Generation of the Cape Ghir upwelling filament: A numerical study

Abstract

Filaments are narrow, shallow structures of cool water originating from the coast. They are typical features of the four main eastern boundary upwelling systems (EBUS). In spite of their significant biological and chemical roles, through the offshore exportation of nutrient-rich waters, the physical processes that generate them are still not completely understood. This paper is a process-oriented study of filament generation mechanisms. Our goal is twofold: firstly, to obtain a numerical solution able to correctly represent the characteristics of the filament off Cape Ghir (30• 38'N, northwest Africa) in the Canary EBUS and secondly, to explain its formation by a simple mechanism based on the balance of potential vorticity.The first goal is achieved by the use of the ROMS model (Regional Ocean Modeling System) with embedded domains around Cape Ghir, with a horizontal resolution going up to 1.5 km for the finest domain. The latter gets its initial and boundary conditions from a parent solution and is forced by climatological, high-resolution atmospheric fields. The modeled filaments display spatial, temporal and physical characteristics in agreement with the available in situ and satellite observations. This model solution is used as a reference to compare the results with a set of process-oriented experiments. These experiments allow us to reach the second objective. The solutions serve to highlight the contributions of various processes to the filament generation. Since the study is focused on general processes present under climatological forcing conditions, inter-annual forcing is not necessary.The underlying idea for the filament generation is the balance of potential vorticity in the Canary EBUS: the upwelling jet is characterized by negative relative vorticity and flows southward along a narrow band of uniform potential vorticity. In the vicinity of the cape, an injection of relative vorticity induced by the wind breaks the existing vorticity balance. The upwelling jet is prevented from continuing its way southward and has to turn offshore to follow lines of equal potential vorticity.The model results highlight the essential role of wind, associated with the particular topography (coastline and bottom) around the cape. The mechanism presented here is general and thus can be applied to other EBUS.

Dominance of unicellular cyanobacteria in the diazotrophic community in the Atlantic Ocean

“…The surface waters in Cape Ghir exhibited the highest concentration of inorganic nutrients (PO 3{ 4 , NO { 3 + NO { 2 ) and primary production among the three legs of the cruise, whereas surface chlorophyll fluorescence was highest in Cape Silleiro (Table 1). Detailed hydrography of the region has been reported in Carracedo et al (2012) and Troupin et al (2012).…”

The Cape Ghir filament system in August 2009 (NW Africa)

“…The first relates to the increase of the relative cyclonic vorticity by the ITE and the consequent detachment of the flow from the coast to conserve its potential vorticity as modeled by Troupin et al . []. The second mechanism may be the advection of coastal upwelled water by the ITE as suggested by the anticyclonic deflection of the surface chlorophyll signal of the cold filament (Figures b and d).…”

“…Troupin et al . [] proposed the origin of the Cape Ghir filament in injection of cyclonic relative vorticity by the surface wind curl in the region of the coastal upwelling jet. This wind vorticity source disturbs the existing potential vorticity balance of the upwelling jet, preventing it from continuing southward and detaching it from the coastal region to give rise to the filament.…”

A location-based approach to the classification of mesoscale oceanic structures in SeaWiFS and Aqua-MODIS images of Northwest Africa

“…It would also be useful for the expert community to locate the upwelling filaments near Cape Ghir or Cap Blanc. Extraordinarily long filaments (at least 200 km offshore), which are important vehicles for nutrient and carbon transport in that zone, are recurrent at Cabo Ghir (Troupin et al (2012)). The filaments and eddies are basically generated by interaction of the shape of the coast and the seafloor bathymetry in their interaction with the wind, and this factor is predominant in their generation.…”

“…Generation of a filament (over 280 km) is observed at Cape Blanc, the genesis and surroundings of which may be a little more complex than at Cape Ghir. Some authors (Troupin et al (2012)) suggest that some filaments, in this case the one at Cape Blanc, may be caused by the interaction between an outside eddy field and upwelling water (Meunier et al (2012)). Therefore, a new classification proposal would have 15 categories.…”