An observational and numerical study of wind stress variations within marginal ice zones

Abstract

Published studies of ocean mesoscale processes in marginal ice zones (MIZs) using numerical coupled ice‐ocean models usually assume that the surface wind speed is constant over the model domain and that wind stress variations are simply proportional to surface roughness variations. We show that this assumption is not realistic in most situations because the surface wind stress is also significantly affected by mesoscale pressure variations, by changes in the surface wind vector, and by changes in surface layer stability. Two numerical case studies, utilizing detailed surface and atmospheric measurements, examine the factors affecting small‐scale (<5 km) variations in wind stress within MIZs. These case studies and surveyed observational and modeling results demonstrate that wind stress fields are qualitatively different from the surface roughness fields. A realistic wind stress scenario consists of a maximum just inside the ice edge and another maximum in the open ocean. Stress minima occur within the pack ice region away from the MIZ and over grease ice, if present. The effect of the rougher MIZ ice is counteracted when wind stresses over the open ocean are enhanced by large surface heat fluxes over the ocean, by a strong low level inversion over the ice, or by a sharp atmospheric front with surface winds paralleling the ice edge. Such situations are common in MIZ regions. Some simple methods for including first‐order atmospheric effects on wind stress variations, which could be incorporated into current ice‐ocean mesoscale models of MIZ regions, are suggested.