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  • 1
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    The Atlantic Subtropical Cells Inferred from Observations (2021)
    Details
    Publication Date: 2021-10-13
    Description: The Atlantic Subtropical Cells (STCs) are shallow wind-driven overturning circulations connecting the tropical upwelling areas to the subtropical subduction regions. In both hemispheres, they are characterized by equatorward transport at thermocline level, upwelling at the equator, and poleward Ekman transport in the surface layer. This study uses recent data from Argo floats complemented by ship sections at the western boundary as well as reanalysis products to estimate the meridional water mass transports and to investigate the vertical and horizontal structure of the STCs from an observational perspective. The seasonally varying depth of meridional velocity reversal is used as the interface between the surface poleward flow and the thermocline equatorward flow. The latter is bounded by the 26.0 kg m−3 isopycnal at depth. We find that the thermocline layer convergence is dominated by the southern hemisphere water mass transport (9.0 ± 1.1 Sv from the southern hemisphere compared to 2.9 ± 1.3 Sv from the northern hemisphere) and that this transport is mostly confined to the western boundary. Compared to the asymmetric convergence at thermocline level, the wind-driven Ekman divergence in the surface layer is more symmetric, being 20.4 ± 3.1 Sv between 10°N and 10°S. The net poleward transports (Ekman minus geostrophy) in the surface layer concur with values derived from reanalysis data (5.5 ± 0.8 Sv at 10°S and 6.4 ± 1.4 Sv at 10°N). A diapycnal transport of about 3 Sv across the 26.0 kg m−3 isopycnal is required in order to maintain the mass balance of the STC circulation.
    Keywords: 551.46 ; Atlantic Subtropical Cells ; wind-driven overturning circulations
    Language: English
    Type: map
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  • 2
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    Interplay of oxygen and hydrography in the eastern tropical North Atlantic on decadal to multi-decadal time scales (2016)
    In:  [Talk] In: EGU General Assembly 2016, 17.-22.04.2016, Vienna, Austria .
    Details
    Publication Date: 2019-09-23
    Description: Ocean observations in the oxygen minimum zone (OMZ) of the eastern tropical North Atlantic (ETNA) are analyzed to study decadal oxygen variability. Corresponding changes in hydrography are discussed and associated with changes in the circulation and ventilation. The data set consists of repeat shipboard hydrographic, oxygen and velocity observations along 23°W as well as of three multi-year long moored observations both acquired during the last decade. It is examined in comparison to historic hydrographic data on a decadal to multi-decadal time scale perspective. During the last decade, a strong deoxygenation was observed at depth of the deep oxycline representing a shallowing of the ETNA OMZ, while oxygen increased below in the OMZ core. Both trends are superimposed with a moderate multi-decadal oxygen decrease over the whole depth range. Water mass analysis indicates that this dipole pattern in the decadal oxygen variability is associated with a shift in the ventilation pathways having their origin either in the northern or southern hemisphere. The decadal and multi-decadal oxygen trend is implemented in the oxygen budget for the ETNA OMZ, which is based on recent estimates of oxygen consumption as well as lateral and diapycnal diffusive oxygen supply. The change in the residual of this oxygen budget derived from multi-decadal and decadal oxygen trend patterns indicates a shallower wind-driven near-surface circulation during the last decade compared to the period before. In contrast, the latitudinally alternating zonal jets that were suggested to generally weaken since the 70ies might have intensified during the last decade providing the enhanced oxygen supply at the core depth of the OMZ.
    Type: Conference or Workshop Item , NonPeerReviewed
    Format: slideshow
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  • 3
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    Winter to summer oceanographic observations in the Arctic Ocean north of Svalbard (2017)
    AGU (American Geophysical Union) | Wiley
    In:  Journal of Geophysical Research: Oceans, 122 (8). pp. 6218-6237.
    Details
    Publication Date: 2020-02-06
    Description: Oceanographic observations from the Eurasian Basin north of Svalbard collected between January and June 2015 from the N-ICE2015 drifting expedition are presented. The unique winter observations are a key contribution to existing climatologies of the Arctic Ocean, and show a ∼100 m deep winter mixed layer likely due to high sea ice growth rates in local leads. Current observations for the upper ∼200 m show mostly a barotropic flow, enhanced over the shallow Yermak Plateau. The two branches of inflowing Atlantic Water are partly captured, confirming that the outer Yermak Branch follows the perimeter of the plateau, and the inner Svalbard Branch the coast. Atlantic Water observed to be warmer and shallower than in the climatology, is found directly below the mixed layer down to 800 m depth, and is warmest along the slope, while its properties inside the basin are quite homogeneous. From late May onwards, the drift was continually close to the ice edge and a thinner surface mixed layer and shallower Atlantic Water coincided with significant sea ice melt being observed.
    Type: Article , PeerReviewed , info:eu-repo/semantics/article
    Format: text
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  • 4
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    Large-scale ocean oxygen changes (2018)
    In:  [Invited talk] In: 4. Effects of Climate Change on the World's Ocean Symposium, 04.-09.06.2018, Washington, USA .
    Details
    Publication Date: 2019-09-23
    Type: Conference or Workshop Item , NonPeerReviewed
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  • 5
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    Climate related multi-decadal variability of tropical oxygen minimum zones (2016)
    In:  [Poster] In: CLIVAR Open Science Conference 2016, 18.-25.09.2016, Qingdao, China .
    Details
    Publication Date: 2019-09-23
    Type: Conference or Workshop Item , NonPeerReviewed
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  • 6
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    Deoxygenation and climate related variability of the ocean's oxygen minimum zones (2016)
    In:  [Poster] In: Ocean ventilation and deoxygenation in a warming world, 12.09.2016, Royal Society, London, UK .
    Details
    Publication Date: 2019-09-23
    Type: Conference or Workshop Item , NonPeerReviewed
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  • 7
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    Decline in global oceanic oxygen content during the past five decades (2017)
    Nature Research
    In:  Nature, 542 (7641). pp. 335-339.
    Details
    Publication Date: 2020-06-18
    Description: Ocean models predict a decline in the dissolved oxygen inventory of the global ocean of one to seven per cent by the year 2100, caused by a combination of a warming-induced decline in oxygen solubility and reduced ventilation of the deep ocean1, 2. It is thought that such a decline in the oceanic oxygen content could affect ocean nutrient cycles and the marine habitat, with potentially detrimental consequences for fisheries and coastal economies3, 4, 5, 6. Regional observational data indicate a continuous decrease in oceanic dissolved oxygen concentrations in most regions of the global ocean1, 7, 8, 9, 10, with an increase reported in a few limited areas, varying by study1, 10. Prior work attempting to resolve variations in dissolved oxygen concentrations at the global scale reported a global oxygen loss of 550 ± 130 teramoles (1012 mol) per decade between 100 and 1,000 metres depth based on a comparison of data from the 1970s and 1990s10. Here we provide a quantitative assessment of the entire ocean oxygen inventory by analysing dissolved oxygen and supporting data for the complete oceanic water column over the past 50 years. We find that the global oceanic oxygen content of 227.4 ± 1.1 petamoles (1015 mol) has decreased by more than two per cent (4.8 ± 2.1 petamoles) since 1960, with large variations in oxygen loss in different ocean basins and at different depths. We suggest that changes in the upper water column are mostly due to a warming-induced decrease in solubility and biological consumption. Changes in the deeper ocean may have their origin in basin-scale multi-decadal variability, oceanic overturning slow-down and a potential increase in biological consumption11, 12.
    Type: Article , PeerReviewed
    Format: text
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  • 8
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    Deoxygenation of the ocean (2017)
    In:  [Talk] In: Goldschmidt Conference 2017, 13.-18.08.2017, Paris, France .
    Details
    Publication Date: 2019-09-23
    Type: Conference or Workshop Item , NonPeerReviewed
    Format: text
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  • 9
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    Patterns of deoxygenation: sensitivity to natural and anthropogenic drivers (2017)
    The Royal Society
    In:  Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences, 375 (2102). p. 20160325.
    Details
    Publication Date: 2020-02-06
    Description: Observational estimates and numerical models both indicate a significant overall decline in marine oxygen levels over the past few decades. Spatial patterns of oxygen change, however, differ considerably between observed and modelled estimates. Particularly in the tropical thermocline that hosts open-ocean oxygen minimum zones, observations indicate a general oxygen decline, whereas most of the state-of-the-art models simulate increasing oxygen levels. Possible reasons for the apparent model-data discrepancies are examined. In order to attribute observed historical variations in oxygen levels, we here study mechanisms of changes in oxygen supply and consumption with sensitivity model simulations. Specifically, the role of equatorial jets, of lateral and diapycnal mixing processes, of changes in the wind-driven circulation and atmospheric nutrient supply, and of some poorly constrained biogeochemical processes are investigated. Predominantly wind-driven changes in the low-latitude oceanic ventilation are identified as a possible factor contributing to observed oxygen changes in the low-latitude thermocline during the past decades, while the potential role of biogeochemical processes remains difficult to constrain. We discuss implications for the attribution of observed oxygen changes to anthropogenic impacts and research priorities that may help to improve our mechanistic understanding of oxygen changes and the quality of projections into a changing future.
    Type: Article , PeerReviewed
    Format: text
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  • 10
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    Eastern boundary circulation and hydrography off Peru during the 2017 Coastal El Nino (2018)
    In:  [Talk] In: Ocean Science Meeting 2018, 12.-16.02.2018, Portland, OR, USA .
    Details
    Publication Date: 2019-09-23
    Description: In early 2017 sea surface temperatures in the far eastern tropical Pacific were anomalously high while central Pacific SST anomalies remained neutral or negative. Associated to this anomaly pattern were strong anomalous precipitation events in northern Peru causing severe flooding. During April and May 2017 the near-coastal temperature anomalies declined. In-situ observations from four consecutive research cruises and a glider survey collected between 12°S and 14°S off the coast of Peru are used to describe the eastern boundary circulation and hydrography during declining surface temperature anomalies. The observational data base consists of ship-board hydrography, oxygen and upper-ocean velocity observations, hydrography from glider surveys and velocity time series from mooring deployments. Hydrography at 12°S shows a pronounced warm anomaly near the surface and on the shelf where the full water column warmed by more than 2°C with respect to climatology. Further offshore, a weaker warming was observed below the surface layer as well. The oxycline was displaced downwards and well-oxygenated waters occupied the upper 50m of the water column. Poleward velocities of the Peru-Chile Undercurrent strongly intensified in late-April and May reaching velocities above 50 cm s-1. During this period, near-surface temperature anomalies decreased but subsurface temperatures on the shelf remained high. The forcing of the observed variability of the eastern boundary circulation and of the hydrography during the late phase of the “Coastal El Niño” event is investigated and related to local and remote processes.
    Type: Conference or Workshop Item , NonPeerReviewed
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