Regional Arctic scenario experiments with RCAO

The Arctic sea ice extent has been very low the last couple of years, a scenario which was not projected to occur before 2030 according to the latest IPCC report. Recently run simulations with the Rossby Centre Atmosphere Ocean climate model (RCAO) suggests that the Arctic will be for the first time almost without sea ice in late summer around 2040 and that no substantial recovery takes place after 2060 any longer.

The resolution of most global climate models is too coarse to adequately resolve important processes such as deep water convection, sea ice melting/freezing, ocean - sea ice - atmosphere interactions at the ice edge and orographical influence on the flow of water and air. As these essential processes are strongly non-linear, climate simulation in Arctic regions suffers from large uncertainties, especially due to processes involving sea ice. This problem has been highlighted recently by the observed extremely low Arctic sea ice extents during late summers 2007 to 2009, which was not expected. The IPCC report is not projecting such a low ice cover before 2030.
In order to complete the puzzle of future Arctic climate change and analyze the mechanisms and impacts, a number of regional Arctic scenario experiments are performed with the Rossby Centre Atmosphere Ocean climate model (RCAO). The regional simulations are based on A1B scenario simulations of the last IPCC Assessment Report from the Norwegian Bergen Climate Model (BCM) and the German Max-Planck-Institute climate model (ECHAM).

The regional simulations show a warmer Arctic, which agrees better to ERA-40 reanalysis data in the 20th century, and a slightly smaller trend in the 21st century compared to their corresponding global simulations.
The large scale change patterns of sea level pressure and air temperature are mainly dominated by the changes in the global models but locally significant modifications occur in the regional simulations. Generally, SLP is reduced in most of the Arctic by 1 to 3 hPa until years 2060-2080. Air temperature increases by 2 to 4 Kelvin in most Arctic regions but up to 10 Kelvin where winter sea ice disappears. In these regions, the response in the regional simulations seems to be more pronounced than in the global simulations.

Arctic summer sea ice extent, RC Newsletter nov 2009
Fig 1. Arctic summer sea ice extent in observations (black), the global model ECHAM5/MPI-OM (blue) and four regional coupled scenario simulations forced by ECHAM5/MPI-OM (dots).
Förstora Bild

The ECHAM-forced runs show generally lower summer sea ice extents and a stronger decrease than the BCM-forced runs in future. Several periods with rapid reductions of summer sea ice extent and partial recovery thereafter occur between the years 2000 and 2050 (Figure 1). The amplitudes of the reduction events compare well to the recently observed sea ice reduction and might indicate that also real-world Arctic sea ice cover can recover to the long-term trend. The observed increasing sea ice extents in summers 2008 and 2009 compared to the 2007 minimum could be part of such a recovery. However, a first analysis of the causes for the simulated rapid sea ice reductions indicate different mechanisms for the different events.
Our simulations suggest that the Arctic will be for the first time almost without sea ice in late summer around 2040 and that no substantial recovery takes place after 2060 any longer.