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Bourgeois, Timothee
Universite de Versailles-Saint-Quentin-En-Yvelines, Batiment Buffon, 55 Avenue de Paris, 78000 Versailles (France); Universite de Paris-Saclay, Espace Technologique Bat. Discovery - RD 128, 2e et, 91190 Saint-Aubin (France); CEA, DRF-LSCE (France)2017
Universite de Versailles-Saint-Quentin-En-Yvelines, Batiment Buffon, 55 Avenue de Paris, 78000 Versailles (France); Universite de Paris-Saclay, Espace Technologique Bat. Discovery - RD 128, 2e et, 91190 Saint-Aubin (France); CEA, DRF-LSCE (France)2017
AbstractAbstract
[en] The coastal ocean suffers from the convergence of multiple anthropogenic stressors with climate change at the forefront. Combined stresses from global warming, ocean acidification, eutrophication and deoxygenation threaten coastal ecosystems and thus their services that humans rely on. Unfortunately, the coastal ocean's large spatiotemporal heterogeneity limits our understanding of the biogeochemical processes involved and their responses to anthropogenic perturbations. The current database of coastal observations remains insufficient, and global biogeochemical ocean models have long been inadequate to the study of the global coastal ocean. Indeed, the spatial resolution of these models has been too coarse to resolve key small-scale coastal processes. However, continual improvements in computational resources now allow global simulations to be made with sufficiently high model resolution that begins to be suitable for coastal ocean studies. In this thesis, we propose to study the evolution of the coastal ocean biogeochemistry at the global scale over recent decades using higher resolution versions of the global physical-biogeochemical model NEMO-PISCES. After evaluating of the global representation of the coastal biogeochemistry in this ocean model, we estimate the current role of the coastal ocean in the ocean uptake of anthropogenic carbon and we study the impact of the anthropogenically driven changes in riverine inputs on the coastal biogeochemistry. From simulations made at 3 different spatial resolutions (200 km, 50 km, 25 km), we esteem that the 50-km model grid offers the best compromise between quality of results and computational cost. The upgrade to 25 km does not appear to provide significant improvement in model skill of simulating coastal biogeochemical fields. After evaluating the model, we provide an estimate of the coastal-ocean sink of anthropogenic carbon, the first study to do so with a global 3-D model. In our simulation, the coastal zone absorbs only 4.5% of the anthropogenic carbon taken up by the global ocean during 1993-2012, less than the 7.5% proportion of coastal-to-global-ocean surface areas. Coastal uptake is weakened due to a bottleneck in offshore transport, which is inadequate to reduce the mean anthropogenic carbon concentration of coastal waters to the average level found in the open-ocean mixed layer. Finally, the anthropogenic perturbation in riverine delivery of nutrients to the ocean has limited impact on the coastal carbon cycle when integrated across all coastal regions, but locally it can induce sharp biogeochemical contrasts. For example, the North Sea shows minor biogeochemical changes following the moderate local trend in nutrient riverine inputs, which is in dramatic contrast to the East China Sea where extensive deoxygenation and acidification are driven by sharp increases in riverine nutrient inputs. (author)
[fr]
L'ocean cotier subit la convergence de nombreuses perturbations anthropiques, avec le changement climatique en premiere ligne. Le rechauffement, l'acidification de l'ocean, l'eutrophisation et la desoxygenation se combinent en menacant les ecosystemes cotiers et les activites humaines associees. Malheureusement, la tres forte heterogeneite spatiale et temporelle de l'ocean cotier limite la comprehension des processus biogeochimiques impliques et leurs reponses face aux perturbations anthropiques. Les bases de donnees actuelles d'observations cotieres sont encore insuffisantes et les modeles biogeochimiques oceaniques globaux ont longtemps ete inadaptes a l'etude de l'ocean cotier global. En effet, la resolution spatiale de ces modeles etait trop grossiere pour resoudre de maniere pertinente les processus de petites echelles. L'augmentation de la puissance de calcul des supercalculateurs permet l'utilisation de grilles de modele plus fines adaptees a l'etude de l'ocean cotier. Dans cette these, nous proposons d'etudier l'evolution au cours des dernieres decennies de la biogeochimie de l'ocean cotier a l'echelle globale a l'aide du modele couple physique-biogeochimie NEMO-PISCES. Apres une evaluation de la representation globale de la biogeochimie cotiere et du cycle du carbone cotier dans notre modele oceanique, nous estimons le role actuel de l'ocean cotier dans l'absorption oceanique de carbone anthropique et nous etudions l'impact de la perturbation anthropique des apports fluviaux sur la biogeochimie cotiere. En utilisant 3 grilles de resolutions spatiales differentes (200 km, 50 km et 25 km), il a ete estime que l'utilisation de la grille de 50 km represente le meilleur compromis entre les trois resolutions testees et que le passage a 25 km ne montre pas d'ameliorations significatives des champs biogeochimiques cotiers evalues. Apres cette premiere evaluation, le puits de carbone anthropique de l'ocean cotier a ete estime pour la premiere fois a partir d'un modele 3D global. L'ocean cotier absorberait ainsi seulement 4,5 % du carbone anthropique absorbe par l'ocean global pour la periode 1993-2012 alors qu'il represente 7,5 % de la surface oceanique globale. L'absorption cotiere est reduite par l'export limite du carbone anthropique vers l'ocean ouvert ne permettant pas de reduire la concentration moyenne de carbone anthropique des eaux cotieres au niveau de celle de la couche de melange de l'ocean ouvert. Enfin, les effets de la perturbation anthropique des apports fluviaux sur la biogeochimie cotiere ont ete juges limites quant integres a l'echelle cotiere globale. Cependant, ces perturbations sont tres contrastees regionalement. La mer du Nord presente des variations biogeochimiques mineures du fait de la tendance locale moderee appliquee aux apports fluviaux en nutriments, comparee a la mer de Chine de l'Est ou la forte augmentation des apports fluviaux provoque d'importants phenomenes de desoxygenation et d'acidification. (author)Original Title
Effets des perturbations anthropiques sur la biogeochimie dans l'ocean cotier a l'echelle globale
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5 May 2017; 192 p; 221 refs.; Available from the INIS Liaison Officer for France, see the INIS website for current contact and E-mail addresses; Meteorologie, Oceanographie, Physique de l'Environnement
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