Metzl¹, Nicolas, Christian Brunet¹, Aurelie Jabaud-Jan¹, Catherine Pierre², Alain Poisson¹ and Bernard Schauer¹

¹LBCM/IPSL, Université Pierre et Marie Curie, Case 134 and ² LODYC/IPSL, Université Pierre et Marie Curie, Case 100, 4, Place Jussieu, 75252 PARIS Cedex 05, France, E-mail: metzl@ccr.jussieu.fr

Natural variabilities and anthropogenic CO₂ in the South Indian Ocean (20°S-60°S)

The air-sea CO₂ fluxes and anthropogenic CO₂ inventories are very uncertain in the Southern Ocean because observations are sparse and concequently large-scale biogeochemical models which present very different results, are poorly constrained and validated. In this study, we offer new analyses of the spatio-temporal variabilities of the carbon dioxide system and air-sea CO₂ fluxes in the South-Western Indian Ocean (20°S-60°S) based on data collected in 1998-2002 during seven cruises conducted onboard the R.V. Marion-Dufresne (IPEV/TAAF) in the frame of the French observational program OISO (IPSL/INSU). One important results concerns the strong seasonality of oceanic fCO₂ observed south of the Polar Front: during winter this region is a strong CO₂ source. Compared to the seasonal signals, significant interannual variations have been also observed. During austral summer the interannual variations of ocean CO₂ sources and sinks are related to large-scale climatic anomalies, but not always directly controled by warming and cooling. For example, in the open ocean zone of the Southern Ocean, the oceanic CO₂ sink is stronger during warm period because the higher primary production dominates the temperature effect on ocean fCO₂. During cold situations the oceanic CO₂ sink is reduced and could even be a CO₂ source as large as the source we observed during austral winter. These strong interannual variabilities, revealed for the first time in the Southern Ocean, create difficulties for capturing the anthropogenic CO₂ in the ocean, as it was well observed in other sectors monitored during the JGOFS era.