Antarctic sea ice plays a key role in the present climate system, providing a regulating balance between the atmosphere and ocean heat fluxes, as well as influencing the salt fluxes and deep water formation over the continental shelves. The severe winter environmental conditions of the Antarctic sea-ice zone make it difficult to observe many of the physical characteristics in a comprehensive way. The inter-relations between the variables mean that much can be learnt from the observations of some features along with detailed numerical modelling of the whole system and the interactions between the variables. This study therefore aims to use numerical modelling of the atmosphere, sea ice and surface mixed-layer ocean in the sea-ice zone, together with observations to simulate a comprehensive range of parameters and their variability through the annual cycle to provide a basis for further observations and model validation for the present climate.

The model includes a coupled atmospheric general circulation model with an interactive dynamic and thermodynamic sea-ice model and surface mixed-layer ocean. The deep ocean and ocean surface conditions outside the sea-ice zone are constrained to the present mean climate conditions to ensure no climatic drift. The sca-ice model is similar to previous published versions, bill has refined schemes for partitioning of the freezing of frazil ice within the leads and under the ice floes, and for rafting. These perform well in both polar regions with the same physics. The model simulates the annual cycle of atmospheric and sea-ice features well in comparison with data from the global atmospheric analyses, the satellite sensing of sea ice, and the limited in situ surface observations.

The output from the model also includes: all components of the heart fluxes, atmospheric profiles and surface temperatures for air, ice and ice-ocean mixtures, open-water fractions, surface snow and snow-ice depths, and the sea-ice convergence-divergence and drift. The comparison of these features with additional observations provides a means for further validating the model and representing the present climate more closely.

The Sea-Ice Model Intercomparison Project (SIMIP) is part of the activities of the Sea Ice-Ocean Modeling Panel (SIOM) of the Arctic Climate System Study (WMO) (ACSYS) that aims to determine the optimal sea-ice model for climate simulations. This investigation is focused on the dynamics of sea ice. A hierarchy of four sea-ice rheologies is applied, including a viscous-plastic rheology, a cavitating-fluid model, a compressible Newtonian fluid, and a simple scheme with a step-function stoppage for ice drift.

For comparison, the same grid, land boundaries and forcing fields are applied to all models. Atmospheric forcing for a 7 year period is obtained from the European Centre for Medium-Range Weather Forecasts (UK) (ECMWF analyses), while occanic forcing consists of annual mean geostrophic currents and heal fluxes into a fixed mixed layer. Daily buoy-drift data monitored by the International Arctic Buoy Program (IABP) and ice thicknesses at the North Pole from submarine upward-looking sonar are available as verification data. The daily drift statistics for separate regions and seasons contribute to an error function showing significant differences between the models. Additionally, Fram Strait ice exports predicted by the different models are investigated. The ice export of the viscous-plastic model amounts to 0.11 Sv. when it is optimized to the mean daily buoy velocities and the observed North Pole ice thicknesses. The cavitating-fluid model yields a very similar Fram Strait outflow, but underestimates the North Pole ice thickness. The two other dynamic schemes predict unrealistically large ice thicknesses in the central Arctic region, while Fram Strait ice exports are too low.

A one-dimensional (1-D), thermodynamic sea-ice model with parameterized ice dynamics is coupled to a mixed-layer ocean model and driven with prescribed atmospheric forcings for the central Arctic. The model is used to calculate the sensitivity of the ice pack to various parameterizations that have traditionally been neglected or considered only implicitly in large-scale sea-ice models. The model includes melt ponds, leads (with summertime stratification), an ice-export term, a stability-dependent air–sea heat-exchange coefficient, a prognostic ocean–ice heat exchange, a crude ice-thickness distribution, and a sophisticated albedo parameterization.

The ice pack is sensitive to the partitioning of solar energy between lateral melting and mixed-layer warming, with the most realistic simulations occurring when the heat is nearly evenly divided between these two processes. Conversely, ice thickness and coverage are fairly insensitive to the amount of lateral mixing within the upper ocean, vertical mixing within leads, and to the partitioning of mixed-layer heat content between warming the water and melting the ice bottom. The ice concentration during summer is strongly dependent on the assumed ice-thickness distribution: the amount of open water during summer is less than half the size of the empirically based distribution used here, compared with one in which ice floes are distributed uniformly across a range of thicknesses.

The Arctic represents a relatively pristine frontier that is vulnerable to pollution. Substances originating at mid latitudes are transported to the Arctic by atmospheric processes, ocean currents and rivers. These pollutants can accumulate in the Arctic environment. Testing of nuclear weapons, dumping of waste and operation of ships, and nuclear power plants also pose threats.

To investigate possible pollutant pathways we used a multi-level primitive-equation ocean model, coupled to a dynamic-thermodynamic sea-ice model. Coupling included conservative transfer of momentum, heat and fresh water. Atmospheric forcing (wind stress, temperature, humidity, radiation and heat fresh-water fluxes) was supplied by datasets or bulk formulae. Open lateral-boundary conditions for the ocean model were supplied either by datasets (temperature and salinity) or from a larger-scale ocean model (momentum). Two integrations were compared — one used a centred-difference advection scheme and viscous damping, while the other used a better representation of an advection scheme and a sub-gridscale eddy parameterization.

Tracer simulations showed (1) the importance of good representation of numerical advection, and (2) the role of eddy interacting with sea-floor topography (the neptune effect).

The effects on ice-thickness distribution of including ridge-related roughness in the drag parameterization is investigated using a sea-ice dynamic and thermodynamic model. A long-term integration of the sea-ice model forced by a 5 day running mean of daily geostrophic winds and steady but spatially varying ocean currents was performed to construct climatological thickness fields. Compared to a constant drag coefficient model, results that include roughness have a significantly different spatial distribution with much thicker ice in the western Arctic. These results match observations better than those where ice roughness was omitted.

Sea ice on the large scale is characterized by leads and ridges that typically have a given orientation. Because of various flaws, we would expect that the ice will form oriented leads and ice-thickness characteristics that control the heat and moisture fluxes into the atmosphere. Prediction of these oriented leads, ridges and slip lines is relevant to understanding the role of ice mechanics in global climate change as they can play a significant role in the ice-thickness distribution.

In this paper we develop a model for the dynamical treatment of leads and oriented flaws in large-scale sea-ice models. Two particular isotropic realizations of this model relevant to climate studies are examined: (a) an isotropic composite with oriented leads in all directions imbedded in thick ice, and (b) a simple "strain hardening" isotropic model where only oriented leads having the potential to open rapidly are allowed. Under applied stress both models yield preferential deformation along a symmetric pair of intersecting leads or ridges with the intersection angles dependent on the confinement stress. The "uniform-orientation" model results in a yield curve that approximates a sine lens, while the "strain hardening" model has a teardrop-like yield curve. How the resulting fracture-based yield curves and non-normal flow rules may be cast in a form usable in numerical investigations of climate is discussed.

The surface radiation budget of the polar regions strongly influences ice growth and melt. Thermodynamic sea-ice models therefore require accurate yet computationally efficient methods of computing radiative fluxes. In this paper a new parameterization of the downwelling shortwave radiation flux at the Arctic surface is developed and compared to a variety of existing schemes. Parameterized llnxes are compared to in situ measurements using data for one year at Barrow, Alaska. Our results show that the new parameterization can estimate the downwelling shortwave flux with mean and root mean square errors of 1 and 5%, respectively, for clear conditions and 5 and 20% for cloudy conditions. The new parameterization offers a unified approach to estimating downwelling shortwave fluxes under clear and cloudy conditions, and is more accurate than existing schemes.

Fluxes of heat and moisture over heterogeneous snow cover are studied using a boundary-layer model. The performance of a “tile” model, suitable for calculating gridbox-average surface fluxes in a GCM, is assessed in comparison with the boundary-layer model. The impact of using a tile representation for heterogeneous snow cover in a single-column version of the Hadley Centre GCM is discussed.

A meteorological-analysis procedure allowing simulation of the snow cover in mountainous regions, where the general circulation model orography differs from the real orography, is described. This procedure uses model outputs to estimate the data needed to force a snow model. Temperature and precipitation deduced from three five-year runs were compared to the snow climatology of the French Alps. The snow cover simulations are very sensitive to temperature at middle elevations. At high elevations, the altitude of the equilibrium line is simulated well.

We have conducted tests of the Canadian Land Surface Scheme (CLASS V2.5) for Arctic tundra applications. Our tests emphasize sensitivities to initial conditions, external forcings and internal parameters, and focus on the Alaskan North Slope during the summer of 1992. Observational data from the National Science foundation (NSF), Arctic Systems Science (ARCSS), Land/Atmosphere/Ice Interactions (LAII) Flux Study is available to serve as forcing and validation for our simulations.

Comparisons of the runs show strong sensitivities to the composition and depth of the soil layers, and we find that a minimum total soil depth of 5.0 m is needed to maintain permafrost. The response of the soil to diurnal variations in forcing is strong, while sensitivities to other internal parameters, as well as to precipitation, were relatively small. Some sensitivity to air temperatures and radiative fluxes, particularly the incoming shortwave flux, was also present. Significant sensitivity to the specification of the initial water and ice contents of the soil was found, while the sensitivity to initial soil temperature was somewhat less.

Although the snow albedo feedback mechanism has been shown to amplify global warming effects in nearly all models of global climate, it continues to be represented as a simplistic parameterization. Here, we demonstrate how changes in snow-pack energy-balance drive the seasonal fluctuations in snow albedo for the Greenland ice sheet. For a detailed, point-based investigation of the relationship between snowpack energy balance and albedo, two models are coupled together; one that calculates snow grain-size and the other that uses those grain-size data as input to a radiative-transfer code to obtain spectral albedo. These data indicate that in the near-infrared wavelengths, albedo values drop nearly 20%, during a 10 day period during which grain-sizes increased dramatically. Satellite data were used to map monthly changes in albedo over the entire Greenland ice sheet during the spring and summer months. These monthly albedo images indicate albedo reductions of as much as 80% in coastal regions. Even in areas that experience little or no melt, albedo decreases of 10–20% were common. From these results, it is clear that snow albedo parameterizations for climate models must incorporate the dynamics of snowpack energy balance.

Significant improvements to the representation of climate forcing and mass-balance response in a coupled two-dimensional global energy balance climate model (EBM) and vertically integrated ice-sheet model (ISM) have led to the prediction of an ice-volume chronology for the most recent ice-age cycle of the Northern Hemisphere that is close to that inferred from the geological record. Most significant is that full glacial termination is delivered by the model without the need for new physical ingredients. In addition, a relatively close match is achieved between the Last Glacial Maximum (LGM) model ice topography and that of the recently-described ICE-4G reconstruction. These results suggest that large-scale climate system reorganization is not required to explain the main variations of the North American (NA) ice sheets over the last glacial cycle. Lack of sea-ice and marine-ice dynamics in the model leaves the situation over the Eurasian (EA) sector much more uncertain.

The incorporation of a gravitationally self-consistent description of the glacial isostatic adjustment process demonstrates that the NA and EA bedrock responses can be adequately represented by simpler damped-relaxation models with characteristic time-scales of 3–5ka and 5 ka, respectively. These relaxation times agree with those independently inferred on the basis of postglacial relative sea-level histories.

A multi-layered snow model, including most physical processes governing the evolution of snowpacks, has been coupled to a global circulation model (GCM) to improve the representation of snow cover in climate simulations. The snow model (Crocus) includes original features to simulate the evolution of snowpack layering that allows a realistic calculation of snow albedo as a function of the type and size of the crystals of the surface layer. The coupling scheme is based on a synchronous run of the GCM and of the snow model with an exchange of the surface fluxes at every time-step. It was tested in a five-year run at a T42 resolution. The impact on the atmosphere was important over most snow-covered regions and the snowpacks simulated in the different regions present a layering that is realistic and very variable in connection with the climate. The simulated snow cover compares satisfactorily with the present snow climatology.

The Laboratoire de Météorologie Dynamique (LMD) variable-grid atmospheric general circulation model (AGCM) was used in this study for a five-year high-resolution simulation of the Antarctic climate. The horizontal resolution is about 100 km over a large part of the ice sheet. This study focuses on the simulated surface mass balance (precipitation-evaporation sublimation-melt) and on the spatial and temporal variability of snowfall in Antarctica. The simulated annual mean surface mass balance for the whole continent is close to the observed value, and the model simulates well the spatial distribution of the surface mass balance. The annual cycle of snowfall exhibits a clear minimum in summer over the high interior plateau as well as for Antarctica as a whole, in agreement with the observations. In the interior of the continent, the model produces a permanent light background snowfall that accounts for about 5% of the total annual precipitation. The bulk of the snowfall is produced irregularly during periods that generally last only two or three days that are caused by cyclones off the coast.

Most spectral general circulation models (GCMs) use an envelope topography to set on land surface elevations. The UK Universities Global Atmospheric Modelling Programme General Circulation Model (UGAMP GCM) uses such a formulation for Antarctica, based on the US Navy 10 are minute charts of the region. In the marginal regions of the continent, an envelope topography consistently overestimates the elevation leading to lower-than-observed surface temperatures. Furthermore, errors in excess of 1000 m exist in the US Navy data, and the UGAMP GCM treats the major ice shelves as sea ice, introducing a 12% reduction in the snow-covered area of the continent. Here, we use a new high-resolution, high-accuracy digital elevation model to improve the representation of Antarctica in the UGAMP GCM. The effect of changing the land–sea mask and the topography on the surface temperature, precipitation and wind held is investigated for both summer and winter runs. Changing the land–sea mask had a dramatic effect on temperature, producing a reduction of 13.3°C for the sector west of the Ross Ice Shelf. Using the new mean topography also introduces substantial differences in temperature, wind speed and precipitation for summer and winter.

Linearizations about two horizontal-dimensional ice sheets are proposed as methods of generating normal mode initializations for ice-sheet models and for computing the short-term response. Linearized models can be generated directly from balance-flux calculations without the need for tuning the rate factor.

A linearized model is compared with the Eismint Benchmark, and the normal modes for two coarse Antarctic digital elevation models are computed and compared. Volumetric relaxation spectra are presented. The slowest mode has a time constant comparable to that computed from scale theory.

Open-water leads in sea ice dominate the exchange of heat between the ocean and atmosphere in ice-covered regions, and so must be included in climate models. A parameterization of leads used in one such model is compared to observations and the results of a detailed Arctic sea-ice model. Such comparisons, however, are hampered by the errors in observed lead fraction, but the parameterization appears to compare better in winter than in summer. Simulations with an atmospheric general circulation model (AGCM), using prescribed sea-surface temperatures and ice extent, are used to illustrate the effect of parameterized lead fraction on atmospheric climate, and so provide some insight into the importance of improved lead-fraction parameterizations and observations. The effect of leads in the AGCM is largest in Northern Hemisphere winter, with zonal mean surface-air temperatures over ice increasing by up to 5 K when lead fraction is increased from 1% to near 5%. The effect of leads on sensible heat loss in winter is more important than the effect on radiative heat gain in summer. No significant effect on sea-level pressure, and hence on atmospheric circulation, is found, however. Indirect effects, due to feedbacks between the atmosphere and ice thickness and extent, were not included in these simulations, but could amplify the response.

The role of dynamics in modifying the response of the Arctic ice pack to inter-annually varying forcings and to climate perturbations is investigated using simulations from a two-dimensional ice model and a global climate model (GCM). Inter-annual variability in ice-covered area for 1985-93 is dominated by ice transport, and different transport regimes affect substantially the response of the ice pack to climate perturbations. The thermodynamic-only simulations are more sensitive to initial ice conditions, and respond less than the dynamk-thermodynamic model to small perturbations, but with a greater response to larger perturbations. Comparisons of GCM simulations that use different ice treatments highlights the importance of considering the distribution of ice thickness and extent in assessing climate-change responses.

The Climate System Model (CSM) developed at the National Center for Atmospheric Research (NCAR) consists of atmosphere, land and ocean models, as well as a dynamic-thermodynamic sea-ice model. The results of sea-ice simulation using the first coupled climate simulation with the CSM is presented. It was found that the simulated total-ice areas in both hemispheres compared well with observations for winter, but were too large for summer. The numerical solution of the cavitating fluid dynamics was found to allow excessive ridging of ice, and an ad hoc correction was implemented. The ice velocities were realistic for the Antarctic, but for the Arctic were turned toward Alaska and Siberia by modeled winds and currents. This ice-drift pattern was reflected by ice thickness, which lacks the observed ridging near Greenland. The results illustrate the sensitivity of sea ice to the simulation of polar climate and the challenge of modeling the entire climate system.

This paper investigates the long-term impact of sea ice on global climate using a global sea-ice–ocean general circulation model (OGCM). The sea-ice component involves state-of-the-art dynamics; the ocean component consists of a 3.5° × 3.5° × 11 layer primitive-equation model. Depending on the physical description of sea ice, significant changes are detected in the convective activity, in the hydrographic properties and in the thermohaline circulation of the ocean model. Most of these changes originate in the Southern Ocean, emphasizing the crucial role of sea ice in this marginally stably stratified region of the world's oceans. Specifically, if the effect of brine release is neglected, the deep layers of the Southern Ocean warm up considerably; this is associated with a weakening of the Southern Hemisphere overturning cell. The removal of the commonly used “salinity enhancement” leads to a similar effect. The deep-ocean salinity is almost unaffected in both experiments. Introducing explicit new-ice thickness growth in partially ice-covered gridcells leads to a substantial increase in convective activity, especially in the Southern Ocean, with a concomitant significant cooling and salinification of the deep ocean. Possible mechanisms for the resulting interactions between sea-ice processes and deep-ocean characteristics are suggested.