Abstract
Abstract. The effects of internal model variability on the simulation of Arctic sea-ice extent and volume have been examined with the aid of a seven-member ensemble with a coupled regional climate model for the period 1948–2008. Beyond general weaknesses related to insufficient representation of feedback processes, it is found that the model's ability to reproduce observed summer sea-ice retreat depends mainly on two factors: the correct simulation of the atmospheric circulation during the summer months and the sea-ice volume at the beginning of the melting period. Since internal model variability shows its maximum during the summer months, the ability to reproduce the observed atmospheric summer circulation is limited. In addition, the atmospheric circulation during summer also significantly affects the sea-ice volume over the years leading to a limited ability to start with reasonable sea-ice volume into the melting period. Furthermore, the sea-ice volume pathway shows notable decadal variability that varies in amplitude among the ensemble members. The scatter is particularly large in periods when the ice volume increases, indicating limited skill in reproducing high-ice years.
Highlights
The Arctic has been subjected to a variety of changes in atmospheric, oceanic, and sea-ice conditions in recent years
The coupled regional climate model HIRHAM-NAOSIM used in this study consists of the regional atmospheric climate model HIRHAM (Christensen et al, 1996; Dethloff et al, 1996) and the high-resolution version of the North Atlantic/Arctic Ocean sea-ice model NAOSIM (Karcher et al, 2003; Kauker et al, 2003)
Kauker et al (2009) showed on the basis of the low scatter of the ensemble members in sea- NAOSIM simulations that about 20 % of the sea-ice anomaly ice thickness and concentration in September 2007 is asso- in September 2007 was determined by the initial ice thickciated with a low scatter in the atmospheric circulation in ness in March, whereas the wind stress in May and June the preceding summer months
Summary
The Arctic has been subjected to a variety of changes in atmospheric, oceanic, and sea-ice conditions in recent years. Even though all coupled climate models involved in the Intergovernmental Panel on Climate Change Fourth Assessment Report (IPCC AR4) showed declining Arctic sea ice over the last 50 yr, only very few individual model simulations showed trends comparable to recent observations (Stroeve et al, 2007). This fact demonstrates that there is still large uncertainty about the Arctic’s actual path into the future. The aim of the study is the identification of limitations in reproducing the observed Arctic sea-ice retreat having regard to unknown initial conditions for ocean and sea ice, internal variability of the atmospheric circulation, and general uncertainties due to insufficient description of Arctic climate processes
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