Study of the Cape Ghir upwelling filament using variational data analysis and regional numerical model

• Main Author: Troupin, Charles
• Other Authors: Donnay, Jean-Paul, Beckers, Jean-Marie, Barth, Alexander, Delhez, Eric, Brasseur, P., Pelegri Llopart, J.L., Sagen, H., Sangra Inciarte, P.
• Format: Text
• Language: unknown
• Published: Universite de Liege 2011
• Subjects: physical oceanography/oceanographie physique; upwelling; numerical modeling/modelisation numerique; Cape Ghir; North Atlantic; North Atlantic oscillation
• Online Access: http://bictel.ulg.ac.be/ETD-db/collection/available/ULgetd-10032011-172627/

Aim: upwelling filaments are shallow, elongated structures of cool waters often visible in the regions of coastal upwelling. The present thesis aims to improve our knowledge on the mechanisms responsible for the generation of filaments. Location: the study is centered on the Canary Current Upwelling system, located off northwest Africa, in particular around Cape Ghir (30°38'N, 9°53'W), where these filaments are frequently observed. Methods: - We compiled historical in situ data to build a high-resolution climatology of temperature and salinity in the region of interest using the Data Interpolating Variational Analysis (DIVA) method. - We ran the experiments with the numerical model ROMS to investigate the processes responsible for the filament generation. - We collected in situ measurements during a cruise in August-September 2009, in the frame of the CAIBEX project, in the filament area and analyzed them to get a detailed description of the Cape Ghir filament, its variability and its biogeochemical characteristics. - We computed anomalies of several variables with respect to a long-term mean to examine the time-evolution of the Canary Current Upwelling system. Results: The climatology showed improvements with respect to existing products, especially in the coastal area, and was a valuable asset to provide initial and boundary conditions for the numerical model. The results of the simulations underlined the role of the wind in the filament generation and confirmed our assumption of a mechanism based on the potential vorticity balance. Preliminary analysis of the cruise data allowed us to characterize the filament physical and biological properties, in particular we observed the signal of a subsurface (at a depth of 300~m) anticyclonic eddy on the northern flank of the filament, also present in the numerical model outputs. Anomalies for the year 2010 revealed a widespread warming over the tropical and subtropical Atlantic Ocean and a diminution of the upwelling activity. We attributed these anomalies to a weakening of the winds, itself related to a strongly negative North Atlantic Oscillation (NAO) index.