Generation of the Cape Ghir upwelling filament: A numerical study

in Ocean Modelling (2012), 41

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 ... [more ▼]

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 well represent the characteristics of the filament off Cape Ghir (30°38'N, northwestern 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) in 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. Their respective solution serves to highlight the contribution of various processes in 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. [less ▲]

High-resolution measurements of an upwelling filament during the CAIBEX survey

Poster (2011, May 02)

Upwelling filaments have been studied for several years. We aim to explain the general process in terms of potential vorticity: An injection of positive vorticity north of the Cape forces the jet to turn ... [more ▼]

Upwelling filaments have been studied for several years. We aim to explain the general process in terms of potential vorticity: An injection of positive vorticity north of the Cape forces the jet to turn offshore, in virtue of the principle of vorticity conservation. The Regional Ocean Modeling System (ROMS) is implemented in the Cape Ghir region, with horizontal resolution lower than 1 km. A set of process-oriented experiments has been designed in order to determine the mechanisms at the origin of the filament. [less ▲]

High-resolution measurements and modelling of the Cape Ghir upwelling filament during the CAIBEX cruise

Conference (2010, April 26)

Seasonal variability of the oceanic upper layer and its modulation of biological cycles in the Canary Island region

in Journal of Marine Systems (2010), 80(3-4), 172-183

The Canary Island region is rich in mesoscale phenomena that affect cycles of physical and biological processes. A 1D version of the Regional Oceanic Modeling System (ROMS) is used south of the Gran ... [more ▼]

The Canary Island region is rich in mesoscale phenomena that affect cycles of physical and biological processes. A 1D version of the Regional Oceanic Modeling System (ROMS) is used south of the Gran Canaria Island to simulate seasonal climatologies of these cycles. The model is forced with monthly air–sea fluxes averaged from 1993 to 2002 and initialized with mean in situ profiles of temperature, salinity, oxygen and nitrate concentrations. The K-Profile Parameterization (KPP) mixed layer submodel is compared with other submodels using idealized numerical experiments. When forced with realistic air–sea fluxes, the model correctly reproduces the annual cycle of temperature (mixed layer depth), with minimum surface values of 18 °C (maximal depth > 105 m) in February during convective mixing resulting from a negative heat flux. Maximum temperatures above 23 °C (minimal depth < 20 m) are simulated from September to October after strong summer heating and a decrease in Trade Winds intensity. A simple ecosystem model is coupled to the physical model, which provides simulated biological cycles that are in agreement with regional observations. A phytoplankton bloom develops in late winter, driven by the injection of new nutrients into the euphotic layer. Simulated chlorophyll shows a deep maximum fluctuating around 100 m with concentrations around 1 mg Chla /m³, while surface values are low (around 0.1 mg Chla /m³) during most of the year. The physical and biological model results are validated by comparisons with data from regional studies, climatological fields and time-series from the ESTOC station. [less ▲]

Modeling and observation of an upwelling filament off Cape Ghir (NW Africa) during the CAIBEX survey

Poster (2010, February 22)

Modeling and observation of an upwelling filament off Cape Ghir (NW~Africa) during the CAIBEX campaign

Poster (2009, November 27)

A new mechanism of upwelling generated filaments based on potential vorticity balance

Poster (2009, April 19)

Climatology and circulation of the Azores-Canary region by Data-Interpolation Variational Analysis

Poster (2008, June 02)

Simulation of annual cycles of phytoplankton, zooplankton and nutrients to the South of Gran Canaria

Poster (2007, May 11)