Interactions between wind waves and frazil ice in the turbulent surface boundary layer of an Antarctic coastal polynya

Coastal (latent heat) polynyas are regions of extremely strong ocean–atmosphere heat, moisture and momentum exchange, often with wind speed and surface turbulent heat flux exceeding 30 m·s^–1 and 1000 W·m^–2, respectively, and air temperature below –20°C. Consequently, polynyas play a very important role in shaping the local and regional weather, are crucial for sea ice production and the associated formation of dense water masses. The ocean mixed layer (OML) during polynya events is highly turbulent, with turbulent dissipation due to wind shear, waves and convective mixing. Crystals of frazil ice forming in those very dynamic conditions are transported throughout the OML along irregular, three-dimensional trajectories. The manifestation of those processes at the surface are characteristic elongated strips with high frazil concentration – so called frazil streaks – forming in convergence zones of the Langmuir circulation (https://egusphere.copernicus.org/preprints/2022/egusphere-2022-1435/). The presence of frazil streaks and open water areas between them leads to high spatial variability of OML and, crucially, sea surface properties. In particular, the bulk water viscosity within streaks is much higher and the sea surface roughness much lower than in open water. This in turn affects the momentum flux from the atmosphere and the evolution of wind waves. Wave breaking is suppressed, and short waves are dissipated by frazil/grease ice. Therefore, the whole spectral energy balance is modified. In this paper, satellite data and spectral wave modelling are used to analyse fetch-limited, deep-water wave growth during selected polynya events in the Terra Nova Bay, Antarctica. It is shown that wave growth in the presence of frazil streaks is slower than in analogous ice-free situations, and that wave–ice interactions are the only plausible explanation for observations. Simulations with a spectral wave model SWAN (Simulating Waves Nearshore) are used to examine different scenarios of how the source terms related to wind input, quadruplet wave–wave interactions, whitecapping, and dissipation in grease ice contribute to the net wave energy growth with distance from shore. Additionally, the role of across-wind variations of wind speed and wave properties is examined in detail, illustrating the inherently two-dimensional character of the polynyas’ wave field. Overall, the study shows that polynya events provide a unique, very valuable setting for studying wave–ice interactions, in many respects fundamentally different from the ‘standard’ case of swell entering the marginal ice zone from the open ocean.