The sensitivity of the Arctic Ocean ice cover on 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 gray 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 about 9 W m-2 from a standard value of D = 103 W m-2 has a dramatic effect, reducing the ice thickness by 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 seasonal ice regime where the first-year ice melts completely. It is shown that the existence of seasonal ice regime is dependent on the surface mixed layer thickness. The model enters a completely ice-free state if the thickness of the mixed layer is increased above a threshold value. The adjustment timescale for the ice cover is 6 years for small positive and negative perturbations in D. For larger positive perturbations of about 10 W m-2, the adjustment timescale is up to 20 years.