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- Title
Turbulent Mixing During Late Summer in the Ice–Ocean Boundary Layer in the Central Arctic Ocean: Results From the MOSAiC Expedition.
- Authors
Kawaguchi, Yusuke; Koenig, Zoé; Nomura, Daiki; Hoppmann, Mario; Inoue, Jun; Fang, Ying‐Chih; Schulz, Kirstin; Gallagher, Michael; Katlein, Christian; Nicolaus, Marcel; Rabe, Benjamin
- Abstract
We examined mixing processes within the ice–ocean boundary layer (IOBL) close to the geographic North Pole, with an emphasis on wind‐driven sea ice drift. Observations were conducted from late August to late September 2020, during the final leg of the international Multidisciplinary drifting Observatory for the Study of Arctic Climate (MOSAiC) expedition. Measurements of ice motion, and profiles of currents, hydrography, and microstructure turbulence were conducted. The multifarious direct observations of sea ice and the upper ocean were used to quantify the transport of momentum, heat, and salt in the IOBL. The ice drift was mostly characterized by the inertial oscillation at a semi‐diurnal frequency, which forced an inertial current in the mixed layer. Observation‐derived heat and salinity fluxes at the ice–ocean interface suggest early termination of basal melting and transitioning to refreezing, resulting from a rise in the freezing point temperature by the presence of freshened near‐surface water. Based on the friction velocity, the measured dissipation rate (ε) of turbulent energy can be approximated as 1.4–1.7 times of the "Law of the Wall" criterion. We also observed a spiraling Ekman flow and find its vertical extent in line with the estimate from ε‐based diffusivity. Following passage of a storm, the enhanced oscillatory motions of the ice drift caused trapping of the near‐inertial waves (NIWs) that exclusively propagated through the base of the weakly stratified mixed layer. We accounted Holmboe instabilities and NIWs for the observed distinct peak of the dissipation rate near the bottom of the mixed layer. Plain Language Summary: We examined how sea ice drift in the Arctic Ocean affects the movement of seawater directly under the ice, and how this impacts freezing and melting of the ice itself. During the Multidisciplinary drifting Observatory for the Study of Arctic Climate expedition, we used a number of different instruments to measure air, sea ice and ocean properties on an ice floe between late August and late September 2020. We recorded the drift tracks of the ice, and linked the ice motion to the currents, temperature and salinity within the upper 50 m of the ocean. Strong winds triggered wavy fluctuations and water mixing, in particular close to where the ice and ocean meet. During a storm in mid‐September, the ice stopped melting and started to refreeze even though the water and the air were still relatively warm. We explain this by the presence of surface water that was less salty, and therefore froze faster at higher temperature, and by the ice moving faster than usually observed in the region. In combination, these factors provided favorable conditions for sea ice formation. Our results suggest that the distribution of sea ice meltwater need to be accounted for in order to better predict Arctic sea ice conditions in the future. Key Points: Enhanced ice drift and near‐surface freshened water jointly promoted early termination of basal melting and preconditioning of refreezingTurbulent dissipation rate can be scaled by 1.4–1.7 times of the "Law of the Wall" criterion, with surface buoyancy flux being negligibleNear‐inertial wave was trapped by weakly stratified water in lower mixed layer, which produced turbulence by the Holmboe instability
- Subjects
NORTH Pole; TURBULENT mixing; BOUNDARY layer (Aerodynamics); SEA ice; ICE floes; SEA ice drift; ARCTIC climate
- Publication
Journal of Geophysical Research. Oceans, 2022, Vol 127, Issue 8, p1
- ISSN
2169-9275
- Publication type
Article
- DOI
10.1029/2021JC017975