Role of physical processes controlling oxycline and suboxic layer structures in the Black Sea

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Abstract
A one‐dimensional coupled physical‐biogeochemical model, together with reanalysis of the available data, are used to provide a new perspective for identification and interpretation of the suboxic layer (SOL) of the Black Sea. While the lower boundary appears to be stable at σt ≈ 16.15 ± 0.10 kg m−3, the upper boundary is found not to be isopycnally uniform as asserted previously. It is found to vary depending on the intensity of vertical diffusive and advective oxygen fluxes across the oxycline. Its position, therefore, does not always correspond to a distinct density surface irrespective of the circulation characteristics. Instead, it changes from σt ≈ 15.55 ± 0.1 in cyclonic to σt ≈ 15.9 ± 0.1 kg m−3 in anticyclonic regions, whereas its position in the peripheral Rim Current transition zone occurs at intermediate density values. These findings imply that vertical oxygen variations cannot be expressed in terms of density without taking into account physical characteristics of the water column. The presence of very strong density stratification is found to prevent ventilation of the SOL. Even intense turbulent mixing generated by a large buoyancy loss during an exceptionally cold winter season is unable to generate sufficiently dense water to overturn and temporarily supply oxygen into the suboxic zone.