Hahn, Johannes (2013) Oxygen Variability and Eddy-driven Meridional Oxygen Supply in the Tropical North East Atlantic Oxygen Minimum Zone. (PhD/ Doctoral thesis), Christian-Albrechts-Universität Kiel, Kiel, Germany, 129 pp.

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Abstract

The distribution of the mean oceanic oxygen concentration results from a balance between ventilation and consumption and, in particular, reveals extended oxygen minimum zones (OMZ) in the eastern tropical Pacific and Atlantic at intermediate depth (300m - 700m). It has been recently shown that OMZs expand in size and are subject to a significant oxygen decrease, where the OMZ in the Tropical North East Atlantic (TNEA) holds the most significant and largest oxygen trend. This study analyzes hydrographic and velocity data from shipboard and moored observations which were acquired along the 23°W section cutting meridionally through the TNEA OMZ, in order to (i) quantify regional differences in the oxygen variability, (ii) identify the role of two mixing processes (mesoscale stirring and diapycnal mixing) in the production of oxygen variance based on the extended Osborn-Cox model and (iii) estimate the role of mesoscale eddies in the meridional ventilation of the TNEA OMZ. Enhanced oxygen variability is found at the boundaries of the OMZ which is predominantly generated by mesoscale stirring along isopycnals and diapycnal mixing across isopycnals. South of the OMZ core (which is located at about 11°N), mesoscale stirring dominates the generation of oxygen variance, whereas above the OMZ core within the deep oxycline (located at about 300m depth) both processes, mesoscale stirring and diapycnal mixing, are found to be of similar importance. Meridional eddy fluxes of oxygen are estimated by using (i) a diffusive flux parameterization based on a lateral eddy diffusion coefficient and the mean isopycnal oxygen gradient, and (ii) a correlation method based on velocity and oxygen time series from moored observations. At the mooring positions 5°N, 23°W and 8°N, 23°W, the results of both methods are in good agreement in the depth range of the OMZ core, yielding a northward oxygen flux from the well-ventilated equatorial region toward the OMZ core. The divergence of the meridional oxygen flux, as obtained from the diffusive flux parameterization, yields an eddy-driven meridional oxygen supply from the south into the OMZ of about 2.4 μmol kg−1 yr−1 at its core depth. Above the OMZ core, mesoscale eddies act to redistribute low-oxygen and high-oxygen waters associated with latitudinally alternating westward and eastward currents. Locally, absolute values of the eddy-driven meridional oxygen supply are greater than 10 μmol kg−1 yr−1 which is likely balanced by mean zonal advection. Combining the above results with recent studies about oxygen consumption, diapycnal oxygen supply and oxygen tendency, a refined oxygen budget for the TNEA OMZ is derived. The eddy-driven meridional oxygen supply accounts for more than 50% of the supply required to balance the estimated oxygen consumption. The oxygen tendency in the TNEA OMZ, as given by the multidecadal oxygen decline, is at maximum slightly above the OMZ core and represents a substantial imbalance of the oxygen budget reaching up to 20% of the magnitude of the eddy-driven oxygen supply. Oxygen data from moored observations was acquired with optode oxygen sensors. To achieve a high quality of these measurements, an in situ calibration procedure is proposed which yields an average measurement error of 4.6 μmol kg−1 with 95% confidence (evaluated for water masses in the upper 1000m of the Tropical Atlantic in combination with onboard lab calibrations against zero oxygen). On the one hand, this calibration procedure yields calibration errors that are worse by roughly a factor of 2 in comparison to sophisticated laboratory calibrations, but on the other hand this simple method is not affected by severe sensor drifts that are frequently observed at some time before or after the field deployment. In case of profiling systems (shown here for a CTD/O2 cast), a time constant correction improves the quality of the measured oxygen profile which is of particular interest for the application in gliders or floats.

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