Arctic sea ice in the mid-Holocene Paleoclimate Modelling Intercomparison Project 2 simulations
1Linné Flow Centre, Dept. of Mechanics, Royal Institute of Technology, Stockholm, Sweden
2Bert Bolin Centre for Climate Research, Stockholm University, Stockholm, Sweden
3Dept. of Meteorology, Stockholm University, Stockholm, Sweden
Abstract. The Arctic sea ice in the mid-Holocene simulations of 11 coupled global circulation models part of the Paleoclimate Modelling Intercomparison Project phase 2 (PMIP2) is analysed in this study. The work includes a comparison of the mid-Holocene simulations to the pre-industrial control simulations for each individual model and also a model-model comparison. The forcing conditions in the mid-Holocene and pre-industrial simulations differ in the atmospheric methane concentration and the latitudinal and monthly distribution of solar insolation (due to differences in the orbital parameters). Other studies have found that the difference in insolation, with increased northern hemisphere summer insolation, explain the major differences between the simulated mid-Holocene and pre-industrial climates. The response of the simulated sea ice extent and thickness to the changes in solar insolation and atmospheric greenhouse gases is investigated. The model-model variation in pre-industrial simulated Arctic sea ice is large, with sea ice area extent ranging from 10.1 to 28.2 (7.01 to 24.6) million km2 in March (September), and the maximum sea ice thickness ranging from 1.5 m to more than 5 m in both September and March. Nevertheless, all models agree on the sign of the difference between mid-Holocene and pre-industrial in both March and September. All models have smaller summer sea ice extent and thinner ice cover in all seasons in the mid-Holocene climate compared to the control (pre-industrial) climate. The reduction in sea ice extent is mostly confined to the sea ice margins, whereas the thinning of the ice occurs over the entire ice cover. In addition, the models also experience an enhanced summer warming north of 60° N. For the central Arctic region, models with thicker ice in the mean state in the control simulation experience the largest change in the mean state between the two climates. Comparison to available Climate Model Intercomparison Project 3 (CMIP3) simulations with the same model version and atmospheric CO2 concentration increased to a doubling has also been performed. The sea ice response in this future scenario is stronger than the response in the mid-Holocene simulation. Again we find that the model with the thickest mean state has the largest response.