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Volume 14, issue 5 | Copyright
Ocean Sci., 14, 971-997, 2018
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 10 Sep 2018

Research article | 10 Sep 2018

A model perspective on the dynamics of the shadow zone of the eastern tropical North Atlantic – Part 1: the poleward slope currents along West Africa

Lala Kounta1,2, Xavier Capet2, Julien Jouanno3, Nicolas Kolodziejczyk4, Bamol Sow5, and Amadou Thierno Gaye1 Lala Kounta et al.
  • 1Laboratoire de Physique de l'Atmosphère et de l'Océan Siméon Fongang, ESP /UCAD, Dakar, Senegal
  • 2LOCEAN Laboratory, CNRS-IRD-Sorbonne Universités-MNHN, Paris, France
  • 3LEGOS Laboratory, IRD-Univ. Paul Sabatier-Observatoire Midi-Pyrénées, Toulouse, France
  • 4Laboratoire d'Océanographie Physique et Spatial, IFREMER-IRD-CNRS-UBO, IUEM, Plouzané, France
  • 5Laboratoire d'Océanographie, des Sciences de l'Environnement et du Climat, UASZ, Ziguinchor, Senegal

Abstract. The West African seaboard is one of the upwelling sectors that has received the least attention, and in situ observations relevant to its dynamics are particularly scarce. The current system in this sector is not well known and understood, e.g., in terms of seasonal variability, across-shore structure, and forcing processes. This knowledge gap is addressed in two studies that analyze the mean seasonal cycle of an eddy-permitting numerical simulation of the tropical Atlantic. Part 1 is concerned with the circulation over the West African continental slope at the southernmost reach of the Canary Current system, between  ∼ 8 and 20°N. The focus is on the depth range most directly implicated in the wind-driven circulation (offshore and coastal upwellings and Sverdrup transport) located above the potential density σt = 26.7kgm−3 in the model (approx. above 250m of depth). In this sector and for this depth range, the flow is predominantly poleward as a direct consequence of positive wind stress curl forcing, but the degree to which the magnitude of the upper ocean poleward transport reflects Sverdrup theory varies with latitude. The model poleward flow also exhibits a marked semiannual cycle with transport maxima in spring and fall. Dynamical rationalizations of these characteristics are offered in terms of wind forcing of coastal trapped waves and Rossby wave dynamics. Remote forcing by seasonal fluctuations of coastal winds in the Gulf of Guinea plays an instrumental role in the fall intensification of the poleward flow. The spring intensification appears to be related to wind fluctuations taking place at shorter distances north of the Gulf of Guinea entrance and also locally. Rossby wave activity accompanying the semiannual fluctuations of the poleward flow in the coastal waveguide varies greatly with latitude, which in turn exerts a major influence on the vertical structure of the poleward flow. Although the realism of the model West African boundary currents is difficult to determine precisely, the present in-depth investigation provides a renewed framework for future observational programs in the region.

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Short summary
The currents along the West African seaboard are poorly known. Based on a carefully evaluated numerical simulation the present study describes these currents in the sector 8–20°N and the physical processes that drive them. Prevailing northward flow with two intensification periods per year is identified. Both local and distant coastal winds (blowing as far as thousands of kilometers away in the Gulf of Guinea) contribute to the circulation in this sector.
The currents along the West African seaboard are poorly known. Based on a carefully evaluated...