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  • AMS (American Meteorological Society)  (20)
  • Nature Research  (8)
  • Kiel : Universitätsbibliothek Kiel  (2)
  • 11
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    Contribution of increased Agulhas leakage to tropical Atlantic warming (2015)
    AMS (American Meteorological Society)
    In:  Journal of Climate, 28 (24). pp. 9697-9706.
    Details
    Publication Date: 2020-08-04
    Description: The upper tropical Atlantic Ocean has markedly warmed since the 1960s. It has been shown that this warming was not due to local heat fluxes, and that the trade winds that drive the coastal and equatorial upwelling have intensified rather than weakened. Remote forcing might thus have played an important role. Here model experiments are used to investigate the contribution from an increased inflow of warm Indian Ocean water through Agulhas leakage. A high-resolution hindcast experiment with interannually varying forcing for the time period 1948 to 2007, in which Agulhas leakage increases by about 45% from the 1960s to the early 2000s, reproduces the observed warming trend. To tease out the role of Agulhas leakage, a sensitivity experiment designed to only increase Agulhas leakage is used. Compared to a control simulation it shows a pronounced warming in the upper tropical Atlantic Ocean. A Lagrangian trajectory analysis confirms that a significant portion of Agulhas leakage water reaches the upper 300m of the tropical Atlantic Ocean within two decades, and that the tropical Atlantic warming in the sensitivity experiment is mainly due to water of Agulhas origin. Therefore, it is suggested that the increased trade winds since the 1960s favor upwelling of warmer subsurface waters, which in parts originate from the Agulhas, leading to higher SSTs in the tropics
    Type: Article , PeerReviewed
    Format: text
    DOI: 10.1175/JCLI-D-15-0258.1
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    OceanRep: Article in a Scientific Journal - peer-reviewed
  • 12
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    Variability and coherence of the Agulhas Undercurrent in a High-resolution Ocean General Circulation Model (2009)
    AMS (American Meteorological Society)
    In:  Journal of Physical Oceanography, 39 . pp. 2417-2435.
    Details
    Publication Date: 2020-08-04
    Description: The Agulhas Current system has been analyzed in a nested high-resolution ocean model and compared to observations. The model shows good performance in the western boundary current structure and the transports off the South African coast. This includes the simulation of the northward-flowing Agulhas Undercurrent. It is demonstrated that fluctuations of the Agulhas Current and Undercurrent around 50–70 days are due to Natal pulses and Mozambique eddies propagating downstream. A sensitivity experiment that excludes those upstream perturbations significantly reduces the variability as well as the mean transport of the undercurrent. Although the model simulates undercurrents in the Mozambique Channel and east of Madagascar, there is no direct connection between those and the Agulhas Undercurrent. Virtual float releases demonstrate that topography is effectively blocking the flow toward the north.
    Type: Article , PeerReviewed
    Format: text
    DOI: 10.1175/2009JPO4184.1
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    OceanRep: Article in a Scientific Journal - peer-reviewed
  • 13
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    The Sensitivity of the Greenland-Scotland Ridge Overflow to Forcing Changes. (2003)
    AMS (American Meteorological Society)
    In:  Journal of Physical Oceanography, 33 . pp. 2307-2319.
    Details
    Publication Date: 2018-04-10
    Description: Processes that influence the volume and heat transport across the Greenland–Scotland Ridge system are investigated in a numerical model with ° horizontal resolution. The focus is on the sensitivity of cross-ridge transports and the reaction of the subpolar North Atlantic Ocean circulation to changes in wind stress and buoyancy forcing on seasonal to interannual timescales. A general relation between changes in wind stress or cross-ridge density contrasts and the overturning transport of Greenland–Iceland–Norwegian Seas source water is established from a series of idealized experiments. The relation is used subsequently to interpret changes in an experiment over the years 1992–97 with realistic forcing. On seasonal and interannual timescales there is a clear correlation between heat flux and wind stress curl variability. The realistic model suggests a steady decrease in the strength of the cyclonic subpolar gyre of the North Atlantic with a corresponding decrease in heat transport during the 1990s
    Type: Article , PeerReviewed
    Format: text
    DOI: 10.1175/1520-0485(2003)033〈2307:TSOTGR〉2.0.CO;2
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    OceanRep: Article in a Scientific Journal - peer-reviewed
  • 14
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    On multidecadal and quasi-decadal North Atlantic variability (2008)
    AMS (American Meteorological Society)
    In:  Journal of Climate, 21 (14). pp. 3433-3452.
    Details
    Publication Date: 2019-09-23
    Description: Observed sea surface temperatures (SSTs) in the North Atlantic from 1958 through 2000, as well as data from an ocean model simulation driven with the atmospheric variability observed during the same period, are examined using multichannel singular spectrum analysis. The two leading oscillatory modes are associated with a multidecadal and a quasi-decadal period. The former is connected to a basinwide uniform SST pattern and changes in the deep North Atlantic meridional overturning circulation. The quasi-decadal mode involves a tripolar SST anomaly pattern forced by atmospheric variability with a spatial structure resembling that of the North Atlantic Oscillation (NAO). The upper ocean’s dynamical response to this NAO variability provides an instantaneous positive feedback to the SST pattern, while a delayed negative feedback is due to shallow overturning circulation anomalies.
    Type: Article , PeerReviewed
    Format: text
    DOI: 10.1175/2007JCLI1800.1
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    OceanRep: Article in a Scientific Journal - peer-reviewed
  • 15
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    Larval behaviour, dispersal and population connectivity in the deep sea (2020)
    Nature Research
    Details
    Publication Date: 2023-02-08
    Description: Ecosystem connectivity is an essential consideration for marine spatial planning of competing interests in the deep sea. Immobile, adult communities are connected through freely floating larvae, depending on new recruits for their health and to adapt to external pressures. We hypothesize that the vertical swimming ability of deep-sea larvae, before they permanently settle at the bottom, is one way larvae can control dispersal. We test this hypothesis with more than 3x108 simulated particles with a range of active swimming behaviours embedded within the currents of a high-resolution ocean model. Despite much stronger horizontal ocean currents, vertical swimming of simulated larvae can have an order of magnitude impact on dispersal. These strong relationships between larval dispersal, pathways, and active swimming demonstrate that lack of data on larval behaviour traits is a serious impediment to modelling deep-sea ecosystem connectivity; this uncertainty greatly limits our ability to develop ecologically coherent marine protected area networks.
    Type: Article , PeerReviewed , info:eu-repo/semantics/article
    Format: text
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    OceanRep: Article in a Scientific Journal - peer-reviewed
  • 16
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    The Submesoscale Kinetic Energy Cascade: Mesoscale Absorption of Submesoscale Mixed-Layer Eddies and Frontal Downscale Fluxes (2020)
    AMS (American Meteorological Society) | Wiley
    Details
    Publication Date: 2023-02-08
    Description: Mesoscale eddies can be strengthened by the absorption of submesoscale eddies resulting from mixed-layer baroclinic instabilities. This is shown for mesoscale eddies in the Agulhas Current system by investigating the kinetic energy cascade with a spectral and a coarse-graining approach in two model simulations of the Agulhas region. One simulation resolves mixed-layer baroclinic instabilities and one does not. When mixed-layer baroclinic instabilities are included, the largest submesoscale near-surface fluxes occur in winter-time in regions of strong mesoscale activity for upscale as well as downscale directions. The forward cascade at the smallest resolved scales occurs mainly in frontogenetic regions in the upper 30 m of the water column. In the Agulhas ring path, the forward cascade changes to an inverse cascade at a typical scale of mixed-layer eddies (15 km). At the same scale, the largest sources of the upscale flux occur. After the winter, the maximum of the upscale flux shifts to larger scales. Depending on the region, the kinetic energy reaches the mesoscales in spring or early summer aligned with the maximum of mesoscale kinetic energy. This indicates the importance of submesoscale flows for the mesoscale seasonal cycle. A case study shows that the underlying process is the mesoscale absorption of mixed-layer eddies. When mixed-layer baroclinic instabilities are not included in the simulation, the open-ocean upscale cascade in the Agulhas ring path is almost absent. This contributes to a 20 %-reduction of surface kinetic energy at mesoscales larger than 100 km when submesoscale dynamics are not resolved by the model.
    Type: Article , PeerReviewed
    Format: text
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    OceanRep: Article in a Scientific Journal - peer-reviewed
  • 17
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    Can Lagrangian Tracking Simulate Tracer Spreading in a High-Resolution Ocean General Circulation Model? (2019)
    AMS (American Meteorological Society)
    In:  Journal of Physical Oceanography, 49 (5). pp. 1141-1157.
    Details
    Publication Date: 2022-01-31
    Description: To model tracer spreading in the ocean, Lagrangian simulations in an offline framework are a practical and efficient alternative to solving the advective–diffusive tracer equations online. Differences in both approaches raise the question of whether both methods are comparable. Lagrangian simulations usually use model output averaged in time, and trajectories are not subject to parameterized subgrid diffusion, which is included in the advection–diffusion equations of ocean models. Previous studies focused on diffusivity estimates in idealized models but could show that both methods yield similar results as long as the deformations-scale dynamics are resolved and a sufficient amount of Lagrangian particles is used. This study compares spreading of an Eulerian tracer simulated online and a cloud of Lagrangian particles simulated offline with velocities from the same ocean model. We use a global, eddy-resolving ocean model featuring 1/20° horizontal resolution in the Agulhas region around South Africa. Tracer and particles were released at one time step in the Cape Basin and below the mixed layer and integrated for 3 years. Large-scale diagnostics, like mean pathways of floats and tracer, are almost identical and 1D horizontal distributions show no significant differences. Differences in vertical distributions, seen in a reduced vertical spreading and downward displacement of particles, are due to the combined effect of unresolved subdaily variability of the vertical velocities and the spatial variation of vertical diffusivity. This, in turn, has a small impact on the horizontal spreading behavior. The estimates of eddy diffusivity from particles and tracer yield comparable results of about 4000 m2 s−1 in the Cape Basin.
    Type: Article , PeerReviewed
    Format: text
    DOI: 10.1175/JPO-D-18-0152.1
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    OceanRep: Article in a Scientific Journal - peer-reviewed
  • 18
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    Loop Current Variability as Trigger of Coherent Gulf Stream Transport Anomalies (2019)
    AMS (American Meteorological Society)
    In:  Journal of Physical Oceanography, 49 (8). pp. 2115-2132.
    Details
    Publication Date: 2022-01-31
    Description: Satellite observations and output from a high-resolution ocean model are used to investigate how the Loop Current in the Gulf of Mexico affects the Gulf Stream transport through the Florida Straits. We find that the expansion (contraction) of the Loop Current leads to lower (higher) transports through the Straits of Florida. The associated surface velocity anomalies are coherent from the southwestern tip of Florida to Cape Hatteras. A simple continuity-based argument can be used to explain the link between the Loop Current and the downstream Gulf Stream transport: as the Loop Current lengthens (shortens) its path in the Gulf of Mexico, the flow out of the Gulf decreases (increases). Anomalies in the surface velocity field are first seen to the southwest of Florida and within 4 weeks propagate through the Florida Straits up to Cape Hatteras and into the Gulf Stream Extension. In both the observations and the model this propagation can be seen as pulses in the surface velocities. We estimate that the Loop Current variability can be linked to a variability of several Sverdrups (1Sv = 10(6) m(3) s(-1)) through the Florida Straits. The exact timing of the Loop Current variability is largely unpredictable beyond a few weeks and its variability is therefore likely a major contributor to the chaotic/intrinsic variability of the Gulf Stream. However, the time lag between the Loop Current and the flow downstream of the Gulf of Mexico means that if a lengthening/shortening of the Loop Current is observed this introduces some predictability in the downstream flow for a few weeks.
    Type: Article , PeerReviewed
    Format: archive
    Format: text
    DOI: 10.1175/JPO-D-18-0236.1
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    OceanRep: Article in a Scientific Journal - peer-reviewed
  • 19
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    Is the thermohaline circulation changing? (2006)
    AMS (American Meteorological Society)
    In:  Journal of Climate, 19 (18). pp. 4631-4637.
    Details
    Publication Date: 2020-08-04
    Description: Analyses of ocean observations and model simulations suggest that there have been considerable changes in the thermohaline circulation (THC) during the last century. These changes are likely to be the result of natural multidecadal climate variability and are driven by low-frequency variations of the North Atlantic Oscillation (NAO) through changes in Labrador Sea convection. Indications of a sustained THC weakening are not seen during the last few decades. Instead, a strengthening since the 1980s is observed. The combined assessment of ocean hydrography data and model results indicates that the expected anthropogenic weakening of the THC will remain within the range of natural variability during the next several decades
    Type: Article , PeerReviewed
    Format: text
    DOI: 10.1175/JCLI3876.1
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    OceanRep: Article in a Scientific Journal - peer-reviewed
  • 20
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    Water mass formation from revised COADS data (1995)
    AMS (American Meteorological Society)
    In:  Journal of Physical Oceanography, 25 (10). pp. 2444-2457.
    Details
    Publication Date: 2018-04-05
    Description: Surface heat and freshwater fluxes from the Comprehensive 0cean-Atmosphere Data Set are revised and used diagnostically to compute air-sea transformation rates on density, temperature, and salinity classes over the domain of the data. Maximum rates occur over the warmest water and over mode waters, which are the dominant result of air-sea interaction. Transformation in different is accordingly distinguished by temperature and salinity, just as water masses in different oceans are so distinguished. Over the entire domain, to about 30°S, approximately 80×106 m3 s−1 of warm cool water are transformed by air-sea fluxes, on annual average. Calculations for several seas in the North Atlantic, where deep water is thought to originate, we also presented.
    Type: Article , PeerReviewed
    Format: text
    DOI: 10.1175/1520-0485(1995)025〈2444:WMFFRC〉2.0.CO;2
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    OceanRep: Article in a Scientific Journal - peer-reviewed
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