Interannual variability of the Canary upwelling system

Principal Investigator

Dr Steven Herbette
LODYC
Université Pierre et Marie Curie
4, place Jussieu
75252 PARIS Cedex 05
France

steven.herbette@lodyc.jussieu.fr

Other researchers: Leo Nykjaer (JRSC-ISPRA) and Cyril Lathuilière (Phd sudent at LOCEAN)

Project description

This project aims at investigating the impact of climate variability on the distribution patterns of pelagic fish in the Northwest African coastal ecosystem.

The Canary Current System (CCS) is one of the four major upwelling regions of the world in terms of primary production; it shelters large fisheries resources and provides an important source of income for the economy of the neighbouring countries. The basic coastal upwelling process is well understood: equator-ward trade winds blow along the African coast and drive an offshore transport of surface waters and subsequent upwelling of cold nutrient rich waters in the coastal area. In the decade extending from 1990 to 2000, large variability of the population of small pelagic fishes has been observed in the region, with migration of species and decrease of fish stocks. This is supposed to be highly correlated with changes in the physical properties of the coastal waters due to anomalies in the upwelling patterns.

This study will focus on the region off Cape Blanc (21N) and the continental shelf located just northward. The circulation in this eastern boundary region is poorly known despite the Dakla-shelf is one of the crucial spawning area for sardines in the Canary ecosystem and some specific oceanographic phenomena occur. Well offshore, there is a confluence zone off Cap Blanc: the equator-ward Canary Current and the northward extension of the North Equatorial Counter Current close respectively the subtropical and tropical gyres and form the Cap Verde density front that separates the north cold and salty waters from the tropical warm and fresh waters. In the coastal waters, the circulation is mainly linked to the coastal upwelling, with the presence of a permanent filament extending offshore and a poleward undercurrent on the continental slope. The latter contributes to the entrainment of the nutrient-rich South Atlantic Central Water (SACW) northward, where it might upwell to the surface and penetrate on the Dakla-shelf. The seasonal and inter-annual variability in time and intensity of the SACW intrusion may have a crucial influence on the enrichment of this area, highly populated with sardines.

This regional modeling study uses the Regional Oceanic Modeling System (ROMS) which is a primitive equation model with a free surface and a terrain-following vertical coordinate. The model was developed with the goal of simulating both coastal and oceanic regions and their interactions (Shchepetkin and Mc Williams 2003). Our strategy for managing the large range of scales from regional down to local scales is a multi-level approach based on the AGRIF (Adapted Grid Refinement in FORTRAN) package developed by Blayo and Debreu (1999). To encompass the whole Canary Basin, a “parent” grid extending from 5N to 41N and from 30W to 5W at a resolution of 25km with 32 sigma levels is first designed. In order to focus on the divergence zone at Cape Blanc, we then build a “child” grid at a resolution of 7km extending from 26W to 8W and from 13N to 31N. The 7km resolution nested grid allows studying mesoscale processes, such as eddies, squirts and filaments that dominate the circulation in the coastal transition zone. However, this resolution is still coarse with regards to the convergence experiments of Marchesiello et al. (2003), and further refinement of the model will be considered. As increasing the resolution increases the computational time, speed up of the model can only be achieved by using parallel high performance computing platforms.

Our modeling approach is of incremental complexity. It starts by addressing the mean circulation, seasonal cycle and mesoscale physics in the Canary Current System and then focuses on the inter-annual variability. The study of the inter-annual variability can be carried out in several ways, depending on whether we choose to investigate for the influence of remote or local forcing. First, we will study the impact of wind fluctuations by specifying surface wind stress calculated from surface winds speed at 10m given by the ERA40 data archive of ECMWF. We will also run a fortran lagrangian analysis tool (Blanke and Raynaud, 1997) in order to diagnose the origins of the water masses on the Dakla continental shelf, and quantify their intrusion.

Blayo, E. and L. Debreu, 1999: Adaptive mesh refinement for finite-difference ocean models: first experiments. J. Phys. Oceanogr., 29, 1239-250.

Marchesiello, P., J.C. McWilliams, and A. Shchepetkin, 2003: Equilibrium structure and dynamics of the California Current System, J. Phys. Oceanogr., 33, 753-783.

Shchepetkin, A. F., and J. C. McWilliams, 2003: A method for computing horizontal pressure-gradient force in an ocean model with a non-aligned vertical coordinate, J. Geophys. Res., 108, article: 3090.

Blanke, B. and S. Raynaud, 1997: Kinematics of the Pacific Equatorial Undercurrent: an eulerian and lagrangian approach from GCM results, J. Phys. Oceanogr., 27, 1038-1053

For more details, please refer to the latest progress report.

Additional information

Project started in 2005.