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  • 1
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    Neutral Density Revisited (1999)
    Elsevier
    In:  Deep Sea Research Part II: Topical Studies in Oceanography, 46 . pp. 33-54.
    Details
    Publication Date: 2019-09-23
    Description: The possibilities of defining and computing an approximately neutral density variable are reexamined in this paper. There are three desirable properties that a neutral density variable should possess. Firstly, the isosurfaces of this variable should coincide with (approximately) neutral surfaces. This would facilitate the analysis of hydrographic data on the most appropriate mixing and spreading surfaces. Secondly, the horizontal gradients of the neutral density should agree with the gradients of the in situ density, and thirdly the vertical gradient of the neutral density variable should be proportional to the static stability of the water column. A density variable that approximates the latter two properties can be used in ocean circulation models based on layer coordinates, and would reduce substantial errors in present isopycnal models due to the use of a potential density variable. No variable can possess all the three properties simultaneously. The variable γn introduced by Jackett and McDougall (1997, J. Phys. Oceanogr. 27, 237–263) satisfies the first of the properties exactly but is not designed for the use in models. Based on climatological data in the North Atlantic, an alternative neutral density variable ν̃(S, Θ) is defined, which is shown to approximate the two gradient criteria much better than any potential density. We suggest that this neutral density variable may be useful in isopycnal ocean models as an alternative to potential density, since it could significantly reduce errors in thermal wind relation and vertical stability
    Type: Article , PeerReviewed
    Format: text
    DOI: 10.1016/S0967-0645(98)00113-1
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  • 2
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    Towards the impact of eddies on the response of the Southern Ocean to climate change (2010)
    Elsevier
    In:  Ocean Modelling, 34 (3-4). pp. 150-165.
    Details
    Publication Date: 2019-01-23
    Description: The sensitivity of the meridional overturning circulation (MOC) of the Southern Ocean (SO) to wind stress changes is discussed. Using an idealised SO model in both non- and eddy-permitting configurations, we assess the effects of both, coarsening the horizontal resolution and implementing different parameterisations for the lateral eddy diffusivity appropriate to the Gent and McWilliams (1990) parameterisation, K. We find that the MOC is characterised by an eddy-driven part ψ* which generally opposes the wind-driven part and that the increase of the MOC diminishes with amplifying winds, with the possibility that the MOC in the SO may become completely insensitive to wind stress changes. However, for moderate wind stress, the MOC is still significantly increasing in our configuration. The diagnosed lateral eddy diffusivity K in the eddy-permitting version shows strong spatial variability and is increasing with increasing wind stress. Similar to the MOC (but in contrast to ψ*) the increase of K diminishes with amplifying winds. It turns out that a small increase in the isopycnal slopes is also relevant in order to capture the correct sensitivity of ψ* on wind stress. This relation also holds in model configurations with coarser but still eddy-permitting horizontal resolution: decreasing the horizontal resolution decreases K, but increases the isopycnal slopes such that the strength of the MOC including its sensitivity to wind stress is almost unchanged. The parameterisations are able to reproduce the MOC for certain wind stresses, but all parameterisations underestimate the sensitivity of K and thus overestimate the sensitivity of the MOC on wind stress. Our results show that it is indispensable to incorporate the correct sensitivity of K into climate models in order to reproduce the correct sensitivity of the MOC to wind stress and that up-to-date parameterisations for K are only partially successful.
    Type: Article , PeerReviewed
    Format: text
    Format: text
    DOI: 10.1016/j.ocemod.2010.05.005
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  • 3
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    Lagrangian and Eulerian lateral diffusivities in zonal jets (2011)
    Elsevier
    In:  Ocean Modelling, 39 (1-2). pp. 114-124.
    Details
    Publication Date: 2017-02-06
    Description: Meridional diffusivities from Lagrangian particle dispersion and Eulerian diffusivities from a flux-gradient relationship are estimated in an idealized primitive equation channel model featuring eddy-driven zonal jets. The Eulerian estimate shows an increase with depth and clear minima of meridional diffusivities within the zonal jets, indicating mixing barriers. The Lagrangian estimates agree with the Eulerian method on the vertical variation and also show indications of meridional mimima, although meridional variations are poorly resolved. We found early maxima in the particle spreading rates which should not be related to diffusivities since they are caused by the meandering zonal jets. The meanders also produce rotational eddy fluxes, which can obscure the Eulerian diffusivity estimates. Zonal particle dispersion rates do not converge within the chosen lag interval, because of shear dispersion by the mean flow, i.e. it is not possible to estimate Lagrangian zonal diffusivities representative for regions of similar size of the zonal jet spacing. Removing the zonal mean flow, zonal and meridional dispersion rates converge and show much higher zonal than meridional diffusivities. Further, the pronounced vertical increase and indications of meridional minima in the Lagrangian meridional diffusivities disappear, pointing towards the importance of shear dispersion by the mean flow for the suppression of meridional mixing by zonal jets. (C) 2011 Elsevier Ltd. All rights reserved.
    Type: Article , PeerReviewed
    Format: text
    DOI: 10.1016/j.ocemod.2011.02.002
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  • 4
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    Multidecadal CO2uptake variability of the North Atlantic (2010)
    AGU (American Geophysical Union)
    In:  Journal of Geophysical Research: Atmospheres, 115 (D12). D12113.
    Details
    Publication Date: 2018-02-06
    Description: The multidecadal variability of air-sea CO(2)fluxes in the North Atlantic under preindustrial atmospheric CO(2) conditions is simulated, using a coupled biogeochemical/circulation model driven by long-term surface forcing reconstructed from the leading modes of sea level pressure observations from 1850 to 2000. Heat fluxes are of great importance for the multidecadal CO(2) fluctuations, about equal in magnitude to wind stress, in contrast to their less prominent role for CO(2) flux variability on interannual timescales. Another difference, compared to higher frequencies, is the dominance of the North Atlantic Oscillation in driving the variability of the air-sea CO(2) fluxes. Two spatially distinct regimes lead to large anomalies in the CO(2) fluxes but compensate to a large degree. The first regime is advective and has its clear signature southeast of Greenland while the second one, in the vicinity of the Labrador Sea and off Newfoundland, is convective. In both regimes, the multidecadal CO(2) fluctuations are driven mainly by variations in temperature, salinity, and DIC content at the sea surface while the role of the biological pump is of minor importance in this particular model. The magnitude of the simulated multidecadal CO(2) uptake changes is on the order of 0.02 Pg C/yr and amounts to 10-15% of the estimated annual anthropogenic CO(2) uptake of the North Atlantic.
    Type: Article , PeerReviewed
    Format: text
    DOI: 10.1029/2009JD012431
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  • 5
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    Effects of biologically induced differential heating in an eddy-permitting coupled ocean-ecosystem model (2009)
    AGU (American Geophysical Union)
    In:  Journal of Geophysical Research: Oceans, 114 (C6).
    Details
    Publication Date: 2018-01-15
    Description: On the basis of integrations of an eddy-permitting coupled physical-biological model of the tropical Pacific we explore changes in the simulated mean circulation as well as its intraseasonal to interannual variability driven by the biologically modulated vertical absorption profiles of solar radiation. Three sensitivity ocean hind-cast experiments, covering the period from 1948 to 2003, are performed. In the first one, simulated chlorophyll affects the attenuation of light in the water column, while in the second experiment, the chlorophyll concentration is kept constant in time by prescribing an empirically derived spatial pattern. The third experiment uses a spatially and temporally constant value for the attenuation depth. The biotically induced differential heating is generated by increased absorption of light in the surface layers, leading to a surface warming and subsurface cooling. The effect is largest in the eastern equatorial Pacific. However, the initial vertical redistribution of heat leads to considerable changes of the near-surface ocean circulation subsequently influencing the near-surface temperature structure. In general, including biophysical coupling improves the model performance in terms of temperature and ocean circulation patterns. In particular, the upwelling in the eastern equatorial Pacific is enhanced, the mixed layer becomes shallower, the warm bias in the eastern Pacific is reduced, and the zonal temperature gradient increases. This leads to stronger La Niña events and an associated increase in the variability of the Niño3 SSTA time series. Furthermore, the eddy kinetic energy (EKE) associated with mesoscale eddies in the eastern equatorial Pacific increases by almost 100% because of enhanced EKE production due to enhanced horizontal and vertical shear of the mean currents.
    Type: Article , PeerReviewed
    Format: text
    DOI: 10.1029/2008JC004936
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  • 6
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    Contribution of oxygen minimum zone waters to the coastal upwelling off Mauritania (2009)
    Elsevier
    In:  Progress in Oceanography, 83 . pp. 143-150.
    Details
    Publication Date: 2016-10-04
    Description: The coastal upwelling off Mauritania and its connection with the oxygen minimum zone (OMZ) in the tropical Atlantic is investigated in an eddy-resolving general circulation model. Two main supply routes for the upwelling are identified. First a southern eastward pathway crossing 23 degrees W between 3 degrees N and 10 degrees N related to the equatorial zonal current system supplies up to 50% of the water upwelled in winter, and about 30% in summer. Second, another eastward pathway crossing 23 degrees W further north between 28 degrees N and 38 degrees N supplies 35% of the upwelled water in spring compared to 25% during the rest of the year. Most of the water of the northern pathway is entrained into the mixed layer already before reaching the upwelling region. Only the southern pathway contributes not recently ventilated waters to the upwelling. The connection with the OMZ is very weak, only about 1% of the upwelling waters originate here. On the other hand, if water from the OMZ reaches the surface mixed layer within 6 years, this mostly (71%) happens in the upwelling region
    Type: Article , PeerReviewed
    Format: text
    DOI: 10.1016/j.pocean.2009.07.015
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  • 7
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    Mid-depth equatorial tracer tongues in a model of the Atlantic Ocean (2006)
    AGU (American Geophysical Union)
    In:  Journal of Geophysical Research: Oceans, 111 (C12). C12025.
    Details
    Publication Date: 2018-04-19
    Description: Observational estimates of middepth tracer tongues in the equatorial Atlantic are reviewed and are compared with results from several eddy-resolving model simulations. Local maxima of chlorofluorocarbon (CFC) concentrations along the equator at around 1500 m depth are related to mean eastward jet structures in the models at similar depth ranges and can also be identified in several simulated tracer distributions. Similar to the observations, strong eastward jets are located in the simulations 1°–2° north and south of the equator. The model simulations show, in addition, consistent with the CFC observations, weaker jets at around 4°–6°N/S and 8°–10°N/S, suggestive of a large-scale alternating eastward/westward current system in the western tropical Atlantic in this depth range. Lagrangian transport estimates in the model using float diagnostics show a transport of 1–3 Sv in each of the eastward jets 1°–2°N/S off the equator compared to 3–12 Sv throughflow into the South Atlantic, with no seasonal cycle apparent in the transport fractioning. Comparing different model solutions reveals the choice of the subgrid-scale mixing parameterization as important for the amplitudes of the jets. Enhanced (reduced) diapycnal mixing is related to stronger (weaker) jets.
    Type: Article , PeerReviewed
    Format: text
    DOI: 10.1029/2006JC003565
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  • 8
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    Role of wind stress and heat fluxes in generating interannual to decadal variability of air-sea CO2 and O2 fluxes in a North Atlantic model (2006)
    AGU (American Geophysical Union)
    In:  Geophysical Research Letters, 33 .
    Details
    Publication Date: 2018-02-19
    Description: A coupled ecosystem-circulation model of the North Atlantic is used to examine the individual contributions by wind stress and surface heat fluxes to naturally driven interannual-to-decadal variability of air-sea fluxes of CO2 and O2 during 1948–2002. The model results indicate that variations in O2 fluxes are mainly driven by variations in surface heat fluxes in the extratropics (15°N to 70°N), and by wind stress in the tropics (10°S to 15°N). Conversely, variations in simulated CO2 fluxes are predominantly wind-stress driven over the entire model domain (18°S to 70°N); while variability in piston velocity and surface heat fluxes is less important. The simulated uptake of O2 by the North Atlantic amounts to 70 ± 11 Tmol yr−1 to which the subpolar region (45°N to 70°N) contributes by 62 ± 10 Tmol yr−1. Whereas the subpolar North Atlantic takes up more than 2/3 of the total carbon absorbed by the North Atlantic in our model (about 0.3 Pg C yr−1), interannual variability of air-sea CO2 fluxes reaches similar values (about 0.01 Pg C yr−1 each) in the subpolar (45°N to 70°N), the subtropical (15°N to 45°N) and the equatorial (10°S to 15°N) Atlantic.
    Type: Article , PeerReviewed
    Format: text
    DOI: 10.1029/2006GL026538
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  • 9
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    Why western boundary currents are diffusive: A link between bottom pressure torque and bolus velocity (2010)
    Elsevier
    In:  Ocean Modelling, 32 . pp. 14-24.
    Details
    Publication Date: 2019-09-23
    Description: The bottom pressure torque is known to vanish in the interior ocean but to play a dominant role in the western boundary layer in balancing the planetary vorticity on spatial scales larger than the Rossby radius of deformation. In this study, the appearance of the bottom pressure torque and thus any deviation of wind-driven flow from classical Sverdrup balance is locally related in steady state to non-zero bolus velocity and/or friction, under the assumption that horizontal density advection is small compared to the lifting of isopycnals. To first order approximation, the vortex stretching by the vertical bolus velocity is related to the bottom pressure torque. The bolus vortex stretching becomes a significant term in the barotropic vorticity budget of the western boundary layer and is formally equivalent to bottom friction as in the classical models of the wind-driven gyre circulation.
    Type: Article , PeerReviewed
    Format: text
    DOI: 10.1016/j.ocemod.2009.07.003
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  • 10
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    Adiabatic reduction of circulation-related CO〈sub〉2〈/sub〉 air-sea flux biases in North Atlantic carbon-cycle models (2006)
    AGU (American Geophysical Union)
    In:  Global Biogeochemical Cycles, 20 (GB2008).
    Details
    Publication Date: 2018-03-22
    Description: Physical transport processes of carbon, alkalinity, heat, and nutrients to a large extent control the partial pressure of CO2 at the sea surface and hence the oceanic carbon uptake. Using a state-of-the-art biogeochemical model of the North Atlantic at eddy-permitting resolution we show that biases in the simulated circulation generate errors in air-sea fluxes of CO2 which are still larger than those associated with the considerable uncertainties in parameterizations of the air-sea gas exchange. A semiprognostic correction method that adiabatically corrects the momentum equations while conserving water mass properties and tracers is shown to yield a more realistic description of the carbon fluxes into the North Atlantic at little additional computational cost. Owing to upper ocean flow patterns in better agreement with observations, simulated CO2 uptake in the corrected regional model is larger by 25% compared to the uncorrected model.
    Type: Article , PeerReviewed
    Format: text
    DOI: 10.1029/2005GB002521
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