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  • AGU (American Geophysical Union)  (6)
  • 1
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    Major role of the equatorial current system in setting oxygen levels in the eastern tropical Atlantic Ocean: a high-resolution model study (2014)
    AGU (American Geophysical Union) | Wiley
    In:  Geophysical Research Letters, 41 . pp. 2033-2040.
    Details
    Publikationsdatum: 2019-09-23
    Beschreibung: Understanding the causes of the observed expansion of tropical ocean's oxygen minimum zones (OMZs) is hampered by large biases in the representation of oxygen distribution in climate models, pointing to incorrectly represented mechanisms. Here we assess the oxygen budget in a global biogeochemical circulation model, focusing on the Atlantic Ocean. While a coarse (0.5°) configuration displays the common bias of too large and too intense OMZs, the oxygen concentration in an eddying (0.1°) configuration is higher and closer to observations. This improvement is traced to a stronger oxygen supply by a more realistic representation of the equatorial and off-equatorial undercurrents, outweighing the concurrent increase in oxygen consumption associated with the stronger nutrient supply. The sensitivity of the eastern tropical Atlantic oxygen budget to the equatorial current intensity suggests that temporal changes in the eastward oxygen transport from the well-oxygenated western boundary region might partly explain variations in the OMZs.
    Materialart: Article , PeerReviewed
    Format: text
    DOI: 10.1002/2013GL058888
    Standort Signatur Einschränkungen Verfügbarkeit
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  • 2
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    Variability in subtropical-tropical cells drives oxygen levels in the tropical Pacific Ocean (2014)
    AGU (American Geophysical Union) | Wiley
    In:  Geophysical Research Letters, 41 (24). pp. 8926-8934.
    Details
    Publikationsdatum: 2019-09-23
    Beschreibung: Previous studies found a negative trend in oxygen concentrations in tropical regions during the last decades. Employing a biogeochemical circulation model, we highlight the importance of wind driven ocean transport associated with the Subtropical-Tropical Cells (STCs) in setting the oxygen levels in the tropical ocean. The observed and simulated slowdown of the STCs by 30 percent from the 1960s to the 1990s caused a decrease in oxygen transport to the tropics. Transport of phosphate was similarly reduced, decreasing export production and respiration. The effects of physical transport and biological consumption partly compensate, damping oxygen interannual and decadal variability. Our results suggest that the observed residual oxygen trend in the tropical Pacific is mainly driven by changes in oxygen transport. Accordingly, the observed recent strengthening of the STCs leads us to expect a pause in the oxygen decrease or even an increase of tropical Pacific oxygen values in the near future.
    Materialart: Article , PeerReviewed
    Format: text
    Format: text
    DOI: 10.1002/2014GL061774
    Standort Signatur Einschränkungen Verfügbarkeit
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  • 3
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    Sensitivity of simulated extent and future evolution of marine suboxia to mixing intensity (2011)
    AGU (American Geophysical Union)
    In:  Geophysical Research Letters, 38 (6). L06607.
    Details
    Publikationsdatum: 2019-09-23
    Beschreibung: Geological and hydrographic records contain evidence of substantial past variations in the oxygenation of the global ocean. Numerical models predicts a future decrease of marine oxygen levels under global warming. Using a global biogeochemical-climate model in which diapycnal mixing is parametrised as the sum of the regionally heterogenous tidal and homogenous background vertical mixing, we here show that simulated total oceanic oxygen content and the extent of marine suboxia are both sensitive to the strength of background vertical mixing. Eight otherwise identical configurations of the model were spun up under pre-industrial conditions for different vertical diffusivities ranging from background values of 0.01 cm 2/s to 0.5 cm 2/s. This range corresponds to various observational estimates and to values currently used in numerical ocean circulation models. Whereas the simulated total oceanic oxygen content is larger for larger mixing intensities, the simulated suboxic volume displays a maximum at intermediate diffusivities of about 0.2 cm 2/s. The intensity of vertical mixing also determines the evolution of suboxic areas under projected 21st century CO 2 emissions: while all model configurations predict a decline in total oceanic oxygen, the simulated extent of marine suboxia shows a 21st century expansion only for mixing rates higher than 0.2 cm 2/s, whereas the suboxic volume declines for lower mixing rates despite an overall loss of marine oxygen. Differences in the poorly constrained mixing parameterisation can thus lead to qualitatively different estimates about the future evolution of marine suboxia under projected climate change.
    Materialart: Article , PeerReviewed
    Format: text
    DOI: 10.1029/2011GL046877
    Standort Signatur Einschränkungen Verfügbarkeit
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  • 4
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    Interannual variability of the Atlantic North Equatorial Undercurrent and its impact on oxygen (2019)
    AGU (American Geophysical Union) | Wiley
    In:  Journal of Geophysical Research: Oceans, 124 (4). pp. 2348-2373.
    Details
    Publikationsdatum: 2022-01-31
    Beschreibung: The North Equatorial Undercurrent (NEUC) has been suggested to act as an important oxygen supply route towards the oxygen minimum zone in the Eastern Tropical North Atlantic. Observational estimates of the mean NEUC strength are uncertain due to the presence of elevated mesoscale activities, and models have difficulties in simulating a realistic NEUC. Here we investigate the interannual variability of the NEUC and its impact onto oxygen based on the output of a high‐resolution ocean general circulation model (OGCM) and contrast the results with an unique data set of 21 ship sections along 23° W and a conceptual model. We find that the interannual variability of the NEUC in the OGCM is related to the Atlantic Meridional Mode (AMM) with a stronger and more northward NEUC during negative AMM phases. Discrepancies between OGCM and observations suggest a different role of the NEUC in setting the regional oxygen distribution. In the model a stronger NEUC is associated with a weaker oxygen supply towards the east. We attribute this to a too strong recirculation between the NEUC and the northern branch of the South Equatorial Current (nSEC) in the OGCM. Idealized experiments with the conceptual model support the idea that the impact of NEUC variability on oxygen depends on the source water pathway. A strengthening of the NEUC supplied out of the western boundary acts to increase oxygen levels within the NEUC. A strengthening of the recirculations between NEUC and the nSEC results in a reduction of oxygen levels within the NEUC.
    Materialart: Article , PeerReviewed
    Format: text
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    DOI: 10.1029/2018JC014760
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  • 5
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    Wind synoptic activity increases oxygen levels in the tropical Pacific Ocean (2019)
    AGU (American Geophysical Union) | Wiley
    In:  Geophysical Research Letters, 46 (5). pp. 2715-2725.
    Details
    Publikationsdatum: 2022-01-31
    Beschreibung: The mechanisms controlling the variability of oxygen levels in the ocean are poorly quantified. We focus here on the impact of wind synoptic variability associated with tropical convective regions and extra‐tropical storms. Removing the wind higher frequencies of variability (2 days – 1 month) in an atmosphere reanalysis used to force an ocean model decreases wind stress by up to 20% in the tropics and 50% in the mid‐latitudes, weakening wind‐driven ocean circulation by 20%. Oxygen levels decrease by up to 10 mmol.m‐3 in tropical oceans and 30 mmol.m‐3 in subtropical gyres mainly due to changes in advective processes. While a large part of the tropical oxygen anomaly has local origins, changes in oxygen levels in the subtropical gyres modulate tropical oxygen distribution. Our study suggests that the “storminess” of the ocean is an important parameter that could determine the future evolution of poorly oxygenated regions.
    Materialart: Article , PeerReviewed
    Format: text
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    DOI: 10.1029/2018GL081041
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  • 6
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    Partitioning of the Open Waters of the Gulf of Mexico Based on the Seasonal and Interannual Variability of Chlorophyll Concentration (2018)
    AGU (American Geophysical Union) | Wiley
    Details
    Publikationsdatum: 2022-03-09
    Beschreibung: The seasonal and interannual variability of chlorophyll in the Gulf of Mexico open waters is studied using a three‐dimensional coupled physical‐biogeochemical model. A 5 years hindcast driven by realistic open‐boundary conditions, atmospheric forcings, and freshwater discharges from rivers is performed. The use of recent in situ observations allowed an in‐depth evaluation of the model nutrient and chlorophyll seasonal distributions, including the chlorophyll vertical structure. We find that different chlorophyll patterns of temporal variability coexist in the deep basin which thereby cannot be considered as a homogeneous region with respect to chlorophyll dynamics. A partitioning of the Gulf of Mexico open waters based on the winter chlorophyll concentration increase is then proposed. This partition is basically explained by the amount of nutrients injected into the euphotic layer which is highly constrained by the dynamic of the winter mixed layer. The seasonal and interannual variability appears to be affected by the variability of atmospheric fluxes and mesoscale dynamics (Loop Current eddies in particular). Finally, estimates of primary production in the deep basin are provided.
    Materialart: Article , PeerReviewed
    Format: text
    Standort Signatur Einschränkungen Verfügbarkeit
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