Ice Sheet Surface Research Articles

Thermobaric Conditions at Ice-Water Interface in Subglacial Lake Vostok, East Antarctica

Thermobaric conditions of subglacial Antarctic environment remain poorly understood, despite recent advances in radar and seismic surveying. The direct accessing to the largest subglacial lake, Lake Vostok, was carried out twice by Russian scientists in February 2012 and January 2015, opening new opportunities for assessing the thermobaric conditions at ice-water interface. According to the assumption that ice sheet is “floating” on the lake, it was predicted that the water would rise 30 - 40 m in the bottom part of the borehole, but in fact the water rose from the lake to a height of more than 500 m. To explain this phenomenon we assume that the pressure in Lake Vostok results from the external pressure of the entire mass of ice above it and the pressure of the water column that is overlaid above the point being considered. Extrapolation of temperature measurements from the deep bore-holes drilled at Vostok Station also confirmed that the bed of the ice sheet is at pressure melting point. As a result of accessing Lake Vostok, the pressure in the lake is reduced that would lead to the formation of a new additional layer of accretion ice on the lower ice sheet surface.

The effects of dating uncertainties on net accumulation estimates from firn cores

AbstractThe mean, trend and variability of net snow accumulation in firn cores are often used to validate model output, develop remote-sensing algorithms and quantify ice-sheet surface mass balance. Thus, accurately defining uncertainties associated with these in situ measurements is critical. In this study, we apply statistical simulation methods to quantify the uncertainty in firn-core accumulation data due to the uncertainty in depth–age scales. The methods are applied to a suite of firn cores from central West Antarctica. The results show that uncertainty in depth–age scales can give rise to spurious trends in accumulation that are the same order of magnitude as accumulation trends reported in West Antarctica. The depth–age scale uncertainties also significantly increase the apparent interannual accumulation variability, so these uncertainties must first be accounted for before using firn-core data to assess such processes as small-spatial-scale variability. Better quantification of error in accumulation will improve our ability to meaningfully compare firn-core data across different regions of the ice sheet, and provide appropriate targets for calibration and/or validation of model output and remote-sensing data.

Response times of ice-sheet surface heights to changes in the rate of Antarctic firn compaction caused by accumulation and temperature variations

AbstractVariations in accumulation rate As(t) and temperature Ts(t) at the surface of firn cause changes in the rate of firn compaction (FC) and surface height H(t) that do not involve changes in mass, and therefore need to be accounted for in deriving mass changes from measured H(t). As the effects of changes in As(t) and Ts(t) propagate into the firn, the FC rate is affected with a highly variable and complex response time. The H(t) during measurement periods depend on the history of As(t) and Ts(t) prior to the measurements. Consequently, knowledge of firn response times to climate perturbations is important to estimate the required length of the time series of As(t) and Ts(t) used in FC models. We use our numerical FC model, which is time-dependent on both temperature and accumulation rate, to examine the response times of both H(t) and the rates of change dH(t)/dt to variations in As(t) and Ts(t) using sample perturbations and climate data for selected sites in Antarctica. Our results show that the response times for dH(t)/dt, which are of particular interest, are much shorter than the responses of H(t). Typical response times of dH(t)/dt are from several years to <20 years. The response times are faster in warmer and higher-accumulation areas such as Byrd Station, West Antarctica (4 years), and slower in colder and lower-accumulation areas such as Vostok, East Antarctica (18 years). The response times to temperature are much faster (0.9 year at Byrd and 2.2 years at Vostok), but the corresponding height changes persist much longer. The associated variations in firn density are significantly preserved in the density–depth profiles. For typical fluctuations of surface weather, the Ts(t) from satellite observations since 1982 and As(t) from meteorological data since 1979 are essentially of sufficient length to correct for FC height changes for measurements beginning in 1992.

Antarctic firn compaction rates from repeat-track airborne radar data: I. Methods

AbstractWhile measurements of ice-sheet surface elevation change are increasingly used to assess mass change, the processes that control the elevation fluctuations not related to ice-flow dynamics (e.g. firn compaction and accumulation) remain difficult to measure. Here we use radar data from the Thwaites Glacier (West Antarctica) catchment to measure the rate of thickness change between horizons of constant age over different time intervals: 2009–10, 2010–11 and 2009–11. The average compaction rate to ~25 m depth is 0.33 m a−1, with largest compaction rates near the surface. Our measurements indicate that the accumulation rate controls much of the spatio-temporal variations in the compaction rate while the role of temperature is unclear due to a lack of measurements. Based on a semi-empirical, steady-state densification model, we find that surveying older firn horizons minimizes the potential bias resulting from the variable depth of the constant age horizon. Our results suggest that the spatio-temporal variations in the firn compaction rate are an important consideration when converting surface elevation change to ice mass change. Compaction rates varied by up to 0.12 m a−1 over distances <6 km and were on average >20% larger during the 2010–11 interval than during 2009–10.

Assessing spatio-temporal variability and trends in modelled and measured Greenland Ice Sheet albedo (2000–2013)

Abstract. Accurate measurements and simulations of Greenland Ice Sheet (GrIS) surface albedo are essential, given the role of surface albedo in modulating the amount of absorbed solar radiation and meltwater production. In this study, we assess the spatio-temporal variability of GrIS albedo during June, July, and August (JJA) for the period 2000–2013. We use two remote sensing products derived from data collected by the Moderate Resolution Imaging Spectroradiometer (MODIS), as well as outputs from the Modèle Atmosphérique Régionale (MAR) regional climate model (RCM) and data from in situ automatic weather stations. Our results point to an overall consistency in spatio-temporal variability between remote sensing and RCM albedo, but reveal a difference in mean albedo of up to ~0.08 between the two remote sensing products north of 70° N. At low elevations, albedo values simulated by the RCM are positively biased with respect to remote sensing products by up to ~0.1 and exhibit low variability compared with observations. We infer that these differences are the result of a positive bias in simulated bare ice albedo. MODIS albedo, RCM outputs, and in situ observations consistently indicate a decrease in albedo of −0.03 to −0.06 per decade over the period 2003–2013 for the GrIS ablation area. Nevertheless, satellite products show a decline in JJA albedo of −0.03 to −0.04 per decade for regions within the accumulation area that is not confirmed by either the model or in situ observations. These findings appear to contradict a previous study that found an agreement between in situ and MODIS trends for individual months. The results indicate a need for further evaluation of high elevation albedo trends, a reconciliation of MODIS mean albedo at high latitudes, and the importance of accurately simulating bare ice albedo in RCMs.

Ice sheet surface lineaments as nonconventional indicators of East Antarctica bedrock tectonics

A recent focus of major international exploration in East Antarctica has been aimed at revealing its bedrock topography and imaging its tectonic architecture and evolution. Here we present the tectonic interpretation of regional-scale lineaments revealed by the Radarsat mosaic of Antarctica on the ice sheet surface in the Vostok–Dome C–Adventure Basin region. These lineaments appear in the radar backscatter textures as alignments of marked tonal variations with lengths of tens to hundreds of kilometers and were identified using an automated methodology. We explore the origin scenarios for the ice sheet surface lineaments by comparing their azimuthal trends and spatial distribution with the main morphotectonic features of the bedrock. Azimuthal analysis reveals that lineaments cluster around two preferential directions interpreted as structural or tectonic domains. These show strong correlations with azimuths of tectonic fabrics in the bedrock. The main lineament domain parallels the morphotectonic features of the study area, namely the Adventure Basin and the Concordia and Aurora Trenches. The second lineament set corresponds to the mean orientation of the Lake Vostok depression. The spatial analyses of the two lineament domains strengthen our findings and interpretations. Comparisons with wind and ice flow directions exclude their influence on the identified lineament pattern. Results reveal the tectonic origin of the lineament domains, and demonstrate the method’s usefulness as a tool for tectonic studies of regions characterized by thick covers. These regions include other areas of the East Antarctic craton such as the Gamburtsev Subglacial Mountains, as well as deserts or surfaces of other planets.

Sensitivity of Greenland Ice Sheet surface mass balance to perturbations in sea surface temperature and sea ice cover: a study with the regional climate model MAR

Abstract. During recent summers (2007–2012), several surface melt records were broken over the Greenland Ice Sheet (GrIS). The extreme summer melt resulted in part from a persistent negative phase of the North Atlantic Oscillation (NAO), favoring warmer atmospheric conditions than normal over the GrIS. Simultaneously, large anomalies in sea ice cover (SIC) and sea surface temperature (SST) were observed in the North Atlantic, suggesting a possible connection. To assess the direct impact of 2007–2012 SIC and SST anomalies on GrIS surface mass balance (SMB), a set of sensitivity experiments was carried out with the regional climate model MAR forced by ERA-Interim. These simulations suggest that perturbations in SST and SIC in the seas surrounding Greenland do not considerably impact GrIS SMB, as a result of the katabatic wind blocking effect. These offshore-directed winds prevent oceanic near-surface air, influenced by SIC and SST anomalies, from penetrating far inland. Therefore, the ice sheet SMB response is restricted to coastal regions, where katabatic winds cease. A topic for further investigation is how anomalies in SIC and SST might have indirectly affected the surface melt by changing the general circulation in the North Atlantic region, hence favoring more frequent warm air advection towards the GrIS.

The role of atmospheric rivers in anomalous snow accumulation in East Antarctica

AbstractRecent, heavy snow accumulation events over Dronning Maud Land (DML), East Antarctica, contributed significantly to the Antarctic ice sheet surface mass balance (SMB). Here we combine in situ accumulation measurements and radar‐derived snowfall rates from Princess Elisabeth station (PE), located in the DML escarpment zone, along with the European Centre for Medium‐range Weather Forecasts Interim reanalysis to investigate moisture transport patterns responsible for these events. In particular, two high‐accumulation events in May 2009 and February 2011 showed an atmospheric river (AR) signature with enhanced integrated water vapor (IWV), concentrated in narrow long bands stretching from subtropical latitudes to the East Antarctic coast. Adapting IWV‐based AR threshold criteria for Antarctica (by accounting for the much colder and drier environment), we find that it was four and five ARs reaching the coastal DML that contributed 74–80% of the outstanding SMB during 2009 and 2011 at PE. Therefore, accounting for ARs is crucial for understanding East Antarctic SMB.

Comparison of Near-Surface Air Temperatures and MODIS Ice-Surface Temperatures at Summit, Greenland (2008–13)

AbstractThe stability of the Moderate Resolution Imaging Spectroradiometer (MODIS) ice-surface temperature (IST) product from Terra was investigated for use as a climate-quality data record. The availability of climate-quality air temperature data TA from a NOAA observatory at Greenland’s Summit Station has enabled this high-temporal-resolution study of MODIS ISTs. During a >5-yr period (July 2008–August 2013), more than 2500 IST values were compared with ±3-min-average TA values from NOAA’s primary 2-m temperature sensor. This enabled an expected small offset between air and ice-sheet surface temperatures (TA > IST) to be investigated over multiple annual cycles. The principal findings of this study show 1) that IST values are slightly colder than the TA values near freezing but that this offset increases as temperature decreases and 2) that there is a pattern in IST–TA differences as the solar zenith angle (SoZA) varies annually. This latter result largely explains the progressive offset from the in situ data at colder temperatures but also indicates that the MODIS cloud mask is less accurate approaching and during the polar night. The consistency of the results over each year in this study indicates that MODIS provides a platform for remotely deriving surface temperature data, with the resulting IST data being most compatible with in situ TA data when the sky is clear and the SoZA is less than ~85°. The ongoing development of the IST dataset should benefit from improved cloud filtering as well as algorithm modifications to account for the progressive offset from TA at colder temperatures.

The pattern of anthropogenic signal emergence in Greenland Ice Sheet surface mass balance

AbstractSurface mass balance (SMB) trends influence observed Greenland Ice Sheet (GrIS) mass loss, but the component of these trends related to anthropogenic forcing is unclear. Here we study the simulated spatial pattern of emergence of an anthropogenically derived GrIS SMB signal between 1850 and 2100 using the Community Earth System Model. We find emergence timing heterogeneity, with a bimodal structure reflecting interior snowfall increases against a background of low SMB variability, and peripheral surface melting increases against a backdrop of high SMB variability. We also find a nonemerging intermediate region. We conclude that (1) a bimodal pattern of GrIS SMB change will unambiguously reflect the impact of anthropogenic forcing; (2) present‐day peripheral and interior SMB trends likely have an underlying anthropogenically forced component; (3) local emergence occurs well before emergence of a spatially integrated signal; and (4) the GrIS summit region may be an ideal location for monitoring regional/global climate change.

Greenland Ice Sheet exports labile organic carbon to the Arctic oceans

Abstract. Runoff from small glacier systems contains dissolved organic carbon (DOC) rich in protein-like, low molecular weight (LMW) compounds, designating glaciers as an important source of bioavailable carbon for downstream heterotrophic activity. Fluxes of DOC and particulate organic carbon (POC) exported from large Greenland catchments, however, remain unquantified, despite the Greenland Ice Sheet (GrIS) being the largest source of global glacial runoff (ca. 400 km3 yr−1). We report high and episodic fluxes of POC and DOC from a large (>600 km2) GrIS catchment during contrasting melt seasons. POC dominates organic carbon (OC) export (70–89% on average), is sourced from the ice sheet bed, and contains a significant bioreactive component (9% carbohydrates). A major source of the "bioavailable" (free carbohydrate) LMW–DOC fraction is microbial activity on the ice sheet surface, with some further addition of LMW–DOC to meltwaters by biogeochemical processes at the ice sheet bed. The bioavailability of the exported DOC (26–53%) to downstream marine microorganisms is similar to that reported from other glacial watersheds. Annual fluxes of DOC and free carbohydrates during two melt seasons were similar, despite the approximately two-fold difference in runoff fluxes, suggesting production-limited DOC sources. POC fluxes were also insensitive to an increase in seasonal runoff volumes, indicating a supply limitation in suspended sediment in runoff. Scaled to the GrIS, the combined DOC (0.13–0.17 Tg C yr−1 (±13%)) and POC fluxes (mean = 0.36–1.52 Tg C yr−1 (±14%)) are of a similar order of magnitude to a large Arctic river system, and hence may represent an important OC source to the near-coastal North Atlantic, Greenland and Labrador seas.

Ice sheet survey over Antarctica using satellite altimetry: ERS-2, Envisat, SARAL/AltiKa, the key importance of continuous observations along the same repeat orbit

From September 2002 to October 2010, the Envisat radar altimeter surveyed Greenland and Antarctica ice sheets on a 35 day repeat orbit, providing a unique data set for ice sheet mass balance studies. Up to 85 repeat cycles are available and the whole Envisat data set may be along-track processed in order to provide height variability and trend with a good spatial resolution for the objectives of ice sheet survey.Soon, a joint Centre National d’Etudes Spatiales/Indian Space Research Organisation mission, SARAL (Satellite with Argos and AltiKa), with the AltiKa payload on board, will be launched on exactly the same orbit (less than 1 km of the nomimal orbit in the across-track direction). This will allow an extension of previous European Remote Sensing (ERS) satellite, ERS-1 and ERS-2, and Envisat missions of the European Space Agency (ESA), in particular from the point of view of ice altimetry. However, AltiKa operates in the Ka band (36.8 GHz), a higher frequency than the classical Ku band (13.6 GHz), leading to important modifications and potential improvements in the interactions between radar wave and snow-pack.In this paper, a synthesis is presented of all available information relevant to ice altimetry scientific purposes as derived from the Envisat mission: mean and temporal derivatives of the height − but also of the backscatter and of the two waveform parameters − snow-pack change corrections, across-track surface slope at 1 km scale, etc. The spatial and temporal variability of ice sheet surface elevation is investigated with the help of the high-resolution Envisat along-track observations. We show that at least 50 repeat cycles are needed to reach the required accuracy for the elevation trend. It is thus advocated as potentially highly beneficial for SARAL/AltiKa as a follow-on mission. Moreover, the novel characteristics of SARAL/AltiKa are promising in improving our understanding of snow penetration impact.

Sedimentation waves on the Martian North Polar Cap: Analogy with megadunes in Antarctica

Complex interactions between katabatic winds and the cryosphere may lead to the formation of sedimentation waves at the surface of ice sheets. These have been first described and named snow megadunes in Antarctica. Here we use topographic data, optical images, subsurface radar soundings and spectroscopic data acquired by Mars orbiters, to show that the surface of the Martian North Polar Cap displays two superimposed sets of sedimentation waves with differing wavelengths. These sedimentation waves have similarities with Antarctic snow megadunes regarding their surface morphology, texture, grain size asymmetry, and internal stratigraphic architecture. Both sets of Martian sedimentation waves present young ice and occasional sastrugi fields, indicative of net accumulation, on their shallow-dipping upwind sides, their tops and the intervening troughs. Old layers of dusty ice, indicative of net ablation, are exhumed on the steep-dipping downwind sides of the larger waves. Smooth surfaces of coarse-grained ice, indicative of reduced accumulation associated with sublimation metamorphism, cover the steep-dipping downwind sides of the smaller waves. These surface characteristics and the internal stratigraphy revealed by radar soundings are consistent with the interpretation that both sets of Martian sedimentation waves grow and migrate upwind in response to the development of periodic accumulation/ablation patterns controlled by katabatic winds. The recognition of these sedimentation waves provides the basis for the development of a common model of ice/wind interaction at the surface of Martian and terrestrial glaciers. Martian smaller waves, characterized by reduced net accumulation on their downwind sides, are analogous to Antarctic snow megadunes that have been described so far. A terrestrial equivalent remains to be discovered for the larger Martian waves, characterized by net ablation on their downwind sides.

A century of variation in the dependence of Greenland iceberg calving on ice sheet surface mass balance and regional climate change.

Iceberg calving is a major component of the total mass balance of the Greenland ice sheet (GrIS). A century-long record of Greenland icebergs comes from the International Ice Patrol's record of icebergs (I48N) passing latitude 48° N, off Newfoundland. I48N exhibits strong interannual variability, with a significant increase in amplitude over recent decades. In this study, we show, through a combination of nonlinear system identification and coupled ocean–iceberg modelling, that I48N's variability is predominantly caused by fluctuation in GrIS calving discharge rather than open ocean iceberg melting. We also demonstrate that the episodic variation in iceberg discharge is strongly linked to a nonlinear combination of recent changes in the surface mass balance (SMB) of the GrIS and regional atmospheric and oceanic climate variability, on the scale of the previous 1–3 years, with the dominant causal mechanism shifting between glaciological (SMB) and climatic (ocean temperature) over time. We suggest that this is a change in whether glacial run-off or under-ice melting is dominant, respectively. We also suggest that GrIS calving discharge is episodic on at least a regional scale and has recently been increasing significantly, largely as a result of west Greenland sources.

Thick sediments beneath Greenland’s ablation zone and their potential role in future ice sheet dynamics

The geological nature of glacier beds plays a key role in ice sheet dynamics. Whereas little is known about Greenland’s subglacial geology, the presence of basal sediments is a necessary condition for fast-flowing Antarctic ice streams. Such sediments sustain subglacial till layers, which if water saturated and under high pore-water pressure provide little resistance to ice flow. Using receiver function modeling of teleseismic P-waves, we report a thick (at least tens of meters) sediment layer beneath a site in Greenland’s ablation zone, ∼15 km away from the western ice sheet margin. Although we do not discuss the origin or detailed nature of these subglacial sediments, we suggest that they are capable of sustaining a till layer. Due to the prevalence of an inefficient, pressurized subglacial drainage system, this till layer would typically be under high pore pressures and fail at the shear stresses transmitted from the overlaying ice. Ice flow resistance is thus focused on regions where till is consolidated or absent, or where form drag over obstacles takes place. In contrast to Antarctica, Greenland’s surface melt now affects practically all latitudes, and efficient hydraulic connections between ice sheet surface and bed are pervasive throughout the ablation zone. This implies that rapid, melt-induced changes in subglacial water pressure are possible. The time required for basal till strength to react to these changes depends on the till’s properties. We estimate that formation and destruction of flow resistance can occur on time scales of less than a few years. This could lead to changes in ice flow that are currently difficult to predict.

Climate change and forest fires synergistically drive widespread melt events of the Greenland Ice Sheet.

In July 2012, over 97% of the Greenland Ice Sheet experienced surface melt, the first widespread melt during the era of satellite remote sensing. Analysis of six Greenland shallow firn cores from the dry snow region confirms that the most recent prior widespread melt occurred in 1889. A firn core from the center of the ice sheet demonstrated that exceptionally warm temperatures combined with black carbon sediments from Northern Hemisphere forest fires reduced albedo below a critical threshold in the dry snow region, and caused the melting events in both 1889 and 2012. We use these data to project the frequency of widespread melt into the year 2100. Since Arctic temperatures and the frequency of forest fires are both expected to rise with climate change, our results suggest that widespread melt events on the Greenland Ice Sheet may begin to occur almost annually by the end of century. These events are likely to alter the surface mass balance of the ice sheet, leaving the surface susceptible to further melting.

Determination of Local Slope on the Greenland Ice Sheet Using a Multibeam Photon-Counting Lidar in Preparation for the ICESat-2 Mission

The greatest changes in elevation in Greenland and Antarctica are happening along the margins of the ice sheets where the surface frequently has significant slopes. For this reason, the upcoming Ice, Cloud, and land Elevation Satellite-2 (ICESat-2) mission utilizes pairs of laser altimeter beams that are perpendicular to the flight direction in order to extract slope information in addition to elevation. The Multiple Altimeter Beam Experimental Lidar (MABEL) is a high-altitude airborne laser altimeter designed as a simulator for ICESat-2. The MABEL design uses multiple beams at fixed angles and allows for local slope determination. Here, we present local slopes as determined by MABEL and compare them to those determined by the Airborne Topographic Mapper (ATM) over the same flight lines in Greenland. We make these comparisons with consideration for the planned ICESat-2 beam geometry. Results indicate that the mean slope residuals between MABEL and ATM remain small ( 0.05 °) through a wide range of localized slopes using ICESat-2 beam geometry. Furthermore, when MABEL data are subsampled by a factor of 4 to mimic the planned ICESat-2 transmit-energy configuration, the results are indistinguishable from the full-data-rate analysis. Results from MABEL suggest that ICESat-2 beam geometry and transmit-energy configuration are appropriate for the determination of slope on ~ 90-m spatial scales, a measurement that will be fundamental to deconvolving the effects of surface slope from the ice-sheet surface change derived from ICESat-2.

Progressive increase in number and volume of ice-marginal lakes on the western margin of the Greenland Ice Sheet

The evolution in number, area and volume of ice-marginal lakes in western Greenland is very poorly documented or understood. It is important to understand ice-marginal lake evolutions because they provide an element of meltwater retention, affect ice-margin character and behaviour, and potentially glacier dynamics. This study uses repeat satellite imagery acquired between 1987 and 2010 to reveal a net 44% (±6.5%) increase in the number of lakes, a net 20% (±6.5%) expansion in total lake surface area and an increase of 12% (±3.3%) in the estimated volume of meltwater retained along a 1300km length of the ice margin in western Greenland. Whilst ~12% (±1.6%) of the ice margin holds lakes at any one time there is considerable complexity in lake evolution; many lakes have coalesced, drained partially or fully, or become detached from the ice margin. The total lake volume equates to 144% of the annual runoff combined from Gothab and Jakobshavn hydrological catchments. The rate of increase in meltwater retention between 1987 and 2010 was similar to the rate of increase in ice sheet surface runoff over the same time period. If the study region is representative of the whole Greenland Ice Sheet margin then as a first-order estimate ~5% of the increased runoff over the last 25years has been intercepted en route to the oceans by the increased ice-marginal lake capacity. Interactions between these ice-marginal lakes, the western Greenland Ice Sheet and climate should be determined to provide insight into future land-terminating ice-marginal conditions, runoff retention and meltwater and sediment fluxes to the oceans.

Interdependence of the growth of the Northern Hemisphere ice sheets during the last glaciation: the role of atmospheric circulation

Abstract. The development of large continental-scale ice sheets over Canada and northern Europe during the last glacial cycle likely modified the track of stationary waves and influenced the location of growing ice sheets through changes in accumulation and temperature patterns. Although they are often mentioned in the literature, these feedback mechanisms are poorly constrained and have never been studied throughout an entire glacial–interglacial cycle. Using the climate model of intermediate complexity CLIMBER-2 coupled with the 3-D ice-sheet model GRISLI (GRenoble Ice Shelf and Land Ice model), we investigate the impact of stationary waves on the construction of past Northern Hemisphere ice sheets during the past glaciation. The stationary waves are not explicitly computed in the model but their effect on sea-level pressure is parameterized. We tested different parameterizations to study separately the effect of surface temperature (thermal forcing) and topography (orographic forcing) on sea-level pressure, and therefore on atmospheric circulation and ice-sheet surface mass balance. Our model results suggest that the response of ice sheets to thermal and/or orographic forcings is rather different. At the beginning of the glaciation, the orographic effect favors the growth of the Laurentide ice sheet, whereas Fennoscandia appears rather sensitive to the thermal effect. Using the ablation parameterization as a trigger to artificially modify the size of one ice sheet, the remote influence of one ice sheet on the other is also studied as a function of the stationary wave parameterizations. The sensitivity of remote ice sheets is shown to be highly sensitive to the choice of these parameterizations with a larger response when orographic effect is accounted for. Results presented in this study suggest that the various spatial distributions of ice sheets could be partly explained by the feedback mechanisms occurring between ice sheets and atmospheric circulation.

Probabilistic parameterisation of the surface mass balance–elevation feedback in regional climate model simulations of the Greenland ice sheet

Abstract. We present a new parameterisation that relates surface mass balance (SMB: the sum of surface accumulation and surface ablation) to changes in surface elevation of the Greenland ice sheet (GrIS) for the MAR (Modèle Atmosphérique Régional: Fettweis, 2007) regional climate model. The motivation is to dynamically adjust SMB as the GrIS evolves, allowing us to force ice sheet models with SMB simulated by MAR while incorporating the SMB–elevation feedback, without the substantial technical challenges of coupling ice sheet and climate models. This also allows us to assess the effect of elevation feedback uncertainty on the GrIS contribution to sea level, using multiple global climate and ice sheet models, without the need for additional, expensive MAR simulations. We estimate this relationship separately below and above the equilibrium line altitude (ELA, separating negative and positive SMB) and for regions north and south of 77° N, from a set of MAR simulations in which we alter the ice sheet surface elevation. These give four "SMB lapse rates", gradients that relate SMB changes to elevation changes. We assess uncertainties within a Bayesian framework, estimating probability distributions for each gradient from which we present best estimates and credibility intervals (CI) that bound 95% of the probability. Below the ELA our gradient estimates are mostly positive, because SMB usually increases with elevation: 0.56 (95% CI: −0.22 to 1.33) kg m−3 a−1 for the north, and 1.91 (1.03 to 2.61) kg m−3 a−1 for the south. Above the ELA, the gradients are much smaller in magnitude: 0.09 (−0.03 to 0.23) kg m−3 a−1 in the north, and 0.07 (−0.07 to 0.59) kg m−3 a−1 in the south, because SMB can either increase or decrease in response to increased elevation. Our statistically founded approach allows us to make probabilistic assessments for the effect of elevation feedback uncertainty on sea level projections (Edwards et al., 2014).