Abstract
Abstract. Two hindcast (1983–2007) simulations are performed with the global, ocean-sea ice models NEMO-LIM2 and NEMO-LIM3 driven by atmospheric reanalyses and climatologies. The two simulations differ only in their sea ice component, while all other elements of experimental design (resolution, initial conditions, atmospheric forcing) are kept identical. The main differences in the sea ice models lie in the formulation of the subgrid-scale ice thickness distribution, of the thermodynamic processes, of the sea ice salinity and of the sea ice rheology. To assess the differences in model skill over the period of investigation, we develop a set of metrics for both hemispheres, comparing the main sea ice variables (concentration, thickness and drift) to available observations and focusing on both mean state and seasonal to interannual variability. Based upon these metrics, we discuss the physical processes potentially responsible for the differences in model skill. In particular, we suggest that (i) a detailed representation of the ice thickness distribution increases the seasonal to interannual variability of ice extent, with spectacular improvement for the simulation of the recent observed summer Arctic sea ice retreats, (ii) the elastic-viscous-plastic rheology enhances the response of ice to wind stress, compared to the classical viscous-plastic approach, (iii) the grid formulation and the air-sea ice drag coefficient affect the simulated ice export through Fram Strait and the ice accumulation along the Canadian Archipelago, and (iv) both models show less skill in the Southern Ocean, probably due to the low quality of the reanalyses in this region and to the absence of important small-scale oceanic processes at the models' resolution (~1°).
Highlights
Current General Circulation Models (GCMs) show large intermodel spread in simulating future characteristics of sea ice (Zhang and Walsh, 2005; Arzel et al, 2006)
Linear trends of ice extent computed from classical regression are excellent in both models, but LIM2 underestimates the magnitude of observed deviations such as in September 1996, 2005 and 2007, whereas LIM3 is skillful in this respect
We have investigated the sensitivity of an ocean-sea ice model to the representation of physics in its sea ice component: two hindcast simulations have been studied over the period 1983–2007, for both Arctic and Antarctic sea ice, with an ocean General Circulation Model driven by atmospheric reanalyses and various climatologies
Summary
Current General Circulation Models (GCMs) show large intermodel spread in simulating future (decadal to centennial) characteristics of sea ice (Zhang and Walsh, 2005; Arzel et al, 2006). Most of those GCMs present large discrepancies with respect to observations over the last decades, in terms of mean seasonal cycle as well as interannual variability, for both hemispheres (Parkinson et al, 2006; Arzel et al, 2006; Holland and Raphael, 2006; Connolley and Bracegirdle, 2007; Lefebvre and Goosse, 2008; Stroeve et al, 2007). The sources of this spread are manifold. The representation of sea ice-related thermodynamical and dynamical processes differ from one model to another, ranging from simple static models with no explicit ice thickness distribution to sophisticated dynamical models, including a snow component and a multi-category ice thickness distribution framework
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