SUBPROJECT - A9-N
Circulation mechanisms for decadal time scale changes in oxygen minimum zones
A major achievement from the first two phases of SFB 754 was the establishment of a plausible budget for the Eastern Tropical North Atlantic oxygen minimum zone (ETNA OMZ, Brandt et al., 2015). We can now roughly quantify the role of diapycnal mixing, lateral mixing by eddies and the role of zonal processes. The latter, associated primarily with zonal advection, were found to play a dominant role in ventilating the ETNA OMZ above 300m. Indeed, a characteristic feature of the latitude band occupied by the ETNA OMZ is the presence of latitudinally alternating zonal jets (LAZJs) with large vertical scale that connect the OMZs to the oxygen-rich western boundary. These currents are also present in the tropical Pacific. Brandt et al. (2010) have suggested that variability in these jets played a role in the notable decrease in oxygen content in the core of the ETNA OMZ between 1972-85 and 1999-2008.
Decadal to multidecadal trends in oxygen in the ETNA and Eastern Tropical South Pacific (ETSP) OMZs can be affected by anthropogenic climate change but also by changes in ocean circulation, and hence by changes in the ventilation of the OMZs that may or may not be related to anthropogenic forcing. The key question to be addressed by A9 is how much of the long-term (decadal to multidecadal) variability in oxygen can be attributed to changes in ocean circulation? We are interested in changes associated with both the eddy-driven LAZJs and the large-scale climate modes (noting that the former may depend on the latter). These include the Atlantic Meridional Overturning Circulation (AMOC), the North Atlantic Oscillation (NAO), the Pacific Decadal/Interdecadal Oscillation (PDO/IPO) and the subtropical cells (STCs), as well as El Nino Southern Oscillation (ENSO), and the tropical Atlantic modes of climate variability. As an example, variability in the transport of the Pacific STCs has recently been shown to play an important role for setting oxygen levels in the eastern tropical Pacific (Duteil et al., 2014), and in the tropical Atlantic Ocean, transport variations of the STCs have been connected to changes in the AMOC (e.g. Chang et al., 2008) with potential consequences for oxygen variability in the OMZs.To address this question, observational data collected during all phases of the SFB 754 (e.g. by A3, A4 and A5), high-resolution ocean model simulations developed by A2, satellite derived data, and reanalysis products for both oceanic and atmospheric variables will be used. They will be analysed by applying standard statistical methods enabling us to pin down that part of the variability in ventilation, especially at the western boundary, that can be related to the large-scale climate modes. To understand and quantify the eddy-driven part of the variability, we plan to run two classes of ocean models using simplified geometry and physics, namely a reduced-gravity shallow water model and the Nucleus for European Modeling of the Ocean (NEMO) ocean model, using the same code as A2 but exploring different model resolution and basin configurations. The nonlinear shallow water model will be used to understand and quantify the variability of the LAZJs in a simple setting. Meanwhile, the NEMO model will be used to study the generation and variability of both the LAZJs and the related equatorial deep jets (EDJs) in a more realistic, three-dimensional setting, building on work started as part of A4 in the 2nd phase. We also plan to develop a high resolution regional model using NEMO for the equatorial and subtropical North Atlantic, taking boundary conditions provided by A2, in order to represent the EDJs and LAZJs and their interaction with the large-scale climate modes as optimally as possible, at the same time providing advice to A2 on ways to improve the representation of the LAZJs and the EDJs in their model version. Advection-diffusion models for a tracer mimicing oxygen will also be run using the flow field taken from the 2-D (shallow water) and 3-D (NEMO) model runs in close collaboration with the modeling efforts in A2.
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 Minumum Zone of the Tropical North Atlantic. J. Phys. Oceanogr., 40, 1784-1801, doi: 10.1175/2010JPO4301.1
Brandt, P., H. W. Bange, D. Banyte, M. Dengler, S.-H. Didwischus, T. Fischer, R. J. Greatbatch, J. Hahn, T. Kanzow, J. Karstensen, A. Körtzinger, G. Krahmann, S. Schmidtko, L. Stramma, T. Tanhua, and M. Visbeck (2015) On the role of circulation and mixing in the ventilation of oxygen minimum zones with a focus on the eastern tropical North Atlantic. Biogeosciences, 12, 489-512, doi: 10.5194/bg-12-489-2015
Chang, P., R. Zhang, W. Hazeleger, C. Wen, X. Wan, L. Ji, R. J. Haarsma, W.-P. Breugem, and H. Seidel (2008) Oceanic link between abrupt changes in the North Atlantic Ocean and the AfricanMonsoon. Nature Geoscience, 1, doi:10.1038/ngeo218.Duteil, O., C. W. Böning, and A. Oschlies (2014) Variability in subtropical-tropical cells drives oxygen levels in the tropical Pacific Ocean. Geophys. Res. Lett., 41, 8926-8934, doi: 10.1002/2014GL061774.