Sea-Ice Generated Internal Waves: Impacts on Ice-Ocean Fluxes of Heat and Momentum

Kevin Thielen and Andrew Wells, University of Oxford, Oxford, United Kingdom
The thickness of sea ice is spatially heterogeneous, with pressure ridges and leads providing some of the most extreme variations of ice draft. Recently developed parameterisations suggest that variability in ice thickness can have a significant impact on the transfer of momentum and heat between atmosphere, ice and the underlying ocean, with potential consequences for sea ice melting rates and Arctic ocean circulation. However, such parameterisations often treat ice-ocean drag in neutrally stable environments, which neglects the dynamical impact of near-surface ocean stratification.

We here examine the potential for large morphological features of sea ice to affect the dynamics of the mixed layer, ice-ocean drag, and the ice-ocean heat flux. We simulate two dimensional stratified ocean flow beneath an ice boundary with a pressure ridge using the Lattice Boltzman method. This allows us to consider fully nonlinear and non-hydrostatic internal wave dynamics. The flow dynamics are characterised in terms of a Froude number, which measures the ratio of fluid inertia to the stabilising buoyancy forces in the stratification. Characterising ice-ocean drag in terms of a drag coefficient, we find that the drag coefficient increases significantly as the Froude number decreases, with both upstream and downstream propagating waves for subcritical flows at sufficiently small Froude number. Whilst the mean floe-averaged heat flux shows relatively weak variation with Froude number, there is a strong spatial variability of the ice-ocean heat fluxes. Melting is particularly enhanced around ridge keels, but also oscillates with the propagating waves, suggesting the potential to generate patterned variability in ice thickness surrounding the ridge. Our results also suggest that ice generated internal waves could provide an important contribution to ice-ocean drag, and the corresponding impact of Ekman pumping on the dynamics of sea-ice covered oceans.