Unexpected changes in the oxic/anoxic interface in the Black Sea

THE Black Sea is the largest anoxic marine basin in the world today1. Below the layer of oxygenated surface water, hydrogen sulphide builds up to concentrations as high as 425 μM in the deep water down to a maximum depth of 2,200 m (ref. 2). The hydrographic regime is characterized by low-salinity surface water of river origin overlying high-salinity deep water of Mediterranean origin1,3. A steep pycnocline, centred at about 50 m is the primary physical barrier to mixing and is the origin of the stability of the anoxic (oxygen/hydrogen sulphide) interface. Here we report new observations, however, that indicate dramatic changes in the oceanographic characteristics of the anoxic interface of the Black Sea over decadal or shorter timescales. The anoxic, sulphide-containing interface has moved up in the water column since the last US cruises in 1969 and 1975. In addition, a suboxic zone overlays the sulphide-containing deep water. The expected overlap of oxygen and sulphide was not present. We believe that these observations result from horizontal mixing or flushing events that inject denser, saltier water into the relevant part of the water column. It is possible that man-made reduction in freshwater inflow into the Black Sea could cause these changes, although natural variability cannot be discounted. © 1989 Nature Publishing Group

On the corrections of ERA-40 surface flux products consistent with the Mediterranean heat and water budgets and the connection between basin surface total heat flux and NAO

This is a study of heat fluxes and heat budget of the Mediterranean Sea using the European Centre for Medium␣Range Weather Forecasts (ECMWF) 45 year reanalysis data set ERA␣40. The simple use of the ERA␣40 surface flux components fails to close the budget and, in particular, the shortwave radiation flux is found to be underestimated with respect to observed data by about 10%. The heat flux terms are recomputed and corrected in order to close the heat and freshwater budgets of the Mediterranean basin over the period 1958 to 2001, thus producing a corrected ERA␣40 surface flux data set. Various satellite and in situ observational data are used to construct spatially varying corrections to the ERA␣40 products needed to compute the air␣sea fluxes. The corrected interannual and climatological net surface heat and freshwater fluxes are ␣7 W/m2 and ␣0.64 m/yr, respectively, which are regarded as satisfactorily closing the Mediterranean heat and water budgets. It is also argued that there is an important contribution from large heat losses associated with a few severe winters over the Mediterranean Sea. This is shown to be related to wind regime anomalies, which strongly affect the latent heat of evaporation that is mainly responsible for the interannual modulation of the total heat flux. Furthermore, the surface total heat flux anomaly time series is compared with the North Atlantic Oscillation (NAO) index, and the result is a positive correlation with ocean warming for positive NAO index periods and ocean cooling associated with negative index periods.The OI␣SST products used in this paper were jointly produced by ENEA Department of Environment, Global Change and Sustainable Development and Gruppo Oceanografia da Satellite (GOS) of the CNR␣ISAC (Istituto di Scienze dell’Atmosfera e del Clima) as part of the EU project MFSTEP (EVK3␣CT␣2002␣00075). The National Center for Atmospheric Research is sponsored by the National Science Foun- dation. The in situ AGIP data were kindly supplied by ENI␣AGIP division, Milan. This work was supported by the European Commission MyOcean Project (SPA.2007.1.1.01, development of upgrade capabilities for existing GMES fast␣track services and related operational services, grant agreement 218812␣1␣FP7␣SPACE 2007).PublishedC060224.6. Oceanografia operativa per la valutazione dei rischi in aree marineJCR Journalreserve