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  • 1
    In: Ocean modelling online, Amsterdam [u.a.] : Elsevier Science, 1999, 29(2009), 4, Seite 269-276, 1463-5011
    In: volume:29
    In: year:2009
    In: number:4
    In: pages:269-276
    Description / Table of Contents: The skill of numerical Lagrangian drifter trajectories in three numerical models is assessed by comparing these numerically obtained paths to the trajectories of drifting buoys in the real ocean. The skill assessment is performed using the two-sample KolmogorovSmirnov statistical test. To demonstrate the assessment procedure, it is applied to three different models of the Agulhas region. The test can either be performed using crossing positions of one-dimensional sections in order to test model performance in specific locations, or using the total two-dimensional data set of trajectories. The test yields four quantities: a binary decision of model skill, a confidence level which can be used as a measure of goodness-of-fit of the model, a test statistic which can be used to determine the sensitivity of the confidence level, and cumulative distribution functions that aid in the qualitative analysis. The ordering of models by their confidence levels is the same as the ordering based on the qualitative analysis, which suggests that the method is suited for model validation. Only one of the three models, a 1/10ʿ two-way nested regional ocean model, might have skill in the Agulhas region. The other two models, a 1/2° global model and a 1/8° assimilative model, might have skill only on some sections in the region.
    Type of Medium: Online Resource
    Pages: graph. Darst
    ISSN: 1463-5011
    Language: English
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  • 2
    In: Geophysical research letters, Hoboken, NJ : Wiley, 1974, 35(2008), 1944-8007
    In: volume:35
    In: year:2008
    In: extent:6
    Description / Table of Contents: The Antarctic Circumpolar Current (ACC) carries water freely around the whole continent of Antarctica, but not without obstructions. Some, such as the Drake Passage, constrict its path, while others, such as mid-ocean ridges, may induce meandering in the current's cores and may cause the genesis of mesoscale turbulence. It has recently been demonstrated that some regions that are only relatively shallow may also have a major effect on the flow patterns of the ACC. This is here shown to be particularly true for the Conrad Rise. Using the trajectories of surface drifters, altimetry and the simulated velocities from a numerical model, we show that the ACC bifurcates at the western side of this Rise. In this process it forms two intense jets at the two meridional extremities of the Rise with a relatively stagnant water body over the Rise itself. Preliminary results from a recent cruise provide compelling support for this portrayal.
    Type of Medium: Online Resource
    Pages: 6 , graph. Darst
    ISSN: 1944-8007
    Language: English
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  • 3
    In: Geophysical research letters, Hoboken, NJ : Wiley, 1974, 36(2009), 1944-8007
    In: volume:36
    In: year:2009
    In: extent:4
    Description / Table of Contents: Time series of transports in the Agulhas region have been constructed by simulating Lagrangian drifter trajectories in a 1/10 degree two-way nested ocean model. Using these 34 year long time series it is shown that smaller (larger) Agulhas Current transport leads to larger (smaller) Indian-Atlantic inter-ocean exchange. When transport is low, the Agulhas Current detaches farther downstream from the African continental slope. Moreover, the lower inertia suppresses generation of anti-cyclonic vorticity. These two effects cause the Agulhas retroflection to move westward and enhance Agulhas leakage. In the model a 1 Sv decrease in Agulhas Current transport at 32°S results in a 0.7 ± 0.2 Sv increase in Agulhas leakage.
    Type of Medium: Online Resource
    Pages: 4 , graph. Darst.
    ISSN: 1944-8007
    Language: English
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  • 4
    In: Ocean science, Katlenburg-Lindau : Copernicus Publ., 2005, 5(2009), 4, Seite 511-521, 1812-0792
    In: volume:5
    In: year:2009
    In: number:4
    In: pages:511-521
    Description / Table of Contents: The relation between the Agulhas Current retroflection location and the magnitude of Agulhas leakage, the transport of water from the Indian to the Atlantic Ocean, is investigated in a high-resolution numerical ocean model. Sudden eastward retreats of the Agulhas Current retroflection loop are linearly related to the shedding of Agulhas rings, where larger retreats generate larger rings. Using numerical Lagrangian floats a 37 year time series of the magnitude of Agulhas leakage in the model is constructed. The time series exhibits large amounts of variability, both on weekly and annual time scales. A linear relation is found between the magnitude of Agulhas leakage and the location of the Agulhas Current retroflection, both binned to three month averages. In the relation, a more westward location of the Agulhas Current retroflection corresponds to an increased transport from the Indian Ocean to the Atlantic Ocean. When this relation is used in a linear regression and applied to almost 20 years of altimetry data, it yields a best estimate of the mean magnitude of Agulhas leakage of 13.2 Sv. The early retroflection of 2000, when Agulhas leakage was probably halved, can be identified using the regression.
    Type of Medium: Online Resource
    Pages: graph. Darst
    ISSN: 1812-0792
    Language: English
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  • 5
    Keywords: Hochschulschrift ; Agulhasstrom
    Type of Medium: Online Resource
    Pages: 1 Online-Ressource (66 Seiten = 6 MB) , Illustrationen, Diagramme, Karten
    Edition: 2021
    Language: English
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  • 6
    Keywords: Forschungsbericht
    Type of Medium: Online Resource
    Pages: Online-Ressource (7 S., 366 KB) , graph. Darst., Kt.
    Language: German
    Note: Förderkennzeichen BMBF 03F0377A/B. - Verbund-Nr. 01021199. - Engl. Berichtsbl. u.d.T.: Clivar marin II- TP B 1-1 decadal variability of the Atlantic thermohaline circulation: modeldata synthesis , Unterschiede zwischen dem gedruckten Dokument und der elektronischen Ressource können nicht ausgeschlossen werden , Auch als gedr. Ausg. vorhanden , Systemvoraussetzungen: Acrobat reader.
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  • 7
    Keywords: Forschungsbericht
    Type of Medium: Online Resource
    Pages: Online-Ressource (5 S., 560 KB) , Ill., graph. Darst.
    Language: English , German
    Note: Förderkennzeichen BMBF 01LD0102. - Literaturverz , Unterschiede zwischen dem gedruckten Dokument und der elektronischen Ressource können nicht ausgeschlossen werden , Auch als gedr. Ausg. vorh , Systemvoraussetzungen: Acrobat reader. , Bericht in engl. Sprache, mit dt. Zsfassung
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  • 8
    In: Journal of climate, Boston, Mass. [u.a.] : AMS, 1988, 21(2008), 24, Seite 6599-6615, 1520-0442
    In: volume:21
    In: year:2008
    In: number:24
    In: pages:6599-6615
    Description / Table of Contents: The causes and characteristics of interannualdecadal variability of the meridional overturning circulation (MOC) in the North Atlantic are investigated with a suite of basin-scale ocean models [the Family of Linked Atlantic Model Experiments (FLAME)] and global oceanice models (ORCA), varying in resolution from medium to eddy resolving (1/2ʿ1/12ʿ), using various forcing configurations built on bulk formulations invoking atmospheric reanalysis products. Comparison of the model hindcasts indicates similar MOC variability characteristics on time scales up to a decade; both model architectures also simulate an upward trend in MOC strength between the early 1970s and mid-1990s. The causes of the MOC changes are examined by perturbation experiments aimed selectively at the response to individual forcing components. The solutions emphasize an inherently linear character of the midlatitude MOC variability by demonstrating that the anomalies of a (noneddy resolving) hindcast simulation can be understood as a superposition of decadal and longer-term signals originating from thermohaline forcing variability, and a higher-frequency wind-driven variability. The thermohaline MOC signal is linked to the variability in subarctic deep-water formation, and rapidly progressing to the tropical Atlantic. However, throughout the subtropical and midlatitude North Atlantic, this signal is effectively masked by stronger MOC variability related to wind forcing and, especially north of 30ʿ-35ʿN, by internally induced (eddy) fluctuations.
    Type of Medium: Online Resource
    Pages: graph. Darst
    ISSN: 1520-0442
    Language: English
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  • 9
    In: Nature 〈London〉, London [u.a.] : Nature Publ. Group, 1869, 462(2009), 7272, Seite 495-498, 1476-4687
    In: volume:462
    In: year:2009
    In: number:7272
    In: pages:495-498
    Description / Table of Contents: The transport of warm and salty Indian Ocean waters into the Atlantic Oceanthe Agulhas leakagehas a crucial role in the global oceanic circulation1 and thus the evolution of future climate. At present these waters provide the main source of heat and salt for the surface branch of the Atlantic meridional overturning circulation (MOC)2. There is evidence from past glacial-to-interglacial variations in foraminiferal assemblages3 and model studies4 that the amount of Agulhas leakage and its corresponding effect on the MOC has been subject to substantial change, potentially linked to latitudinal shifts in the Southern Hemisphere westerlies5. A progressive poleward migration of the westerlies has been observed during the past two to three decades and linked to anthropogenic forcing6, but because of the sparse observational records it has not been possible to determine whether there has been a concomitant response of Agulhas leakage. Here we present the results of a highresolution ocean general circulation model7,8 to show that the transport of Indian Ocean waters into the South Atlantic via the Agulhas leakage has increased during the past decades in response to the change in wind forcing. The increased leakage has contributed to the observed salinification9 of South Atlantic thermocline waters. Both model and historic measurements off South America suggest that the additional Indian Ocean waters have begun to invade the North Atlantic, with potential implications for the future evolution of the MOC.
    Type of Medium: Online Resource
    Pages: graph. Darst
    ISSN: 1476-4687
    Language: English
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  • 10
    Publication Date: 2015-02-26
    Description: A set of experiments utilizing different implementations of the global ORCA-LIM model with horizontal resolutions of 2°, 0.5° and 0.25° is used to investigate tropical and extra-tropical influences on equatorial Pacific SST variability at interannual to decadal time scales. The model experiments use a bulk forcing methodology building on the global forcing data set for 1958 to 2000 developed by Large and Yeager (2004) that is based on a blend of atmospheric reanalysis data and satellite products. Whereas representation of the mean structure and transports of the (sub-) tropical Pacific current fields is much improved with the enhanced horizontal resolution, there is only little difference in the simulation of the interannual variability in the equatorial regime between the 0.5° and 0.25° model versions, with both solutions capturing the observed SST variability in the Niño3-region. The question of remotely forced oceanic contributions to the equatorial variability, in particular, the role of low-frequency changes in the transports of the Subtropical Cells (STCs), is addressed by a sequence of perturbation experiments using different combinations of fluxes. The solutions show the near-surface temperature variability to be governed by wind-driven changes in the Equatorial Undercurrent. The relative contributions of equatorial and off-equatorial atmospheric forcing differ between interannual and longer, (multi-) decadal timescales: for the latter there is a significant impact of changes in the equatorward transport of subtropical thermocline water associated with the lower branches of the STCs, related to variations in the off-equatorial trade winds. A conspicuous feature of the STC variability is that the equatorward transports in the interior and along the western boundary partially compensate each other at both decadal and interannual time scales, with the strongest transport extrema occurring during El Niño episodes. The behaviour is rationalized in terms of a wobbling in the poleward extents of the tropical gyres, which is manifested also in a meridional shifting of the bifurcation latitudes of the North and South Equatorial Current systems.
    Type: Article , PeerReviewed
    Format: text
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