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  • 1
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    In:  [Talk] In: IAPSO-IAMAS-IAGA Joint Assembly 2017, 27.08.-01.09.2017, Cape Town, South Africa .
    Publication Date: 2017-11-22
    Type: Conference or Workshop Item , NonPeerReviewed
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  • 2
    Publication Date: 2017-11-27
    Type: Conference or Workshop Item , NonPeerReviewed
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  • 3
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    In:  [Talk] In: DRAKKAR 2018 Annual Workshop, 22.-24.01.2018, Grenoble, France .
    Publication Date: 2018-02-26
    Type: Conference or Workshop Item , NonPeerReviewed
    Format: slideshow
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  • 4
    Publication Date: 2019-02-25
    Description: Long‐term observations from a 17 year long mooring array at the exit of the Labrador Sea at 53°N are compared to the output of a high‐resolution model (VIKING20). Both are analyzed to define robust integral properties on basin and regional scale, which can be determined and evaluated equally well. While both, the observations and the model, show a narrow DWBC cyclonically engulfing the Labrador Sea, the model's boundary current system is more barotropic than in the observations and spectral analysis indicates stronger monthly to interannual transport variability. Compared to the model, the observations show a stronger density gradient, hence a stronger baroclinicity, from center to boundary. Despite this, the observed temporal evolution of the temperature in the central Labrador Sea is reproduced. The model results yield a mean export of North Atlantic Deep Water (NADW) (33.0 +/‐ 5.7 Sv), which is comparable to the observed transport (31.2 +/‐ 5.5 Sv) at 53°N. The results also include a comparable spatial pattern and March mixed layer depth in the central Labrador Sea (maximum depth ∼ 2000 m). During periods containing enhanced deep convection (1990's) our analyses show increased correlation between LSW and LNADW model transport at 53°N. Our results indicate that the transport variability in LSW and LNADW at 53°N is a result of a complex modulation of wind stress and buoyancy forcing on regional and basin wide scale.
    Type: Article , PeerReviewed , info:eu-repo/semantics/article
    Format: text
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  • 5
    Publication Date: 2019-02-01
    Description: Large solitary meanders are arguably the dominant mode of variability in the Agulhas Current. Observational studies have shown that these large meanders are associated with strong upwelling velocities and affect the shelf circulation for over 100 days per year. Here 10-year time series from two ocean general circulation models are used to create a composite picture of the Agulhas Current and its interactions with the shelf circulation in meandering and nonmeandering modes. Both models show good agreement with the size, propagation speed, and frequency of observed meanders. These composite meanders are then used to examine the response of shelf waters to the onset of large meanders, with the use of model output enabling the dynamics at depth to be explored. Results show a composite mean warming of up to 3°C of depth-averaged temperature along the shelf edge associated with an intrusion of the current jet onto the shelf driven by an intensification of the flow along the leading edge of large meanders. However, this intensification of flow results in cooling of bottom waters, driving cold events at the shelf break of 〈10°C at 100 m. Thus, the intensification of the current jet associated with large meander events appears to drive strong up and downwelling events across the inshore front of the Agulhas Current, facilitating shelf-slope exchange.
    Type: Article , PeerReviewed
    Format: text
    Format: text
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  • 6
    Publication Date: 2019-04-01
    Description: Benthic storms are important for both the energy budget of the ocean and for sediment resuspension and transport. Using 30 years of output from a high-resolution model of the North Atlantic, it is found that most of the benthic storms in the model occur near the western boundary in association with the Gulf Stream and the North Atlantic Current, in regions that are generally co-located with the peak near-bottom eddy kinetic energy. A common feature are meander troughs in the near-surface jets that are accompanied by deep low pressure anomalies spinning up deep cyclones with near-bottom velocities of up to more than 0.5 m/s. A case study of one of these events shows the importance of both baroclinic and barotropic instability of the jet, with energy being extracted from the jet in the upstream part of the meander trough and partly returned to the jet in the downstream part of the meander trough. This motivates examining the 30-year time mean of the energy transfer from the (annual mean) background flow into the eddy kinetic energy. This quantity is shown to be co-located well with the region in which benthic storms and large increases in deep cyclonic relative vorticity occur most frequently, suggesting an important role for mixed barotropic-baroclinic instability driven cyclogenesis in generating benthic storms throughout the model simulation. Regions of largest energy transfer and most frequent benthic storms are found to be the Gulf Stream west of the New England Seamounts and the North Atlantic Current near Flemish Cap.
    Type: Article , PeerReviewed
    Format: text
    Format: text
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  • 7
    Publication Date: 2018-11-22
    Description: The Greater Agulhas Region is one of the regions with highest mesoscale activity in the world ocean. Among other drivers, the interactions of the mesoscale eddies drive energetic submesoscale dynamics in the surface boundary layer. Here we derive near-surface horizontal wavenumber spectra from high-resolution satellite products and from the output of a double nested (1/20°, 1/60°) NEMO configuration with 120 vertical levels. The nests are embedded via AGRIF in a ORCA025 base model. The regional distribution of inertial range and mesoscale spectral slopes can be attributed to the different dynamical regimes prevalent in the Greater Agulhas Region. Further, we emphasize the use of horizontal wavenumber spectra to identify an appropriate choice of the subgrid closure for the model.
    Type: Conference or Workshop Item , NonPeerReviewed
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  • 8
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    In:  [Talk] In: DRAKKAR 2018 Annual Workshop, 22.-24.01.2018, Grenoble, France .
    Publication Date: 2018-11-22
    Description: Two AGRIF configurations based on NEMO3.6 using LIM2 with high-resolution focus onto the Atlantic Ocean at 1/20°, embedded in ORCA025, have been established. VIKING20X is the successor of VIKING20 with extended high-resolution area over the whole Atlantic from about 68°N to the southern tip of Africa at 34°S. The nest region of INALT20 extends from 10°N, reaching into the southern Ocean at 63°N, also covering the western Indian Ocean. The two configurations are set up as complementary pair to help to disentangle the influences resulting from an improved representation either of the North Atlantic with its deep water formation contributing to the lower limb of the AMOC or the South Atlantic with its impact from the Agulhas system to the upper limb. Several sensitivity experiments have been conducted focusing on the influence of lateral boundary conditions (free/no-slip) as well as the applied wind stress formulation (relative/absolute winds).
    Type: Conference or Workshop Item , NonPeerReviewed
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  • 9
    Publication Date: 2018-11-22
    Description: Many questions in ocean and climate modelling require the combined use of high resolution, global coverage and multi-decadal integration length. For this combination, even modern resources limit the use of traditional structured-mesh grids. Here we compare two approaches: A high-resolution grid nested into a global model at coarser resolution (NEMO with AGRIF) and an unstructured-mesh grid (FESOM) which allows to variably enhance resolution where desired. The Agulhas system around South Africa is used as a testcase, providing an energetic interplay of a strong western boundary current and mesoscale dynamics. Its open setting into the horizontal and global overturning circulations also requires global coverage. Both model configurations simulate a reasonable large-scale circulation. Distribution and temporal variability of the wind-driven circulation are quite comparable due to the same atmospheric forcing. However, the overturning circulation differs, owing each model’s ability to represent formation and spreading of deep water masses. In terms of regional, high-resolution dynamics, all elements of the Agulhas system are well represented. Owing to the strong nonlinearity in the system, Agulhas Current transports of both configurations and in comparison with observations differ in strength and temporal variability. Similar decadal trends in Agulhas Current transport and Agulhas leakage are linked to the trends in wind forcing. Although the number of 3D wet grid points used in FESOM is similar to that in the nested NEMO, FESOM uses about two times the number of CPUs to obtain the same model throughput (in terms of simulated model years per day). This is feasible due to the high scalability of the FESOM code.
    Type: Conference or Workshop Item , NonPeerReviewed
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  • 10
    Publication Date: 2018-11-22
    Type: Conference or Workshop Item , NonPeerReviewed
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