Mixing in the Black Sea detected from the temporal and spatial variability of oxygen and sulfide – Argo float observations and numerical modelling

Abstract

Abstract. The temporal and spatial variability of the upper ocean hydrochemistry in the Black Sea is analysed using data originating from profiling floats with oxygen sensors and carried out with a coupled three-dimensional circulationbiogeochemical model including 24 biochemical state variables. Major focus is on the dynamics of suboxic zone which is the interface separating oxygenated and anoxic waters. The scatter of oxygen data seen when plotted in density coordinates is larger than those for temperature, salinity and passive tracers. This scatter is indicative of vigorous biogeochemical reactions in the suboxic zone, which acts as a boundary layer or internal sink for oxygen. This internal sink affects the mixing patterns of oxygen compared to the ones of conservative tracers. Two different regimes of ventilation of pycnocline were clearly identified: a gyre-dominated (cyclonic) regime in winter and a coastal boundary layer (anticyclonic eddy)-dominated regime in summer. These contrasting states are characterized by very different pathways of oxygen intrusions along the isopycnals and vertical oxygen conveyor belt organized in multiple-layered cells formed in each gyre. The contribution of the three-dimensional modelling to the understanding of the Black Sea hydro-chemistry, and in particular the coast-to-open-sea mixing, is also demonstrated. Evidence is given that the formation of oxic waters and of cold intermediate waters, although triggered by the same physical process, each follow a different evolution. The difference in the depths of the temperature minimum and the oxygen maximum indicates that the variability of oxygen is not only just a response to physical forcing and changes in the surface conditions, but undergoes its own evolution.

Decline of the Black Sea oxygen inventory

“…It is also essential (1) to determine to which extent the shoaling of the oxygen penetration depth entrains a shoaling of the sulfidic onset depth, (2) to set up a continuous monitoring of the Black Sea oxygen inventory and the intensity of winter convective ventilation (through CIL cold content), and (3) to clarify and quantify the interplay of diapycnal and isopycnal ventilation mechanisms and, in particular, the role played by the peripheral permanent/semipermanent mesoscale structures and how this relates to the intensity of the Rim Current (Stanev et al, 2014;Kubryakov and Stanichny, 2015). We propose that these objectives might be answered by maintaining in the Black Sea a minimum population of both moored and drifting autonomous profilers equipped with oxygen and sulfidic sensors.…”

“…The threshold of 20 µM of oxygen was applied later to analyze historical oxygen data of lower quality. The upper (O 2 disappearance) and lower (H 2 S onset) interfaces of this suboxic layer are controlled by different biogeochemical and physical processes Stanev et al, 2014), and undergo uncorrelated vertical migrations (Konovalov and Murray, 2001). Sinking organic matter is mainly respirated aerobically within the oxycline: the lower part of the oxygenated layer where oxygen concentration decreases downwards to 20 µM.…”

“…In addition to these biogeochemical factors, the dynamics of the upper and lower interfaces of the suboxic layer are controlled by physical processes Stanev et al, 2014). In the Black Sea, dense waters formed by winter cooling and mixing (Staneva and Stanev, 2002) do not sink to the deepest layer, as in the Mediterranean Sea, but accumulate on top of the permanent halocline (Fig.…”

“…In the presence of large gradients, uneven data distribution may induce artificial signals when interannual trends are assessed from direct annual averages. In the stratified Black Sea, properties expressed in terms of depth coordinates (m) present a high spatial variability due to mesoscale features (Kempe et al, 1990) and to the general curvature of Black Sea isopycnals (Öszoy and Ünlüata, 1997;Stanev et al, 2014). As an alternative, using density (isopycnal levels, σ θ ) as vertical coordinate is generally considered a stable solution to assess the vertical migration of the chemocline on a decadal scale (Tugrul et al, 1992;Saydam et al, 1993;Murray et al, 1995).…”

Black Sea thermohaline properties: Long‐term trends and variations

“…Not only the area covered by CIL shrinks but also the CIL thickness decreases. Several authors [ Oguz et al ., ; Stanev et al ., ] suggested CIL disappears due to the atmosphere warming, for example in 2001 or 2011 because in both years the WSST is higher than 9°C. Our simulations (Figure ) coincide with the experimental and previous modelling evidence that CIL temperature increases considerably and less area is covered with intermediate layers of temperature lower than 8°С in the last decades.…”