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Abstract 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 the⁷Be 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⁻¹and correspondingly low vertical heat fluxes. 

Key Points Convective instabilities may govern vertical ice front melting at low ambient currents but are ignored in existing parameterizations We propose a novel melt parameterization for vertical fronts continuous across convective and shear regimes constrained by observations The contribution of background melting as opposed to melting within subglacial discharge plumes might be larger than previously thought