Ocean feedback to wind‐driven coastal polynyas

Abstract

Abstract. For lack of observations, a coupled numerical model of an ice layer and a coastal ocean is used to conduct thought experiments on the evolution of wind-driven coastal polynyas. Attention is focused on the possible ocean feedback to polynya developments before and after wind relaxation. The coupled system is initially motionless, having an ice layer of uniform concentration and thickness. In the ocean, the salinity is initially uniform and the temperature is at the freezing point. Subsequent cold air outbreaks of several days in duration push the ice layer offshore. Frazil is produced and collected in the polynya owing to continuous heat loss to the atmosphere. Ocean feedback is particularly effective after wind relaxation. Under large-scale seaward winds, the displaced ice edge does not return landward after wind abatement if the wind event is strong and long. Over a sizable coastal bottom obstacle, however, anticyclonic circulation develops and its landward arm may force a landward return of the ice edge after wind relaxation. Further, dipole vortices in the ocean may develop under a mesoscale seaward wind event. Landward ocean currents around the rim of a dipole may also force a postwind return of the ice edge. The ocean feedback is generally stronger and faster in shallower basins; this can be demonstrated by a one-dimensional, stress-driven coastal ocean model. IntroductionCoastal polynyas are openings of ice-free regions in the coastal ocean. In winter, coastal polynyas are often produced by seaward winds. If the air and water are cold enough to produce ice, the new frazil racing toward the seaward retreating ice layer may limit the maximum width of a coastal polynya [Lebedev, 1968]. A simple ice layer model [Pease, 1987] was formulated following this idea and compared favorably with limited observations. Pease's model is kinematic, assuming that newly formed frazil in open water is instantaneously collected at the ice edge. Her model was later generalized to incorporate finite drift speed of frazil [Ou, 1988]