Simulating the evolution of the Arctic climate during the last millennium

Ensemble simulations have been performed with a global three-dimensional atmosphere-sea-ice-ocean model driven by both natural (solar and volcanic) and anthropogenic (increase in greenhouse gas concentrations and tropospheric aerosols) forcings during the 2nd millennium AD. The model consists of ECBILT, a spectral T21, 3-level quasi-geostrophic atmospheric model, coupled to CLIO, a coarse resolution sea-ice ocean general circulation model. All 5 ensemble members display relatively mild temperatures averaged over the Arctic during the first centuries of the simulation and then a cooling that starts as early as 1250. This gradually leads to minimum temperatures that occur in the model during the periods 1670-1700 and 1800-1830. Subsequently, during the 19th and 20th century, the Arctic climate warms under the influence of both natural and anthropogenic forcings. If the latter are not included, the model reached a local maximum in the 1940’s followed by a cooling. When taking into account all forcings, the absolute maximum of the ensemble mean is reached at the end of the simulation. The analysis of the ensemble mean shows that the response of the high latitudes to the forcings is significantly stronger than at lower latitudes. In addition to this forced variability, the model displays unforced variations. The relative contribution of those two types of variability can be estimated at large and regional scales from the spread of the ensemble members, enabling a more sound comparison with observations.