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Shawn J. Marshall and Martin J. Sharp

1. Introduction Glacier and ice sheet models simulate glacier dynamics as a function of the bedrock topography, ice thickness, a constitutive relationship for ice rheology, and parameterizations of flow (i.e., sliding) at the base of the glacier. These models require detailed surface mass balance fields for simulations of glacier response to climate change: estimates of snow accumulation and snow/ice melt on scales of hundreds of meters to tens of kilometers, depending on the ice mass of

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Garry K. C. Clarke, Etienne Berthier, Christian G. Schoof, and Alexander H. Jarosch

observer and the valley walls and the steepness of the surrounding topography. The fact that such a guess is possible and has some observational basis suggests that the depth estimation process can be formalized and even automated. As illustrated in Fig. 1 , our task starts with a glacierized digital elevation model, and entails the estimation of ice thickness for each of the ice-covered cells. By assembling these estimates, an ice-denuded DEM ( Fig. 1c ) can be constructed for the same terrain. We

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Martin Sharp and Libo Wang

assess the uncertainties associated with regional-scale mass balance estimates. Because much of the interannual and longer-term variability in the surface mass balance of Arctic glaciers arises from variability in the summer balance [i.e., from rates of surface melt rather than rates of snow accumulation; see Koerner (2005) and Gardner and Sharp (2007) ], knowledge of regional-scale patterns of summer melt and their temporal variability could form a basis for upscaling procedures. It will also

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Alex S. Gardner, Martin J. Sharp, Roy M. Koerner, Claude Labine, Sarah Boon, Shawn J. Marshall, David O. Burgess, and David Lewis

influence future glacier contributions to global eustatic sea level ( Gregory and Oerlemans 1998 ; Braithwaite and Raper 2002 ; Marshall et al. 2005 ; Bougamont et al. 2005 ; Hanna et al. 2005 ). Mass balance models calculate snow and ice melt using two main approaches: the energy balance approach and the temperature-index or “degree day” approach. The latter approach assumes an empirical relationship between melting and near-surface air temperature, while the former involves the assessment of all

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Marc d’Orgeville and W. Richard Peltier

the Bering Sea anomaly and from it to justify the 20-yr time scale of the PDO. In the development of the PDO, the positive anomaly in sea ice extent appears to propagate from the coast of Alaska into the Bering Sea (lags −2 to 6 yr; Fig. 9g ). This westward propagation is in turn linked to the North American snow cover anomaly, which is characterized by a positive anomaly that propagates from the Rocky Mountains to Alaska (lags −4 to 4 yr; Fig. 9f ). Such propagation is not expected to be an

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Michael S. Pritchard, Andrew B. G. Bush, and Shawn J. Marshall

limitations of degree-day melt modeling, which is unable to account for energy balance processes such as variations in shortwave and longwave radiation that accompany differences in cloud cover. Coupled ice sheet and climate models need to move to a fully coupled energy balance rather than parameterized temperature index models for snow and ice melt. This will also help to reduce uncertainties associated with the choice of temperature lapse rate; degree-day melt models are very sensitive to this choice

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J. Paul Spence, Michael Eby, and Andrew J. Weaver

-conserving ice–snow thermodynamics with a two-category thickness distribution ( Hibler 1979 ) and an elastic–viscous–plastic rheology ( Hunke and Dukowicz 1997 ). The model predicts ice thickness, areal fraction, and surface temperature. The UVic ESCM employs a vertically integrated energy–moisture balance atmospheric model for computational efficiency. Precipitation occurs when the relative humidity exceeds 90%, and on land it is treated by a simple bucket model described in Matthews et al. (2003) . The

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Garry K. C. Clarke, Andrew B. G. Bush, and John W. M. Bush

regions the surface albedo is fixed at 0.6, though this can increase if there is fresh snow cover. Freshwater additions associated with the Lake Agassiz flood and the rerouting of glacial meltwater ( Fig. 1 ) are assumed to have no salinity (0 psu) and a temperature identical to that of the ocean grid cell to which the water is added. Salinity is monitored at every time step to detect negative values and set these to zero. Simulations were run on a Silicon Graphics, Inc. (SGI), Origin 2400 (6

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Guido Vettoretti, Marc d’Orgeville, William R. Peltier, and Marek Stastna

climate regimes the THC is observed to recover in a nontrivial manner, with the recovery occurring fastest with modern conditions, followed by the 4 × CO 2 experiment, and then by the LGM experiment. In their study, the LGM THC recovered most slowly because of the expansion of sea ice cover in the North Atlantic, which reduced surface heat loss and increased freshwater supply over the normal sites of North Atlantic Deep Water (NADW) production. 3. Results The results are organized in the following

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