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- Title
Upper‐Ocean Turbulence Structure and Ocean‐Ice Drag Coefficient Estimates Using an Ascending Microstructure Profiler During the MOSAiC Drift.
- Authors
Fer, Ilker; Baumann, Till M.; Koenig, Zoé; Muilwijk, Morven; Tippenhauer, Sandra
- Abstract
Sea ice mediates the transfer of momentum, heat, and gas between the atmosphere and the ocean. However, the under‐ice boundary layer is not sufficiently constrained by observations. During the Multidisciplinary drifting Observatory for the Study of the Arctic Climate (MOSAiC), we collected profiles in the upper 50–80 m using a new ascending vertical microstructure profiler, resolving the turbulent structure within 1 m to the ice. We analyzed 167 dissipation rate profiles collected between February and mid‐September 2020, from 89°N to 79°30′N through the Amundsen Basin, Nansen Basin, Yermak Plateau, and Fram Strait. Measurements covered a broad range of forcing (0–15 m s−1 wind and 0–0.4 m s−1 drift speeds) and sea ice conditions (pack ice, thin ice, and leads). Dissipation rates varied by over 4 orders of magnitude from 10−9 W kg−1 below 40 m to above 10−5 W kg−1 at 1 m. Following wind events, layers with dissipation O10−6 $\mathcal{O}\left(1{0}^{-6}\right)$ W kg−1 extended down to 20 m depth under pack ice. In leads in the central Arctic, turbulence was enhanced 2–10 times relative to thin ice profiles. Under‐ice dissipation profiles allowed us to estimate the boundary layer thickness (4 ± 2 m), and the friction velocity (1–15 mm s−1, 4.7 mm s−1 on average). A representative range of drag coefficient for the MOSAiC sampling site was estimated to (4–6) × 10−3, which is a typical value for Arctic floe observations. The average ratio of drift speed to wind speed was close to the free‐drift ratio of 2% with no clear seasonal or regional variability. Plain Language Summary: Turbulence in the ocean mixes water masses and redistributes heat, nutrients and dissolved gases. In the Arctic Ocean, the difference between the sea ice drift and the ocean currents below is a major source of turbulence. The turbulent layer under ice controls the exchange between ice and ocean. Unfortunately, conventional profiling instruments cannot measure turbulence within a few meters of the sea ice. Here we report on measurements collected during the Multidisciplinary drifting Observatory for the Study of the Arctic Climate (MOSAiC), using an instrument specially designed to profile upward from 50 to 80 m depth to the ice‐water interface. We show that this instrument works satisfactorily in the Arctic conditions, delivering unique measurements of turbulence to within 1 m under ice. Measurements were taken in varying wind speed, ice drift, and sea ice conditions, and showed large variability. Energetic turbulent layers following wind events reached to 20 m depth under pack ice. In patches of open waters in the central Arctic, turbulence was more energetic compared to the profiles when the surface was covered by thin ice. Characteristic values that describe the exchange between the ice drift and turbulence are estimated and compared to earlier Arctic observations. Key Points: A novel ascending microstructure profiler successfully measured dissipation rates to within 1 m under the sea iceDissipation rate estimates in the Arctic Ocean during the Multidisciplinary drifting Observatory for the Study of the Arctic Climate (MOSAiC) drift varied by more than four orders of magnitudeA representative range of ice‐ocean drag coefficient for the MOSAiC sampling site was (4–6) × 10−3
- Subjects
NANSEN Basin; ARCTIC regions; WATER masses; DRAG coefficient; ICE prevention &; control; WIND speed; SEA ice drift; SEA ice; TURBULENCE; SURFACE waves (Seismic waves)
- Publication
Journal of Geophysical Research. Oceans, 2022, Vol 127, Issue 9, p1
- ISSN
2169-9275
- Publication type
Article
- DOI
10.1029/2022JC018751