Background

The circulation in the northeastern tropical Atlantic is dominated by a broad and weak southward eastern boundary current (named the Canary Current, the eastern limb of the North Atlantic subtropical gyre) and by strong upwelling dynamics triggered by northeasterly winds. The upwelled relatively cold and nutrient-rich coastal waters mostly flow offshore in filaments rooted at specific locations along the African coast, such as off Cape Blanc and off Dakar. Coastal surface current and undercurrent have also been observed (Hagen, 2000). Upwelling varies seasonally and is stronger in boreal winter when the winds are stronger and the ITCZ has migrated more to the south. Inter-annual variations associated with ENSO can also be substantial.

The coast of Senegal extends between 12°N and 17°N. It is characterized by a major Cape (Pointe des Amaldies near Dakar) which is the most westward location of the African continent. The bathymetry off Senegal consists of a continental shelf with width of ~10 km (off Dakar) increasing to 50 (100) km near the northern (southern) border. Further offshore, the continental slope descends to 3000 m depths within 100-150 km of the coast. The Senegal River outflows into the Atlantic through an elongated estuary near the city of Saint Louis near the northern border of the country. The Senegal River has a drainage basin of 483,181 km², a length of 1790 km and an estimated annual discharge of 8 million km³. The Senegal River and the coastal waters represent a important crucial and critical economic resource for the country, mostly in terms of fresh water supply (used for irrigation) and of sardine fisheries (Demarcq and Faure, 2000).

Surface drifter observations in the Canary Current and off Senegal are rather scarce, mainly due to the limited number of drifters deployed there. However, they reveal a complex surface circulation with weak equatorward flow and energetic mesoscale activity including eddies and filaments (Barton et al., 2004; Lumpkin and Garzoli, 2005). This variability dominated by coastal upwelling dynamics is similar to the one found in other eastern boundary currents (e.g., off California).

Despite the significant cloud coverage associated with the ITCZ, satellite images can be helpful to describe the spatial structure and temporal evolution of the mesoscale features off Senegal. Data from infrared radiometers onboard the NOAA polar-orbiting satellites (Kostianoy and Zatsepin, 1996) and the Meteosat geostationary satellite (Marullo et al., 1989; Demarcq and Faure, 2000) show the complex sea surface temperature (SST) signature of the cold upwelled water progressing offshore in filaments or being trapped in eddies and the temporal evolution of the upwelling-related features at scales from days to years. In particular, Demarcq and Faure (2000) have defined an upwelling index from the SST difference between the coastal and open waters and have shown its variations at seasonal and inter-annual scales. Ocean color satellite monitoring have also provided a precious help for understanding the variability of the near-surface chlorophyll concentration and of the phytoplankton productivity during upwelling off Senegal (Dupouy and Demarq, 1987).

Objectives

Our general objective is to investigate the coastal dynamics off NW Africa in the tropical Atlantic Ocean. In particular, we propose

• to perform an extensive review of existing knowledge on the oceanography in the northeastern Tropical Altantic Ocean.

• to study the near-surface dispersion and circulation off the coast of Senegal, a region strongly influenced by coastal upwelling dynamics and affected by the runoff of an important river. It is planned to study the surface circulation dynamics off Senegal at temporal scales ranging from inertial/tidal to seasonal using drifter observations and ancillary satellite data (SST and ocean color) over about a year (from spring 2009 to winter 2010), in collaboration with local oceanographers.

• to analyze the drifter data in concert with satellite images (SST and ocean color) in order to describe qualitatively the surface dynamics, with particular focus on mesoscale circulation features such as eddies and filaments.

• to train local Senegalese oceanographers and organize capacity building activities to teach how to operate drifters, to process their data, and to analyze the scientific results obtained from them.

Tasks

The COCES project is organized into the following tasks:

T1: Literature review

T2: Planning of drifter operations and drifter procurements

T3: Drifter deployments and recoveries

T4: Drifter data processing

T5: Data analyses and interpretation

Collaborations

The COCES project involves effective collaboration between the P.I. and the following researchers:

1. Drs. Niiler and Centurioni with SIO/UCSD (San Diego, California) and with Hydor Technologies, Inc., who have experience in describing and studying eastern boundary current systems, such as the California Current System, mainly using surface drifters.

2. Dr. Alban Lazar of CNRS/IRD (Paris, France) and LPAOF/UCAD (Dakar, Senegal) who is an young dynamic oceanographer based in Dakar to promote oceanographic research in Senegal.

3. Dr. Bamol Sow of Univ. Ziguinchor (Senegal) who is a young Senegalese professor interested in local observational oceanography and modeling.

4. Dr. Basirou Diaw and Mr. Saliou Faye of CRODT (Dakar, Senegal) respectively senior scientist and graduate student working in physical oceanography.

Reports

2008
Report 2008

2009
Report 2009

2010
Report 2010

2011
Report 2011

Bibliography

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Centurioni, L. R., J. C. Ohlmann, P. P. Niiler, 2008: Permanent Meanders in the California Current System. J. Phys. Oceanogr., 38, 1690-1710.

Davis, R. E. Drifter observations of coastal currents during CODE. The method and descriptive view, Journal of Geophysical Research, 90, 4741-4755, 1985.

Demarcq, H. and V. Faure. Coastal upwelling and associated retention indices derived from satellite SST. Application to Octopus vulgaris recruitment. Oceanologica Acta, 23, 391-408, 2000.

Dupouy, C: and H. Demarcq. CZCS as an aid for understanding modalities of the phytoplankton productivity during upwelling off Senegal. Adv. Space. Res., 7(2), 63-71, 1987.

Hagen, E. Northwest African upwelling scenario. Oceanologica Acta, 24, S113-S128, 2001.

Hansen, D. V. and P.-M. Poulain. Processing of WOCE/TOGA drifter data/. /J. Atmos. Ocean. Tech., 13, 900-909, 1996.

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Lumpkin, R. and S. L. Garzoli. Near-surface circulation in the Tropical Atlantic Ocean. Deep-Sea Research I, 52, 495-518, 2005.

Marullo, S., R. Santoleri, G. L. Liberti and G. Dalu. Observations of coastal upwelling off northwest Africa using meteosat data. Il Nuovo CImento C, 12, 151-161, 1989.

Poulain, P.-M. and P. P. Niiler. Statistical analysis of the surface circulation in the California Current System using satellite-tracked drifters. J. Phys. Oceanogr, 19, 1588-1603, 1989.

Poulain, P.-M., E. Mauri and L. Ursella. Unusual upwelling event and current reversal off the Italian Adriatic coast in summer 2003. Geophys. Res. Lett., Vol. 31, L05303,

doi:10.1029/2003GL019121, 2004.

Sybrandy, A. L. and P. P. Niiler. WOCE/TOGA Lagrangian drifter construction manual. SIO Ref. 91/6, WOCE REP. 63, Scripps Institution of Oceanography, La Jolla, California, 58p., 1991.