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.

‘Observations and Modelling of Cold-air Advection over Arctic Sea Ice’

“…Data on the spatial variations have been obtained by surface-based measurements on the upwind and downwind sides of leads (Andreas et al, 1979) or on a more extensive observation grid, as in SHEBA, and by aircraft observations. The latter have most frequently been made over the ice-edge zone, with a particular interest in the effects on the ABL arising from the large thermal differences between the ice and open water (e.g., Fairall and Markson, 1987;Guest et al, 1995;Dru¨e and Heinemann, 2001). Studies on spatial variability have also been carried out over compact sea ice; these have mostly concentrated on cases of warm-air advection over sea ice (Bennett and Hunkins, 1986;Bru¨mmer et al, 1994;Bru¨mmer and Thiemann, 2002;Vihma et al, 2003), during which the spatial change is related to the gradual cooling of the air mass and the development of a stable internal boundary layer.…”

The Effect of the Sea-ice Zone on the Development of Boundary-layer Roll Clouds During Cold Air Outbreaks

“…Based on observations from the Marginal Ice Zone Experiment (MIZEX) and the Coordinate Eastern Artic Experiment (CEAREX), Guest et al (1995) numerically investigated the wind stress variations within the marginal sea-ice zone and provided an improved method for implementing wind stress forcing over sea ice. Dare and Atkinson (1999; used a two-dimensional model to study the response of the atmospheric boundary layer to large leads and polynyas in the sea-ice zone of the Southern Ocean.…”

Impact of Wind-Driven Mixing in the Arctic Ocean

“…Observational and numerical studies show that the wind stress is often at a local maximum near the edge of the ice (Guest et al, 1995). Indeed, the stress associated with …”