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

[摘要] The temporal and spatial variability of the upper ocean hydrochemistry inthe Black Sea is analysed using data originating from profiling floats withoxygen sensors andcarried out with a coupledthree-dimensional circulation-biogeochemical model including 24 biochemicalstate variables. Major focus is on the dynamics of suboxic zone which is theinterface separating oxygenated and anoxic waters. The scatter ofoxygen data seen when plotted in density coordinates is larger than thosefor temperature, salinity and passive tracers. This scatter is indicative ofvigorous biogeochemical reactions in the suboxic zone, which acts as aboundary layer or internal sink for oxygen. This internal sink affects themixing patterns of oxygen compared to the ones of conservative tracers. Twodifferent regimes of ventilation of pycnocline were clearly identified: agyre-dominated (cyclonic) regime in winter and a coastal boundary layer(anticyclonic eddy)-dominated regime in summer. These contrasting states arecharacterized by very different pathways of oxygen intrusions along theisopycnals and vertical oxygen conveyor belt organized in multiple-layeredcells formed in each gyre. The contribution of the three-dimensionalmodelling to the understanding of the Black Sea hydro-chemistry, and inparticular the coast-to-open-sea mixing, is also demonstrated. Evidence isgiven that the formation of oxic waters and of cold intermediate waters,although triggered by the same physical process, each follow a differentevolution. The difference in the depths of the temperature minimum and theoxygen maximum indicates that the variability of oxygen is not only just aresponse to physical forcing and changes in the surface conditions, butundergoes its own evolution.

[学科分类] 地球化学与岩石