The impact of dynamic sea-ice on the climatology and climate sensitivity of a GCM: a study of past, present, and future climates

We assess two parametrisations of sea-ice in a coupled atmosphere-mixed lay er ocean-sea-ice model. One parametrisation represents the thermodynamic pr operties of sea-ice formation alone (THERM), while the other also includes advection of the ice (DYN). The inclusion of some sea-ice dynamics improves the model's simulation of the present day sea-ice cover when compared to o bservations. Two climate change scenarios are used to investigate the effec t of these different parametrisations on the model's climate sensitivity. T he scenarios are the equilibrium response to a doubling of atmospheric CO2 and the response to imposed glacial boundary conditions. DYN produces a sma ller temperature response to a doubling of CO2 than THERM. The temperature response of THERM is more similar to DYN in the glacial case than in the 2 x CO2 case which implies that the climate sensitivity of THERM and DYN vari es with the nature of the forcing. The different responses can largely be e xplained by the different distribution of Southern Hemisphere sea-ice cover in the control simulations, with the inclusion of ice dynamics playing an important part in producing the differences. This emphasises the importance of realistically simulating the reference climatic state when attempting t o simulate a climate change to a prescribed forcing. The simulated glacial sea-ice cover is consistent with the limited palaeodata in both THERM and D YN, but DYN simulates a more realistic present day sea-ice cover. We conclu de that the inclusion of simple ice dynamics in our model increases our con fidence in the simulation of the anomaly climate.