Seasonal variation in a transient response of sea-ice thickness to perturbed thermal forcing

Understanding the stability and sensitivity of a sea-ice cover subjected to climate change requires an examination of a sea ice response under realistic forcing conditions in a wide range of time scales. As a step toward this goal, we investigate a transient response of ice thickness to simple and idealized forcing conditions from an equilibrium state in seasonal to interannual time scales. More specifically, we ask the following questions: Does the sea-ice response vary depending upon temporal characteristics of perturbations in excess to the canonical seasonal cycle in thermal forcing. If it does, what mechanisms control this temporal dependence of the sea ice response, and how? In order to address these questions we made a series of sensitivity experiments using a one-dimensional thermodynamic sea ice model. The experiments were initialized with an equilibrium state with respect to the forcing using the standard seasonal cycle. A control run was then made with a constant amount (i.e. 1 Wm-2) of extra downward radiation superimposed on the standard forcing "year-round". Then for seasonal dependence we ran the model using the same initial condition but perturbed differently. Instead of a year-long perturbation only a month-long perturbation was applied for each of 12 different months at a time while fixing the same total perturbed amount of heat over the annual cycle (i.e. 12 Wm-2 during January first, then February and so on). Any systematic seasonal difference in ice thickness from these experiments is then interpreted as evidence for seasonal dependence in the transient response to this simple seasonally dependent perturbation.

The results from these experiments show systematic seasonal dependence where the response is much stronger in summer months than in the rest of the year, i.e. more than a factor difference in the change in the annual average of ice thickness between the summer and winter cases. This is a direct consequence of combined effects from a relatively rapid thermal response at the surface during winter and from a fixed surface temperature during the surface ablation period. That is, during winter much of the increase in the downward radiation is counterbalanced by an increase in the upward radiation resulting from a higher surface temperature. A ratio of the net increase in the net radiation input to the amount of perturbation reaches only about 15% during winter. In contrast, almost all of the extra heat is used to melt ice in summer. In this presentation we will also discuss results from other cases with decreasing downward radiation and varying perturbed magnitudes in the context of the stability and sensitivity of sea-ice under different climate change scenarios.