Fast ice controls on turbulent mixing rates on the West Antarctic Peninsula shelf

The ocean surface boundary layer (OSBL) experiences both momentum and buoyancy input from the atmosphere, which are strongly implicated in the seasonal development of the mixed layer. However, in polar shelf seas during winter, the presence of a rigid layer of fast ice can render a profoundly different OSBL, where these fluxes are strongly suppressed. While the under-ice OSBL has been extensively studied in the Arctic, no study, to the best of our knowledge, has quantified the dissipation rate at the same location under contrasting conditions of fast-ice covered and fast-ice free conditions in the Antarctic. In this study, we report on a set of hydrographic and turbulence observations taken in February and August 2016 in Ryder Bay, West Antarctica, alongside accompanying meteorological and velocity observations. The results yield a profoundly different OSBL in the two seasons. Dissipation rates in the top 100 m are strongly enhanced in the ice-free season compared with the fast-ice covered season (values of ~6 x 10^-9 W kg^-1 compared with ~1.5 x 10^-9 W kg^-1). Analysis of a neighbouring moored ADCP suggests that this is attributable to a significant increase in wind-generated near-inertial energy in the upper ocean during summer. By contrast, in winter, dissipation in the top 30 m of the water column appears to exhibit a law-of-the -wall type scaling, and is sufficiently elevated to reduce tidal flow amplitude by ~40%. While the upwards heat fluxes implied by these levels of dissipation are modest (typically 1.5 W m^-2), the rapid reduction in West Antarctic fast ice observed over the last 30 years may lead to a change in the dynamics, dissipation rates and heat fluxes of the OSBL.