On the role of circulation and mixing in the ventilation of oxygen minimum zones with a focus on the eastern tropical North Atlantic

Ocean observations carried out in the framework of the Collaborative Research Center 754 (SFB 754) "Climate-Biogeochemistry Interactions in the Tropical Ocean" are used to study (1) the structure of tropical oxygen minimum zones (OMZs), (2) the processes that contribute to the oxygen budget, and (3) long-term changes in the oxygen distribution. The OMZ of the eastern tropical North Atlantic (ETNA), located between the well-ventilated subtropical gyre and the equatorial oxygen maximum, is composed of a deep OMZ at about 400 m depth with its core region centred at about 20° W, 10° N and a shallow OMZ at about 100 m depth with lowest oxygen concentrations in proximity to the coastal upwelling region off Mauritania and Senegal. The oxygen budget of the deep OMZ is given by oxygen consumption mainly balanced by the oxygen supply due to meridional eddy fluxes (about 60%) and vertical mixing (about 20%, locally up to 30%). Advection by zonal jets is crucial for the establishment of the equatorial oxygen maximum. In the latitude range of the deep OMZ, it dominates the oxygen supply in the upper 300 to 400 m and generates the intermediate oxygen maximum between deep and shallow OMZs. Water mass ages from transient tracers indicate substantially older water masses in the core of the deep OMZ (about 120-180 years) compared to regions north and south of it. The deoxygenation of the ETNA OMZ during recent decades suggests a substantial imbalance in the oxygen budget: about 10% of the oxygen consumption during that period was not balanced by ventilation. Long-term oxygen observations show variability on interannual, decadal and multidecadal time scales that can partly be attributed to circulation changes. In comparison to the ETNA OMZ the eastern tropical South Pacific OMZ shows a similar structure including an equatorial oxygen maximum driven by zonal advection, but overall much lower oxygen concentrations approaching zero in extended regions. As the shape of the OMZs is set by ocean circulation, the widespread misrepresentation of the intermediate circulation in ocean circulation models substantially contributes to their oxygen bias, which might have significant impacts on predictions of future oxygen levels.

Long-term eddy-covariance measurements from FINO2 platform above the Baltic Sea

The estimation of the carbon dioxide (CO2) fluxes above the open ocean plays an important role for the determination of the global carbon cycle. A frequently used method therefore is the eddy-covariance technique, which is based on the theory of the Prandl-layer with height-constant fluxes in the atmospheric boundary layer. To test the assumption of the constant flux layer, in 2008 measurements of turbulent heat and CO2 fluxes were started within the project Surface Ocean Processes in the Anthropocene (SOPRAN) at the research platform FINO2. The FINO2 platform is situated in the South-west of the Baltic Sea, in the tri-border region between Germany, Denmark, and Sweden. In the frame of the Research project SOPRAN, the platform was equipped with additional sensors in June 2008. A combination of 3-component sonic anemometers (USA-1) and open-path infrared gas analyzers for absolute humidity (H2O) and CO2 (LICOR 7500) were installed at a 9m long boom directed southward of the platform in two heights, at 6.8 and 13.8m above sea surface. Additionally slow temperature and humidity sensors were installed at each height. The gas analyzer systems were calibrated before the installation and worked permanently without any calibration during the first measurement period of one and a half years. The comparison with the measurements of the slow sensors showed for both instruments no significant long-term drift in H2O and CO2. Drifts on smaller time scales (in the order of days) due to the contamination with sea salt, were cleaned naturally by rain. The drift of both quantities had no influence on the fluctuation, which, in contrast to the mean values, are important for the flux estimation. All data were filtered due to spikes, rain, and the influence of the mast. The data set includes the measurements of all sensors as average over 30 minutes each for one and a half years, June 2008 to December 2009, and 10 month from November 2011 to August 2012. Additionally derived quantities for 30 minutes intervals each, like the variances for the fast-sensor variables, as well as the momentum, sensible and latent heat, and CO2 flux are presented.