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  • Nature Publishing Group  (6)
  • Geophysical Fluid Dynamics Laboratory (GFDL)  (2)
  • ELSEVIER SCI LTD
  • 1
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    Ocean currents generate large footprints in marine palaeoclimate proxies (2015)
    Nature Publishing Group
    In:  Nature Communications, 6 (6521).
    Details
    Publikationsdatum: 2019-02-25
    Beschreibung: Fossils of marine microorganisms such as planktic foraminifera are among the cornerstones of palaeoclimatological studies. It is often assumed that the proxies derived from their shells represent ocean conditions above the location where they were deposited. Planktic foraminifera, however, are carried by ocean currents and, depending on the life traits of the species, potentially incorporate distant ocean conditions. Here we use high-resolution ocean models to assess the footprint of planktic foraminifera and validate our method with proxy analyses from two locations. Results show that foraminifera, and thus recorded palaeoclimatic conditions, may originate from areas up to several thousands of kilometres away, reflecting an ocean state significantly different from the core site. In the eastern equatorial regions and the western boundary current extensions, the offset may reach 1.5 °C for species living for a month and 3.0 °C for longer-living species. Oceanic transport hence appears to be a crucial aspect in the interpretation of proxy signals.
    Materialart: Article , PeerReviewed
    Format: text
    DOI: 10.1038/ncomms7521
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  • 2
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    Agulhas leakage dynamics affects decadal variability in Atlantic overturning circulation (2008)
    Nature Publishing Group
    In:  Nature, 456 . pp. 489-492.
    Details
    Publikationsdatum: 2019-09-23
    Beschreibung: Predicting the evolution of climate over decadal timescales requires a quantitative understanding of the dynamics that govern the meridional overturning circulation (MOC)1. Comprehensive ocean measurement programmes aiming to monitor MOC variations have been established in the subtropical North Atlantic2, 3 (RAPID, at latitude 26.5° N, and MOVE, at latitude 16° N) and show strong variability on intraseasonal to interannual timescales. Observational evidence of longer-term changes in MOC transport remains scarce, owing to infrequent sampling of transoceanic sections over past decades4, 5. Inferences based on long-term sea surface temperature records, however, supported by model simulations, suggest a variability with an amplitude of plusminus1.5–3 Sv (1 Sv = 106 m3 s-1) on decadal timescales in the subtropics6. Such variability has been attributed to variations of deep water formation in the sub-arctic Atlantic, particularly the renewal rate of Labrador Sea Water7. Here we present results from a model simulation that suggest an additional influence on decadal MOC variability having a Southern Hemisphere origin: dynamic signals originating in the Agulhas leakage region at the southern tip of Africa. These contribute a MOC signal in the tropical and subtropical North Atlantic that is of the same order of magnitude as the northern source. A complete rationalization of observed MOC changes therefore also requires consideration of signals arriving from the south.
    Materialart: Article , PeerReviewed
    Format: text
    DOI: 10.1038/nature07426
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  • 3
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    Water column methanotrophy controlled by a rapid oceanographic switch (2015)
    Nature Publishing Group
    In:  Nature Geoscience, 8 (5). pp. 378-382.
    Details
    Publikationsdatum: 2020-11-23
    Beschreibung: Large amounts of the greenhouse gas methane are released from the seabed to the water column1, where it may be consumed by aerobic methanotrophic bacteria2. The size and activity of methanotrophic communities, which determine the amount of methane consumed in the water column, are thought to be mainly controlled by nutrient and redox dynamics3–7. Here, we report repeated measurements of methanotrophic activity and community size at methane seeps west of Svalbard, and relate them to physical water mass properties and modelled ocean currents. We show that cold bottom water, which contained a large number of aerobic methanotrophs, was displaced by warmer water with a considerably smaller methanotrophic community within days. Ocean current simulations using a global ocean/sea-ice model suggest that this water mass exchange is consistent with short-term variations in the meandering West Spitsbergen Current. We conclude that the shift from an offshore to a nearshore position of the current can rapidly and severely reduce methanotrophic activity in the water column. Strong fluctuating currents are common at many methane seep systems globally, and we suggest that they affect methane oxidation in the water column at other sites, too.
    Materialart: Article , PeerReviewed
    Format: text
    DOI: 10.1038/ngeo2420
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  • 4
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    Atlantic multi-decadal oscillation covaries with Agulhas leakage (2015)
    Nature Publishing Group
    In:  Nature Communications, 6 (10082).
    Details
    Publikationsdatum: 2019-02-25
    Beschreibung: The interoceanic transfer of seawater between the Indian Ocean and the Atlantic, ‘Agulhas leakage’, forms a choke point for the overturning circulation in the global ocean. Here, by combining output from a series of high-resolution ocean and climate models with in situ and satellite observations, we construct a time series of Agulhas leakage for the period 1870–2014. The time series demonstrates the impact of Southern Hemisphere westerlies on decadal timescales. Agulhas leakage shows a correlation with the Atlantic Multi-decadal Oscillation on multi-decadal timescales; the former leading by 15 years. This is relevant for climate in the North Atlantic
    Materialart: Article , PeerReviewed
    Format: text
    Format: text
    DOI: 10.1038/ncomms10082
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  • 5
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    Stevens Open Boundary Conditions (1996)
    Geophysical Fluid Dynamics Laboratory (GFDL)
    In:  In: MOM 2 Version 2.0 (Beta) - Documentation User’s Guide and Reference Manual - Release date: Nov 7,1996. , ed. by Pacanowski, R. C. GDFL Ocean Technical Report, 3.2 . Geophysical Fluid Dynamics Laboratory (GFDL), Princeton, USA, pp. 141-150.
    Details
    Publikationsdatum: 2020-04-24
    Materialart: Book chapter , NonPeerReviewed
    Format: text
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  • 6
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    Stevens Open Boundary Conditions (1999)
    Geophysical Fluid Dynamics Laboratory (GFDL)
    In:  In: MOM 3.0 Manual. , ed. by Pacanowski, R. C. and Griffies, S. M. GFDL Ocean Group Technical Report, 4 . Geophysical Fluid Dynamics Laboratory (GFDL), Princeton, USA, pp. 245-252.
    Details
    Publikationsdatum: 2020-03-24
    Materialart: Book chapter , NonPeerReviewed
    Format: text
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  • 7
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    Increase in Agulhas leakage due to poleward shift of Southern Hemisphere westerlies (2009)
    Nature Publishing Group
    In:  Nature, 462 . pp. 495-498.
    Details
    Publikationsdatum: 2019-09-23
    Beschreibung: The transport of warm and salty Indian Ocean waters into the Atlantic Ocean—the Agulhas leakage—has 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 high-resolution 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.
    Materialart: Article , PeerReviewed
    Format: text
    DOI: 10.1038/nature08519
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  • 8
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    On the role of the Agulhas system in ocean circulation and climate (2011)
    Nature Publishing Group
    Details
    Publikationsdatum: 2022-02-18
    Beschreibung: The Atlantic Ocean receives warm, saline water from the Indo-Pacific Ocean through Agulhas leakage around the southern tip of Africa. Recent findings suggest that Agulhas leakage is a crucial component of the climate system and that ongoing increases in leakage under anthropogenic warming could strengthen the Atlantic overturning circulation at a time when warming and accelerated meltwater input in the North Atlantic is predicted to weaken it. Yet in comparison with processes in the North Atlantic, the overall Agulhas system is largely overlooked as a potential climate trigger or feedback mechanism. Detailed modelling experiments—backed by palaeoceanographic and sustained modern observations—are required to establish firmly the role of the Agulhas system in a warming climate.
    Materialart: Article , PeerReviewed
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  • 9
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    Simulating the Agulhas system in global ocean models – nesting vs. multi-resolution unstructured meshes (2018)
    ELSEVIER SCI LTD
    In:  EPIC3Ocean Modelling, ELSEVIER SCI LTD, 121, pp. 117-131, ISSN: 1463-5003
    Details
    Publikationsdatum: 2018-02-28
    Beschreibung: 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.
    Repository-Name: EPIC Alfred Wegener Institut
    Materialart: Article , isiRev
    Format: application/pdf
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