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  • 1
    Online Resource
    Online Resource
    Intraseasonal variability of the equatorial Atlantic Ocean (2016)
    Kiel
    Details Availability
    Keywords: Hochschulschrift
    Type of Medium: Online Resource
    Pages: 1 Online-Ressource (77 Blatt = 8 MB) , Diagramme
    URL: http://oceanrep.geomar.de/38081/
    Language: English
    Note: Zusammenfassung in deutscher und englischer Sprache
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  • 2
    Online Resource
    Online Resource
    The Gulf Stream Extension Region : its dynamics and interaction with the atmosphere (2011)
    Kiel
    Details Availability
    Keywords: Hochschulschrift
    Type of Medium: Online Resource
    Pages: 1 Online-Ressource (75 Blatt = 14 MB)
    URL: https://oceanrep.geomar.de/12505
    Language: English
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  • 3
    Online Resource
    Online Resource
    Internally generated circulation and variability in the tropical Atlantic Ocean (2021)
    Kiel : Universitätsbibliothek Kiel
    Details Availability
    Keywords: Hochschulschrift
    Type of Medium: Online Resource
    Pages: 1 Online-Ressource (vi, 110, ix Seiten) , Illustrationen, Diagramme
    URL: https://nbn-resolving.org/urn:nbn:de:gbv:8:3-2021-00687-6
    URL: https://d-nb.info/1245860925/34
    URL: https://macau.uni-kiel.de/receive/macau_mods_00002050
    URN: urn:nbn:de:gbv:8:3-2021-00687-6
    DDC: 551.462
    Language: English
    Note: Kumulatives Verfahren, enthält 3 Zeitschriftenaufsätze
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  • 4
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    The emergence of equatorial deep jets in an idealised primitive equation model: an interpretation in terms of basin modes (2017)
    Springer
    In:  Ocean Dynamics, 67 (12). pp. 1511-1522.
    Details
    Publication Date: 2020-02-06
    Description: Ocean circulation models do not generally exhibit equatorial deep jets (EDJs), even though EDJs are a recognised feature of the observed ocean circulation along the equator and they are thought to be important for tracer transport along the equator and even equatorial climate. EDJs are nevertheless found in nonlinear primitive equation models with idealised box geometry. Here we analyse several such model runs. We note that the variability of the zonal velocity in the model is dominated by the gravest linear equatorial basin mode for a wide range of baroclinic vertical normal modes and that the EDJs in the model are dominated by energy contained in vertical modes between 10 and 20. The emergence of the EDJs is shown to involve the linear superposition of several such neighbouring basin modes. Furthermore, the phase of these basin modes is set at the start of the model run and, in the case of the reference experiment, the same basin modes can be found in a companion experiment in which the amplitude of the forcing has been reduced by a factor of 1000. We also argue that following the spin-up, energy must be transferred between different vertical modes. This is because the model simulations are dominated by downward phase propagation following the spin-up whereas our reconstructions imply episodes of upward and downward propagation. The transfer of energy between the vertical modes is associated with a decadal modulation of the EDJs.
    Type: Article , PeerReviewed , info:eu-repo/semantics/article
    Format: text
    DOI: 10.1007/s10236-017-1111-y
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    OceanRep: Article in a Scientific Journal - peer-reviewed
  • 5
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    Evidence for the maintenance of slowly varying equatorial currents by intraseasonal variability (2018)
    AGU (American Geophysical Union) | Wiley
    In:  Geophysical Research Letters, 45 (4). pp. 1923-1929.
    Details
    Publication Date: 2021-05-18
    Description: Recent evidence from mooring data in the equatorial Atlantic reveals that semi-annual and longer time scale ocean current variability is close to being resonant with equatorial basin modes. Here we show that intraseasonal variability, with time scales of 10's of days, provides the energy to maintain these resonant basin modes against dissipation. The mechanism is analogous to that by which storm systems in the atmosphere act to maintain the atmospheric jet stream. We demonstrate the mechanism using an idealised model set-up that exhibits equatorial deep jets. The results are supported by direct analysis of available mooring data from the equatorial Atlantic Ocean covering a depth range of several thousand meters. The analysis of the mooring data suggests that the same mechanism also helps maintain the seasonal variability.
    Type: Article , PeerReviewed , info:eu-repo/semantics/article
    Format: text
    Format: text
    DOI: 10.1002/2017GL076662
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    OceanRep: Article in a Scientific Journal - peer-reviewed
  • 6
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    Reconstructing Tropical Pacific Sea Level Variability for the Period 1961-2002 Using a Linear Multimode Model (2018)
    AGU (American Geophysical Union) | Wiley
    In:  Journal of Geophysical Research: Oceans, 123 (3). pp. 2037-2048.
    Details
    Publication Date: 2021-03-19
    Description: Monthly mean sea level anomalies in the tropical Pacific for the period 1961-2002 are reconstructed using a linear, multi-mode model driven by monthly mean wind stress anomalies from the NCEP/NCAR and ERA-40 reanalysis products. Overall, the sea level anomalies reconstructed by both wind stress products agree well with the available tide gauge data, although with poor performance at Kanton Island in the western-central equatorial Pacific and reduced amplitude at Christmas Island. The reduced performance is related to model error in locating the pivot point in sea level variability associated with the so-called “tilt” mode. We present evidence that the pivot point was further west during the period 1993-2014 than during the period 1961-2002 and attribute this to a persistent upward trend in the zonal wind stress variance along the equator west of 160° W throughout the period 1961-2014. Experiments driven by the zonal component of the wind stress alone reproduce much of the trend in sea level found in the experiments driven by both components of the wind stress. The experiments show an upward trend in sea level in the eastern tropical Pacific over the period 1961-2002, but with a much stronger upward trend when using the NCEP/NCAR product. We argue that the latter is related to an overly strong eastward trend in zonal wind stress in the eastern-central Pacific that is believed to be a spurious feature of the NCEP/NCAR product.
    Type: Article , PeerReviewed , info:eu-repo/semantics/article
    Format: text
    DOI: 10.1002/2017JC013652
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    OceanRep: Article in a Scientific Journal - peer-reviewed
  • 7
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    Energy budget-based backscatter in a shallow water model of a double gyre basin (2018)
    Elsevier
    In:  Ocean Modelling, 132 . pp. 1-11.
    Details
    Publication Date: 2021-02-08
    Description: The parameterization of sub-grid scale processes is one of the key challenges towards improved numerical simulations of the atmospheric and oceanic circulation. Numerical weather prediction models as well as climate models would benefit from more sophisticated turbulence closures that allow for less spurious dissipation at the grid-scale and consequently higher and more realistic levels of eddy kinetic energy (EKE). Recent studies propose to use a hyperviscous closure in combination with an additional deterministic forcing term as a negative viscosity to represent backscatter of energy from unresolved scales. The sub-grid EKE is introduced as an additional prognostic variable that is fed by dissipation at the grid scale, and enables recycling of EKE via the backscatter term at larger scales. This parameterization was previously shown to work well in zonally re-entrant channel configurations. Here, a generalization in the form of a Rossby number-dependent scaling for the strength of the backscatter is introduced to represent the emergence of a forward energy-cascade in unbalanced flows near the boundaries. We apply the parameterization to a shallow water model of a double gyre basin and provide evidence for its general applicability. In terms of mean state and variability, a low resolution model is considerably improved towards a high resolution control run at low additional computational cost.
    Type: Article , PeerReviewed , info:eu-repo/semantics/article
    Format: text
    DOI: 10.1016/j.ocemod.2018.09.006
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    OceanRep: Article in a Scientific Journal - peer-reviewed
  • 8
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    Annual and semiannual cycle of equatorial Atlantic circulation associated with basin mode resonance (2016)
    AMS (American Meteorological Society)
    In:  Journal of Physical Oceanography, 46 (10). pp. 3011-3029.
    Details
    Publication Date: 2020-08-04
    Description: Seasonal variability of the tropical Atlantic circulation is dominated by the annual cycle, but semi-annual variability is also pronounced, despite weak forcing at that period. Here we use multi-year, full-depth velocity measurements from the central equatorial Atlantic to analyze the vertical structure of annual and semi-annual variations of zonal velocity. A baroclinic modal decomposition finds that the annual cycle is dominated by the 4th mode and the semi-annual cycle by the 2nd mode. Similar local behavior is found in a high-resolution general circulation model. This simulation reveals that the annual and semi-annual cycles of the respective dominant baroclinic modes are associated with characteristic basin-wide structures. Using an idealized linear reduced-gravity model to simulate the dynamics of individual baroclinic modes, it is shown that the observed circulation variability can be explained by resonant equatorial basin modes. Corollary simulations of the reduced-gravity model with varying basin geometry (i.e. square basin versus realistic coastlines) or forcing (i.e. spatially uniform versus spatially variable wind) show a structural robustness of the simulated basin modes. A main focus of this study is the seasonal variability of the Equatorial Undercurrent (EUC) as identified in recent observational studies. Main characteristics of the observed EUC including seasonal variability of transport, core depth, and maximum core velocity can be explained by the linear superposition of the dominant equatorial basin modes as obtained from the reduced-gravity model.
    Type: Article , PeerReviewed , info:eu-repo/semantics/article
    Format: text
    DOI: 10.1175/JPO-D-15-0248.1
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    OceanRep: Article in a Scientific Journal - peer-reviewed
  • 9
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    Influence of the equatorial deep jets on the north equatorial countercurrent (2015)
    Springer
    In:  Ocean Dynamics, 65 . pp. 1095-1102.
    Details
    Publication Date: 2021-05-19
    Description: An ocean circulation model is run using two different idealized equatorial basin configurations under steady wind forcing. Both model versions produce bands of vertically alternating zonal flow at depth, similar to observed Equatorial Deep Jets (EDJs) and with a time scale corresponding to that of the gravest equatorial basin mode for the dominant baroclinic vertical normal mode. Both model runs show evidence for enhanced variability in the surface signature of the North Equatorial Counter Current (NECC) with the same time scale. We also find the same link between the observed NECC and the EDJs in the Atlantic by comparing the signature of the EDJ in moored zonal velocity data at 23° W on the equator with the signature of the NECC in geostrophic velocities from altimeter data. We argue that the presence of a peak in variability in the NECC associated with the EDJ basin mode period is evidence that the influenceatthis time scale is upward, from the EDJ to the NECC
    Type: Article , PeerReviewed , info:eu-repo/semantics/article
    Format: text
    DOI: 10.1007/s10236-015-0855-5
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    OceanRep: Article in a Scientific Journal - peer-reviewed
  • 10
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    Influence of the barotropic mean flow on the width and the structure of the Atlantic Equatorial Deep Jets (2014)
    AMS (American Meteorological Society)
    In:  Journal of Physical Oceanography, 44 . pp. 2485-2497.
    Details
    Publication Date: 2020-08-04
    Description: A representation of an equatorial basin mode excited in a shallow water model for a single high order baroclinic vertical normal mode is used as a simple model for the equatorial deep jets. The model is linearized about both a state of rest and a barotropic mean flow corresponding to the observed Atlantic Equatorial Intermediate Current System. We found that the eastward mean flow associated with the North and South Intermediate Counter Currents (NICC and SICC, respectively) effectively shields the Equator from off-equatorial Rossby waves. The westward propagation of these waves is blocked and focusing on the Equator due to beta dispersion is prevented. This leads to less energetic jets along the Equator. On the other hand, the westward barotropic mean flow along the Equator reduces the gradient of absolute vorticity and hence widens the cross-equatorial structure of the basin mode. Increasing lateral viscosity predominantly affects the width of the basin modes’ Kelvin wave component in the presence of the mean flow while the Rossby wave is confined by the flanking NICC and SICC. Independent of the presence of the mean flow, the application of sufficient lateral mixing also hinders the focusing of off-equatorial Rossby waves, which is hence an unlikely feature of a low-frequency basin mode in the real ocean.
    Type: Article , PeerReviewed
    Format: text
    DOI: 10.1175/JPO-D-14-0056.1
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    OceanRep: Article in a Scientific Journal - peer-reviewed
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