Etude de la variabilité de la fugacité du CO2 dans l'Atlantique tropical: de l'échelle diurne à saisonnière.

Abstract : This thesis lies within the scope of studies on the carbon cycle, and in particular, on the role of the ocean in the continuous increase of carbon dioxide (CO2) concentration in the atmosphere. The ocean absorbs a total of 25 % of the atmospheric CO2 and englobes both regions of CO2 absorption (or 'sinks') and degassing (or 'sources'). The tropical Atlantic Ocean is a source region of CO2 which is still badly known. In order to better document this region, an observational network has been set up. In particular, a sensor CO2 CARIOCA was installed on a buoy instrumented at 6ﰂS, 10ﰂW since June 2006, in order to measure the fugacity of CO2 (fCO2) at the surface. The objective of this thesis is to understand the physical and biogeochemical mechanisms responsible for the variability of fCO2 observed at the buoy from 2006 to 2009. From June to September, fCO2 is affected by the coastal and equatorial upwellings, which form a cold tongue water able to propagate towards the buoy. The fCO2 time series presents a strong diurnal variability over a wide period of the year, as well as a seasonal variability related to the dynamics of the upwellings. The thermodynamical or biological processes which may be responsible for the diurnal variability in this region have been examined using a one-dimensional model. The time periods which seem dominated by the thermodynamical processes are mainly observed outside of the upwelling season and are characterized by an important warming of the upper mixed layer and weak surface and subsurface currents. The biological periods, on the other hand, are marked by a weaker warming of the mixed layer and a diurnal cycle of the inorganic dissolved carbon (DIC). These 1D simulations have shown that the biological activity may dominate the variability of fCO2 when there is a sufficient input of nitrates concentration at the surface. The analysis of the physical parameters and their time series has also suggested a possible influence from internal waves. Indeed, the mixing induced by these waves could represent a significant source of nutrients. The net community production ( NCP) at the location of the mooring ranges from 0,17 to 0,77 μmol.kgﰁ1.dﰁ1, in agreement with some available measures for this region. At the seasonal time scale, the horizontal advection seems to mostly affect the distribution of fCO2 and salinité de surface de mer (SSS). Using outputs from the DRAKKAR three- dimensional model between January and May, we were able to determine the origin of the water which is advected towards the buoy by following its signature in salinity. The model suggests that low salinity waters observed at the buoy essentially stem from the Northeast part of the Guinea Gulf.
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Gaëlle Parard. Etude de la variabilité de la fugacité du CO2 dans l'Atlantique tropical: de l'échelle diurne à saisonnière.. Océan, Atmosphère. Université Pierre et Marie Curie - Paris VI, 2011. Français. ⟨tel-00673734⟩

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