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    Annual and semiannual cycle of equatorial Atlantic circulation associated with basin-mode resonance

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    jpo-d-15-0248%2E1.pdf (4.521Mb)
    Date
    2016-10-05
    Author
    Brandt, Peter  Concept link
    Claus, Martin  Concept link
    Greatbatch, Richard J.  Concept link
    Kopte, Robert  Concept link
    Toole, John M.  Concept link
    Johns, William E.  Concept link
    Böning, Claus W.  Concept link
    Metadata
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    Citable URI
    https://hdl.handle.net/1912/8539
    As published
    https://doi.org/10.1175/JPO-D-15-0248.1
    DOI
    10.1175/JPO-D-15-0248.1
    Keyword
     Atlantic Ocean; Ocean circulation; In situ oceanic observations; Ocean models; Seasonal cycle; Tropical variability 
    Abstract
    Seasonal variability of the tropical Atlantic circulation is dominated by the annual cycle, but semiannual variability is also pronounced, despite weak forcing at that period. This study uses multiyear, full-depth velocity measurements from the central equatorial Atlantic to analyze the vertical structure of annual and semiannual variations of zonal velocity. A baroclinic modal decomposition finds that the annual cycle is dominated by the fourth mode and the semiannual cycle is dominated by the second mode. Similar local behavior is found in a high-resolution general circulation model. This simulation reveals that the annual and semiannual cycles of the respective dominant baroclinic modes are associated with characteristic basinwide 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 vs realistic coastlines) or forcing (i.e., spatially uniform vs 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.
    Description
    Author Posting. © American Meteorological Society, 2016. This article is posted here by permission of American Meteorological Society for personal use, not for redistribution. The definitive version was published in Journal of Physical Oceanography 46 (2016): 3011-3029, doi:10.1175/JPO-D-15-0248.1.
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    • Physical Oceanography (PO)
    Suggested Citation
    Journal of Physical Oceanography 46 (2016): 3011-3029
     

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