Upper‐Ocean Turbulence Structure and Ocean‐Ice Drag Coefficient Estimates Using an Ascending Microstructure Profiler During the MOSAiC Drift

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 scriptO()10−6 $\mathcal{O}\left(1{0}^{-6}\right)$ 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.

The Transition Layer and Remnant Transition Layer of the Western Arctic Ocean: Stratification, Vertical Diffusivity, and Pacific Summer Water Heat Fluxes

“…Tens to hundreds of meters beneath the transition layer, finescale parameterizations and estimates from microstructure returned eddy diapycnal diffusivity estimates ranging between 10 −7 and 10 −5 m 2 /s (Fer, 2009; Guthrie et al., 2013; Lincoln et al., 2016; Lique et al., 2014; Rainville & Windsor, 2008; Shaw & Stanton, 2014). These diffusivity values are similar to those recently obtained at depths of the transition layer from tracer observations of 10 −6 m 2 /s (Schulz et al., 2023), and consistent with weak turbulent dissipation rate estimates at these depths (Fer et al., 2022). These values are generally smaller than at lower latitudes, consistent with the finding that the surface layer stratification in the Arctic is strong and persistent, impermeable even to storm activity (e.g., Lincoln et al., 2016).…”

The Eurasian Arctic Ocean along the MOSAiC drift in 2019-2020: An interdisciplinary perspective on physical properties and processes

“…The presence of meltwater results in a very strong stratification in the uppermost meters, up to two orders of magnitude stronger compared to the halocline. Measurements with an uprising turbulence profiler also show drastically reduced turbulent mixing in the near-surface layer when meltwater layers were present Fer et al (2022). Details on the dynamics and implications of meltwater layers can be found in Smith et al (2023,2022); Salganik et al (2023a); Nomura et al (2023).…”

Temporal evolution of under-ice meltwater layers and false bottoms and their impact on summer Arctic sea ice mass balance

“…The thermodynamic model from Alexandrov and Nizovtseva (2008) was able to accurately describe false bottom thickness evolution (Figure 8a). Fer et al (2022) reported an average value of friction velocity of 0.47 cm s -1 for MOSAiC observations and estimated an average value of ice-ocean drag coefficient 𝐶 𝑑 as 5×10 −3 , which gives much higher average value of friction velocity of 1.65 cm s -1 , more than two times higher than measured by microstructure profiler in July. We found the best fit of in situ observations and model predictions of false bottom thickness for the average friction velocity of 0.57 cm s -1 and corresponding ice-ocean drag coefficient of 6×10 −4 .…”

“…This model requires measurements of seawater temperature, salinity and friction velocity. Ice-ocean friction velocity was estimated based on the 6-hour average ice drift velocity measurements from Polarstern using ice-ocean drag coefficient 𝐶 𝑑 , while seawater physical parameters were measured by the Polarstern thermosalinograph at a depth of 11 m. The estimated values of ice-ocean friction velocity and drag coefficient were compared with in situ measurements from vertical microstructure profiler by Fer et al (2022). For false bottom modelling, the physical parameters of sea ice were taken from Alexandrov and Nizovtseva (2008).…”