Winter Vertical Diffusion Rates in the Arctic Ocean, Estimated From 7Be Measurements and Dissipation Rate Profiles

• Published in: Journal of geophysical research. Oceans Vol. 128; no. 2
• Main Authors: Schulz, K., Kadko, D., Mohrholz, V., Stephens, M., Fer, I.
• Format: Journal Article
• Language: English
• Published: 01.02.2023
• Subjects: Arctic Ocean; halocline; microstructure profiling; turbulent diffusivity; vertical mixing; winter
• ISSN: 2169-9275; 2169-9291
• DOI: 10.1029/2022JC019197

Ocean turbulent mixing is a key process affecting the uptake and redistribution of heat, carbon, nutrients, oxygen and other dissolved gasses. Vertical turbulent diffusivity sets the rates of water mass transformations and ocean mixing, and is intrinsically an average quantity over process time scales. Estimates based on microstructure profiling, however, are typically obtained as averages over individual profiles. How representative such averaged diffusivities are, remains unexplored in the quiescent Arctic Ocean. Here, we compare upper ocean vertical diffusivities in winter, derived from the 7Be tracer‐based approach to those estimated from direct turbulence measurements during the year‐long Multidisciplinary drifting Observatory for the Study of Arctic Climate (MOSAiC) expedition, 2019–2020. We found that diffusivity estimates from both methods agree within their respective measurement uncertainties. Diffusivity estimates obtained from dissipation rate profiles are sensitive to the averaging method applied, and the processing and analysis of similar data sets must take this sensitivity into account. Our findings indicate low characteristic diffusivities around 10−6 m2 s−1 and correspondingly low vertical heat fluxes. Plain Language Summary Ocean turbulent mixing plays an important role in the uptake and redistribution of heat, carbon, nutrients, oxygen and other properties. For example, this process delivers nutrients to the sunlit surface ocean where they are utilized to produce plants (phytoplankton) for the ecosystem food web. However, strong changes in density within the upper Arctic Ocean hinder vertical transport of nutrients, such that nutrient fluxes are generally smaller than those observed elsewhere in the world ocean. Furthermore, low vertical transport rates isolate the surface ocean from heat input from below which helps protect the ice from melting. Here, we compare the strength of upper ocean mixing, an important parameter for the calculation of vertical transport, derived from two independent methods during the MOSAiC (Multidisciplinary drifting Observatory for the Study of Arctic Climate) ice drift experiment, 2019–2020. This comparison allows us to better quantify the vertical diffusivity, and in turn also the vertical transport of for example, heat and nutrients in the ocean. Key Points Arctic Ocean vertical diffusivity (Kz) in the upper halocline in winter is O(10−6) m2 s−1 Diffusivity estimates from 7Be measurements and ocean microstructure profiling agree within a factor of 2 Kz estimates from turbulent dissipation rate profiles are sensitive to the averaging method