Turbulent Mixing During Late Summer in the Ice–Ocean Boundary Layer in the Central Arctic Ocean: Results From the MOSAiC Expedition

• Published in: Journal of Geophysical Research: Oceans
• Main Authors: Kawaguchi, Yusuke, Koenig, Zoé, Nomura, Daiki, Hoppmann, Mario, Inoue, Jun, Fang, Ying‐Chih, Schulz, Kirstin, Gallagher, Michael, Katlein, Christian, Nicolaus, Marcel, Rabe, Benjamin
• Format: Article in Journal/Newspaper
• Language: unknown
• Published: American Geophysical Union (AGU) 2022
• Subjects: Arctic; Arctic Ocean; Holmboe; North Pole; Sea ice
• Online Access: https://epic.awi.de/id/eprint/58801/; https://epic.awi.de/id/eprint/58801/1/JGR%20Oceans%20-%202022%20-%20Kawaguchi%20-%20Turbulent%20Mixing%20During%20Late%20Summer%20in%20the%20Ice%20Ocean%20Boundary%20Layer%20in%20the%20Central%20Arctic.pdf; https://doi.org/10.1029/2021jc017975; https://hdl.handle.net/10013/epic.64eeff98-20d5-44bd-a96f-a64484145ea0

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.