Estimating vertical mixing in the deep north Aegean Sea using argo data corrected for conductivity sensor drift

Successive CTD profiles of an Argo float trapped within a deep (~1250 m) sub-basin of the North Aegean Sea during 2014–2015 enabled identification of deep-water ventilation episodes and inference of a bulk (budget-based) vertical eddy diffusivity during the period that followed these events. Independent effective eddy diffusivity profiles were computed under the assumption that the turbulent exchange of heat, salt and buoyancy through each isobath below 400 m will balance the rate of change of the corresponding variable's content of the water volume within the basin below that depth. The initially estimated thermal, saline and density diffusivity profiles were significantly different from each other. Assuming turbulence was the dominant mixing process between the intermediate and deep water masses led to identification of conductivity sensor drift rate of 4.6 × 10−6 S m−1 day−1. After correcting the sensor's drift, bulk eddy diffusivities Kρ, KS and KT were found to span (2–30) × 10−3 m2 s−1, in close agreement below 400 m depth. These estimates are at least one order of magnitude higher than eddy diffusivities based on a finescale internal-wave strain parameterization. Enhanced boundary mixing and double diffusion are examined as candidates to explain the high effective diffusivity values. A hypsometric correction appears to provide sufficient diapycnal transport along the boundaries to reproduce bulk buoyancy transports.