[A-OS16] Frontiers of Ocean Mixing Research

[AOS16-03] Observations of ice-ocean boundary fluxes associated with near-inertial internal waves in the central Arctic Ocean

*Yusuke Kawaguchi¹, Zoé Koenig^2,6, Daiki Nomura³, Mario Hoppman⁴, Jun Inoue⁵, Ying-Chih Fang^4,7, Kirstin Schulz⁴, Michael Gallagher^8,9, Christian Katlein⁴, Marcel Nicolaus⁴, Benjamin Rabe⁴ (1.Atmosphere and Ocean Research Institute, The University of Tokyo , 2.Norwegian Polar Institute, 3.Field Science Center for Northern Biosphere, Hokkaido University, 4.The Alfred Wegener Institute, Helmholz Centre for Polar and Marine Research, 5.National Institute of Polar Research, 6.Geophysical Institute and Bjerknes Center for Climate Research, The University of Bergen, Bergen, 7.National Sun Yat-sen University, 8.Physical Sciences Laboratory, National Oceanic and Atmospheric Administration, 9.Cooperative Institute for Research in Environmental Science)

Keywords:Ice-ocean boundary layer , the Arctic Ocean, Inertial drift of sea ice , Law of the Wall , Holmboe instability

This study examined mixing processes within the ice-ocean boundary layer (IOBL) with emphasis on wind-driven ice drift near the North Pole of the Arctic. Observations were conducted for one month from late August to late September, 2020, as last leg of the international Multidisciplinary drifting Observatory for Arctic Climate (MOSAiC). The multifarious direct observations of sea ice and surface water quantified the exchange of momentum, heat, and salt in IOBL. Along with the tracked ice motion, sequential profiles of current, hydrography and microstructures were investigated. The ice drift was mostly characterized by the inertial oscillation at semi-diurnal frequency, which resonated the inertial current in mixed layer. According to the local turbulence closure, heat and salinity fluxes at the ice–ocean interface suggested early termination of basal melting and transitioning to refreezing. This is because of the freshened near-surface water that resulted in a rise of freezing point. Based on the friction velocity, u₀*, measured dissipation rate ε of turbulence energy can be approximated by 0.7–3.2 times of Law of the Wall criterion. In the scaling, the unstable boundary condition served less importantly. The SML-integrated ε supported good correlations with the under-ice energy flux E₀ and the wind work E₁₀: ε/E₀ ≈ 0.088 and E₁₀/ε≈ 0.012. Also, we observed a spiraling feature of Ekman current, whose vertical depth scale matched with that by ε-based diffusivity defined by δ_E(K_z). Following a storm passage, the oscillatory motions of ice drift caused the near-inertial waves that exclusively propagated through a weakly stratified water in lower mixed layer. We found a distinct peak of ε near bottom of mixed layer as a consequence of near-inertial waves likely through the Holmboe instability.