On the Response of the Arctic Ocean ice thickness distribution to changes in external forcing

The sensitivity of the Arctic Ocean ice cover to the atmospheric poleward energy flux, D, is studied using a coupled column model of the ocean, ice, and atmosphere. In the model, the ice cover is described by a thickness distribution and the atmosphere is a simple two stream grey body, in radiative equilibrium. It is shown that the thickness distribution in combination with the albedo function gives a strong nonlinear response to positive perturbations of D. The response on D is sensitive to the albedo parameterization and the shape of the thickness distribution, controlled by ridging and divergence. An increase of 9 W m^-2 from a standard value of D=103 W m^-2 has a dramatic effect, reducing the ice thickness with more than 2 m and generates a large open water fraction during summer. The reduction of ice thickness is characterized by a clear transition between two regimes, going from a regime where first year ice survives the next summer melt period, to a regime where the first year ice melts completely. The adjustment time scale of the ice cover towards a new equilibrium is 6 years for negative and small positive perturbations of D. For larger positive perturbations the adjustment take up to 20 years. A somewhat speculative conclusion is that even if the high sensitivity of the ice cover on D may result in quite large changes of the ice thickness, a feedback on the poleward energy flux prevents the ice cover to reach extreme states with very much thinner or thicker ice.