Vertical diffusion processes in the Eastern Mediterranean — Black Sea System Sotiris Kioroglou a,b, ⁎, Elina Tragou b , Vassilis Zervakis b , Dimitris Georgopoulos a , Barak Herut d , Isaak Gertman d , Vedrana Kovacevic c , Emin Özsoy e , Ersin Tutsak e a Hellenic Center for Marine Research, Athens Greece b Department of Marine Sciences University of Aegean, Mytilene, Greece c Instituto Nazionale di Oceanografia e di Geofisica Sperimentale, OGS, Italy d Israel Oceanographic and Limnological Research e Institute of Marine Sciences, Technical University of Middle East, Erdemli, Turkey abstract article info Article history: Received 15 December 2011 Received in revised form 1 August 2013 Accepted 26 August 2013 Available online 16 October 2013 Keywords: Overturns Diffusion Thermohaline staircase Mixing Mediterranean Black Sea The identification and examination of ‘complete’ potential density overturns in CTD profiles, within the framework of SESAME project, are employed to assess vertical eddy diffusivities, mostly in the top 100 m of the water column, for a broad area covering the East Mediterranean, the Turkish Straits and the Black Sea. The implementation of this method shows that, mixing induced by mechanical turbulence is enhanced in frontal areas, in the proximity of straits and inside anticyclones; furthermore, that mechanical turbulence is insignificant, down to the scale of CTD resolution, within areas of double diffusive staircases, encountered in deep layers of the water column. Consequently, only laminar theories about double diffusion are applied for assessing diffusivities therein. Susceptibility to different types of double diffusion seems to be related to the interaction of different types of water masses. © 2013 Elsevier B.V. All rights reserved. 1. Introduction Turbulent and/or double diffusive mixing have proven to be significant for physical chemical and biological oceanic processes (e.g., Cullen et al., 1983; Gargett, 1984; Gargett and Holloway, 1984) and their parameterization to play a key role in the effectiveness of numerical models in ‘simulating’ oceanic processes (McDougall and Ruddick, 1992). In this work we attempt to assess basic parameters of vertical mixing (eddy diffusivities and dissipation rates) for the broader area encompassing the Eastern Mediterranean and the Black Sea, based on CTD data which were acquired within the framework of the EU funded project SESAME. We estimate diffusivities not only for areas characterized solely by mechanical turbulence, but also for areas where double diffusion might be combined with mechanical turbulence. The ‘ state of the art’ instruments for resolving the full turbulent spectrum, almost down to the Kolmogorov microscale (a few cm), at which turbulence decays due to molecular dissipation, are the vertically free falling (Gregg et al., 1978; Osborn and Crawford, 1980; Wolk et al., 2001) or horizontally towed microprofilers (Mormorino et al., 1987), equipped with fast response vertical shear, temperature and conductivity probes. These have been invented in the seventies and since then are being continually improved and sparsely deployed in the ocean, due to their high cost and requirements for highly specialized personnel. However vertical profiles of temperature and salinity, (and hence density) that could be valuable for vertical mixing estimates are also provided by standard CTDs, routinely used in oceanographic expeditions since the second half of the 20th century. Admittedly, their resolution and accuracy cannot compete with the data quality provided by a microprofiler. Still, the abundance of available CTD data from the world's oceans and their comparatively low cost acquisition provide a strong motive for exploiting them as a tool for vertical mixing estimates. The basis of the CTD use in assessing mixing is that turbulence, being three-dimensional, unavoidably results in the ‘overturning’ of the water column, a situation in which heavier fluid particles lay instantaneously over lighter ones, forming a temporary, statically unstable, Z-shaped configuration in the vertical potential density (σ θ ) profile. We applied the overturn method for areas of pure mechanical turbulence and also for checking whether mechanical turbulence could be combined with double diffusion (Laurent and Schmitt, 1998). For the second case in particular where mechanical turbulence coexists with double diffusion, laminar theories are not sufficient and modifications accounting for both mixing processes should be implemented (e.g., McDougall and Ruddick, 1992). In this context we first verified that no valid overturns were present within double diffusive areas, before selecting the suitable laminar models for evaluating double diffusive fluxes and diffusivities. Journal of Marine Systems 135 (2014) 53–63 ⁎ Corresponding author at: Hellenic Center for Marine Research, PS 16343, 46.7 of Athinon Souniou Avenue, Athens, Greece. E-mail address: skior@hcmr.gr (S. Kioroglou). 0924-7963/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jmarsys.2013.08.007 Contents lists available at ScienceDirect Journal of Marine Systems journal homepage: www.elsevier.com/locate/jmarsys