SUBPROJECT - A4
Trends and variability of oxygen inventory, supply and consumption in the eastern tropical North Atlantic oxygen minimum zone
The main focus of the ongoing analysis of temporal and spatial variability of oxygen inventory and supply will be the identification of dominant variability patterns in recent and to be continued observations and their evaluation with respect to dynamical considerations and model results. Starting from a first quantification of the oxygen budget for the Eastern Tropical North Atlantic (ETNA) oxygen minimum zone (OMZ), we will evaluate the role of circulation and water mass variability as identified in the observational dataset to better understand changes in oxygen concentration in the OMZ with a characteristic pattern of increasing and decreasing oxygen concentrations during the SFB 754 period. Improved constraints on the oxygen supply due to vertical and lateral mixing will help to substantiate the results of the budget analysis. Using mooring and shipboard data from a slightly downscaled continuation of the SFB 754 observational program, we will analyse i) short-term oxygen variability (time scales associated with internal waves and tides, inertial oscillations, up to the mesoscale eddy field) and its seasonal to longer-‐term changes and ii) long-‐term oxygen changes/trends for the entire SFB 754 period (in cooperation with A9). We will deliver a synthesis of the available, and to be further increased, dataset from shipboard and moored observations for the ETNA OMZ that will be analysed in comparison to the SFB 754 model results from A2, A9, and B1.
A second focus will be on data analysis and model experiments to better constrain oxygen consumption, which is the least well-‐constrained term in our oxygen budget of the ETNA OMZ. There are systematic differences in estimates of oxygen utilization rates (OUR) employing water mass renewal time scales and water mass ages. These systematic differences are also found in numerical modelling. During the third phase, we therefore want to carry out a focussed study to improve quantification methods of OUR and provide improved OUR estimates: Existing and new high-‐resolution data for transient tracers (CFC-12, SF6) along our standard 23°W section, acquired in cooperation with A3, will be used to estimate water mass ages, to compare the results of different methods for the estimation of such water mass ages, to synthesize these estimates with OUR estimates from other subprojects (e.g., B4 and B8 using incubations and particle flux methods) and finally to derive an improved oxygen budget for the ETNA OMZ. The results of this focussed OUR study will be further explored in the frame of the model environment available within SFB 754 (A2, B1).
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Brandt, P., V. Hormann, A. Körtzinger, M. Visbeck, G. Krahmann, L. Stramma, R. Lumpkin and C. Schmid (2010) Changes in the ventilation of the oxygen minimum zone of the tropical North Atlantic. J. Phys. Oceanogr., 40, 1784–1801, doi: 10.1175/2010JPO4301.1
Brandt, P., V. Hormann, B. Bourlès, J. Fischer, F. Schott, L. Stramma and M. Dengler (2008) Oxygen tongues and zonal currents in the equatorial Atlantic. J. Geophys. Res., 113, C04012, doi: 10.1029/2007JC004435
Fischer, T., D. Banyte, P. Brandt, M. Dengler, G. Krahmann, T. Tanhua and M. Visbeck (2013) Diapycnal oxygen supply to the tropical North Atlantic oxygen minimum zone. Biogeosciences, 10, 5079–5093, doi: 10.5194/bgd-9-14291-2012
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Karstensen, J., B. Fiedler, F. Schütte, P. Brandt, A. Körtzinger, G. Fischer, R. Zantopp, J. Hahn, M. Visbeck, and D. Wallace (2015) Open ocean dead-zone in the tropical North Atlantic Ocean. Biogeosciences, 12, 2597-2605, doi: 10.5194/bg-12-2597-2015
Stramma, L., M. Visbeck, P. Brandt, T. Tanhua and D. Wallace (2009) Deoxygenation in the oxygen minimum zone of the eastern tropical North Atlantic. Geophys. Res. Lett., 36, L20607, doi: 10.1029/2009GL039593
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