Oxygen Variability and Eddy-driven Meridional Oxygen Supply in the Tropical North East Atlantic Oxygen Minimum Zone

Hahn, Johannes (2013) Oxygen Variability and Eddy-driven Meridional Oxygen Supply in the Tropical North East Atlantic Oxygen Minimum Zone (Doctoral thesis/PhD), 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.

Document Type: Thesis (Doctoral thesis/PhD)
Thesis Advisors: Brandt, Peter and Greatbatch, Richard John
Keywords: Physical Oceanography, Oxygen Minimum Zone, OMZ, Tropical Atlantic, Tropical North East Atlantic, Optode, Calibration, Oxygen Variance, Oxygen Flux, Eddy Flux, Oxygen Supply, Oxygen Budget
Research affiliation: OceanRep > SFB 754
OceanRep > SFB 754 > A4
OceanRep > GEOMAR > FB1 Ocean Circulation and Climate Dynamics > FB1-TM Theory and Modeling
OceanRep > GEOMAR > FB1 Ocean Circulation and Climate Dynamics > FB1-PO Physical Oceanography
Projects: SFB754
Date Deposited: 09 Jan 2014 13:29
Last Modified: 13 Jan 2016 12:24
URI: http://eprints.uni-kiel.de/id/eprint/22943

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