can be maintained over lake-outburst