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Flagship Eur-Oceans

Par Webmaster Legos Dernière modification 08/09/2014 09:52

Objectives:

Assessment of the respective contributions of physical and biogeochemical processes involved in transport and maintaining of the Oxygen Minimum Zone (OMZ) off Peru in the Humboldt Current System (HCS).

Analysis of the sensitivity of the OMZ dynamics to the equatorial remote and local forcings.

Tools:

Development and validation (with AMOP in situ observations) of several configurations at different domain and horizontal resolutions of a regional coupled physical-biogeochemical ocean modelling platform of the HCS.

Perspectives:

Analysis of temporal fluctuations in the oxygen content of the Eastern Tropical Pacific over the last 50 years with models and in situ observations and determination of their impact on marine denitrification and anammox processes and productivity.

Investigation of the role of the HCS in the greenhouse gas emission global budget in the context of regional climatic projections.

 

Scientific and societal issues

 

The evolution of climate and global and environmental quality in the next century will be intimately linked to biogeochemical interactions and to human activities as drivers of biogeochemical fluxes. Our ability to manage and improve the quality of both natural and human systems will depend ultimately on our understanding of these interactions. The scientific basis of forecasts of future climate, climate variability, and quantitative estimates of uncertainty in future projections can only be provided by a tight merging of marine biogeochemistry, ocean physics, and atmospheric physics and chemistry. “Ocean Deoxygenation” is the decrease of the oceans dissolved oxygen content, or trend towards hypoxia, which can reach levels that are detrimental to many marine organisms, and which results both from a decreased O2 solubility at higher water temperature and from an increased stratification or reduced mixing of ocean superficial and deep waters –caused by global warming which reduces the supply of O2 to the ocean interior. This EOC Flagship was granted to better understand and predict the present and future trends of ocean deoxygenation in response to global warming.

 

Ocean deoxygenation could be compensated by a decreased primary production (hence a reduced oxygen consumption) in a more stratified ocean. Currently we do not know if the two processes (reduced supply or reduced consumption) would balance or if one of them would dominate. However current ocean models predict declines of 1-7% in the global ocean inventory in the next century with further declines continuing for 1,000 years or more into the future. An important consequence could be an expansion, in area and volume, of the so called Oxygen Minimum Zones (OMZs) where O2 levels are too low to support most macrofaunal life and where profound changes in biogeochemical cycling occur. If this is verified, there are profound implications for ocean productivity, nutrient cycling, carbon cycling, and marine habitats, in particular the coastal ones.

 

General organization

 

The Flagship took place in France, Germany and Peru over the post-doc period.

Partners

LEGOS (Toulouse), GEOMAR (Kiel, Germany), IGP (Lima, Peru), IMARPE (Callao, Peru)

Financial support

The EOC Flaghship on “Ocean Deoxygenation” was granted to LEGOS and GEOMAR over the period 2011-2013. It was accompanied by a funding complement from GEOMAR and SFB754 to cover a 3 years post-doc fellowship.

Results

The connection between the equatorial mean circulation and the Oxygen Minimum Zone off Peru is investigated through sensitivity experiments with a high resolution coupled physical-biogeochemical model. A validation against in situ observations indicates a realistic simulation of the vertical and horizontal oxygen distribution by the model. Two sets of climatological open-boundary conditions for the physical variables (ROccam and RSoda), which differ slightly with respect to the intensity and vertical structure of the Equatorial Current system, are shown to lead to contrasting characteristics of the simulated OMZ dynamics. Indeed the figure below shows that the OMZ dynamics is very sensitive to the transport of deoxygenated waters by the 2nd Tsuchiya Jet, with a factor of 4.5 in the simulated eastward oxygen flux between both model ROMS-BioEBUS configurations. In addition, at the regional scale, variability in the rate of O2 change due to the ocean dynamics is one order of magnitude larger than the variability associated with the biogeochemical contribution. Close to the coast, where vertical mixing is enhanced and upwelling the most pronounced, the OMZ characteristics depend on the balance between physical and biogeochemical processes (Montes et al., 2014).

flagship_fig1

Vertical sections of annual mean oxygen at 88°w (from 5°N to 20°S) extracted from the ROccam, RSoda configurations and the CARS climatology. Oxygen concentration of 20 mol l-1 is depicted as a white thick contour. Mean eastward zonal velocities are contoured with black lines. Eastward oxygen fluxes (in blue) are indicated for the EUC (Equatorial Undercurrent), pSSCC (primary Southern Subsurface Countercurrent or 1st Tsuchiya jet) and sSSCC (Secondary Southern Subsurface Countercurrent or 2nd Tsuchiya jet) for each configuration.

Publications

Montes et al. (2014) : http://onlinelibrary.wiley.com/doi/10.1002/2014JC009858/abstract

 



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