Wave-Induced Drift Force in the Marginal Ice Zone

Abstract

Wind waves are commonly ignored when modeling the ice motion in the marginal ice zone. In order to estimate the importance of the wave forcing, an expression for the second-order wave-induced drift force on a floe exposed to a full directional wave spectrum is obtained in terms of a quadratic transfer function. For a given floe shape, the transfer function generally augments with the incident wave frequency, with a sharp increase near the resonant frequency of the pitch motion. The short wave limit of this function is determined by the shape of the horizontal contour of the floe. The value corresponding to the truncated cylindrical floe used here is two-thirds of the value obtained by the two-dimensional approximation. The total drift force is computed for two situations; an off-ice wind over a large polynya, and an on-ice wind at the extreme ice edge. In the first case, the drift force induced by the short fetch waves represents a significant fraction of the direct wind forcing and may be partly responsible for the formation of ice edge bands. In the second case, the very large drift force on a floe exposed to the high frequency components of the open water spectrum rapidly decreases (in the first few hundred meters) as these short waves are efficiently attenuated by the ice. This rapid decrease of the force generates a large compressive stress that is important in compacting the ice at the extreme ice edge.