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  • 1
    Publication Date: 2018-11-09
    Description: The sea surface temperature (SST) in the eastern tropical Atlantic exhibits pronounced variability on interannual time scales being associated with wind and rainfall anomalies within the tropical Atlantic region. It has been proposed that the interannual variability of SST is partly driven by the variability of the deep equatorial zonal circulation, the so-called equatorial deep jets (EDJs). The EDJs may be described as a superposition of quasi-resonant equatorial basin modes and the direction of vertical phase propagation implies that their energy is propagating towards the surface. Furthermore, recent findings revealed that the EDJs in turn are maintained by intra-seasonal waves that are generated by the barotropic and baroclinic instability of the near-surface circulation. This talk will present the relevant mechanisms that are involved in the conversion of energy from one type of variability to another, i.e. from chaotic intra-seasonal surface variability via deep interannual zonal variability to interannual surface climate variability, with a special focus on the maintenance of the EDJs by intra-seasonal waves. Since EDJs, a key component of the mechanism discussed above, are not well represented in state-of-the-art Ocean General Circulation Models, preliminary findings on the sensitivity of the EDJs to model parameters and configuration are presented.
    Type: Conference or Workshop Item , NonPeerReviewed
    Format: slideshow
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  • 2
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    In:  [Poster] In: PIRATA 22 - PREFACE - TAV Meeting, 05.-10.11.2017, Fortaleza, Brasil .
    Publication Date: 2018-11-09
    Type: Conference or Workshop Item , NonPeerReviewed
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  • 3
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    American Meteorological Society
    In:  Journal of Physical Oceanography, 48 (2). pp. 261-281.
    Publication Date: 2019-02-01
    Description: Multi-year moored velocity observations of the Angola Current near 11°S reveal a weak southward mean flow superimposed by substantial intraseasonal to seasonal variability, including annual and semiannual cycles with distinct baroclinic structures. In the equatorial Atlantic these oscillations are associated with basin-mode resonances of the fourth and second baroclinic modes, respectively. Here, the role of basin-mode resonance and local forcing for the Angola Current seasonality are investigated. A suite of linear shallow-water models for the tropical Atlantic is employed, each model representing a single baroclinic mode forced at a specific period. The annually and semiannually oscillating forcing is given by 1) an idealized zonally uniform zonal forcing restricted to the equatorial band corresponding to a remote equatorial forcing or 2) realistic, spatially-varying Fourier components of wind stress data that include local forcing off Angola, particularly alongshore winds. Model-computed modal amplitudes are scaled to match moored velocity observations from the equatorial Atlantic. The observed annual cycle of alongshore velocity at 11°S is well reproduced by the remote equatorial forcing. Including local forcing slightly improves the agreement between observed and simulated semiannual oscillations at 11°S compared to the purely equatorial forcing. However, the model-computed semiannual cycle lacks amplitude at mid-depth. This could be the result of either underestimating the strength of the second equatorial basin-mode of the fourth baroclinic mode or other processes not accounted for in the shallow-water models. Overall, our findings underline the importance of large-scale linear equatorial wave dynamics for the seasonal variability of the boundary circulation off Angola.
    Type: Article , PeerReviewed , info:eu-repo/semantics/article
    Format: text
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  • 4
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    In:  [Talk] In: PREFACE International Conference & Final Assembly, 17.-20.04.2018, Arrecife, Lanzarote, Spain .
    Publication Date: 2018-11-09
    Description: Besides the zonal flow that dominates the seasonal and long-term variability in the equatorial Atlantic, energetic intraseasonal meridional velocity fluctuations are observed in large parts of the water column. 15 years of full-depth velocity data from an equatorial mooring at 23°W are used to investigate intraseasonal variability and specifically the downward propagation of intraseasonal energy from the surface into the deep ocean. Near the surface (20 to 50 m), intraseasonal variability at 23°W peaks at periods between 30 to 40 days. It is associated with westward propagating Tropical Instability Waves, which undergo an annual intensification in August. Enhanced energy levels of equatorial intraseasonal variability are observed down to about 2000 m. A frequency-vertical mode decomposition shows that meridional velocity fluctuations are more energetic than the zonal ones for periods 〈 50 days. The energy peak at 30 to 40 days and vertical modes 2 to 5 excludes equatorial Rossby or gravity waves and suggests Yanai waves to be associated with the observed intraseasonal energy. Yanai waves that are considered to be generated by Tropical Instability Waves propagate their energy from near the surface west of 23°W down- and eastward to eventually reach the mooring location. The distribution of intraseasonal energy depends largely on the dominant frequency and the time, depth, and longitude of excitation with the dominant vertical mode of the Yanai waves playing only a minor role. Observations also indicate the presence of weaker intraseasonal variability at 23°W below 2000 m that is not associated with Tropical Instability Waves.
    Type: Conference or Workshop Item , NonPeerReviewed
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  • 5
    Publication Date: 2019-02-01
    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
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  • 6
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    In:  (Diploma thesis), Christian-Albrechts-Universität, Kiel, Germany, 65 pp
    Publication Date: 2012-07-06
    Description: The Gulf Stream Extension Region is a key region for meridional heat transport of the Earth system. Its importance for the climate and for seasonal prediction has been increasingly recognized in recent years. However, the dynamics of the ocean in this region are not understood sufficiently. There are basically three mechanisms to explain the observed increase of barotropic transport and the formation of recirculation gyres in the Gulf Stream Extension Region: (i) advection of potential vorticity, (ii) Joint Effect of Baroclinicity and Relief and (iii) eddy momentum flux, of which the latter is investigated in the present work. 16 years of satellite derived surface velocities of weekly resolution are used to compute the eddy momentum flux and its seasonal and NAO related variability. These fluxes were used to drive a diagnostic linear shallow water model to estimate the mean barotropic transport and its variability associated with turbulent momentum flux. The results show that eddy momentum flux is able to drive significant transport in the Gulf Stream Extension Region, enough to explain its observed increase. The barotropic transport streamfunction exhibits northern and southern recirculation gyres between the separation point at Cape Hatteras and the Newfoundland Ridge and also resemble mesoscale circulation features like the Mann eddy. The seasonal transport variability is shown to be large upstream of the Grand Banks of Newfoundland with the strongest signal where the Gulf Stream crosses the New England Seamounts at about 62°W. The NAO appears to significantly affect the strength of the northern and southern recirculation gyres during the winter season leading to an increased mean transport in winters with a positive NAO index.
    Type: Thesis , NonPeerReviewed
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  • 7
    Publication Date: 2012-07-06
    Type: Conference or Workshop Item , NonPeerReviewed
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  • 8
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    In:  [Talk] In: EGU General Assembly 2013, 07.-12.04.2013, Vienna, Austria .
    Publication Date: 2013-07-08
    Description: The equatorial deep jets (EDJ) are a striking feature of the equatorial ocean circulation. In the Atlantic Ocean, the EDJ are associated with a vertical scale of between 300 and 700 m, a time scale of roughly 4.5 years and upward energy propagation to the surface. It has been found that the meridional width of the EDJ is roughly 1.5 times larger than expected based on their vertical scale. Here a representation of a equatorial basin mode excited in a linear shallow water model for a high order baroclinic vertical normal mode is used as a simple model for the EDJ. We argue that mixing of momentum along isopycnals can explain the enhanced width and a lateral eddy viscosity of 300 m^2s^-1 is found to be sufficient to account for the width implied by observations. Additionally, the effect of barotropic mean flow on the spatial and temporal structure of the wave field is studied. A mean flow resembling the Atlantic Equatorial Intermediate Current System with eastward jets at 2°N/S and westward flow in between results in a wave shielding of the equatorial band from adjacent regions.
    Type: Conference or Workshop Item , NonPeerReviewed
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  • 9
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    In:  [Talk] In: EGU General Assembly 2016, 17.-22.04.2016, Vienna, Austria .
    Publication Date: 2016-09-28
    Type: Conference or Workshop Item , NonPeerReviewed
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  • 10
    Publication Date: 2019-02-01
    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
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