Observing and understanding the seasonal, interannual and decadal variations of the oceanic carbon cycle is crucial to better estimate the global carbon budget and understand its evolution, to investigate the global ocean acidification, and to validate diagnostic and prognostic climate models (e.g. Global Carbon Project, Global Ocean Acidification Observation Network, IPCC). To this aim, the OISO program (Océan Indien Service d'Observations), initiated in 1998, collects measurements of pCO2 and associated parameters (DIC, TA, nutrients, Chl-a, ?13CDIC, ...) along the repeated lines of R.V. Marion Dufresne in the South-West Indian and Southern Oceans; this coverage is an important complement to the international CO2 observing system. The OISO program is linked to national and international programs (LEFE/CO2sink, SOERE/GREATGASES, IGBP/SOLAS//IMBER, SOCAT, GO-SHIPS, GOA-ON, ...). Collaborations with French and foreign scientists (CSIRO, MRI, WHOI, LDEO, Scripps, Princeton Univ., IMCS/Rutgers Univ., Univ. East Anglia, SOC, Univ. Bruxelles, Univ Liege...) are engaged since the start of the project in 1998, either to conduct process studies, to deploy autonomous floats (e.g. SOCLIM and SOCCOM projects, coll. R. Cowley CSIRO), or to use the data to evaluate the global carbon budget (Global Carbon Project) and to validate model simulations and data collected by autonomous floats, moorings and sea mammal sensors (e.g. SOCLIM, SOCCOM, MEOP). To this aim, the OISO data are regularly included in international syntheses (LDEO, SOCAT, CARINA/GLODAP).

The objectives of the OISO cruise in 2021 are i) to re-occupy the transects and stations that were frequently sampled over the last 3 decades (during the GEOSECS, INDIGO, MINERVE, OISO, KEOPS, SOCLIM cruises) and ii) to complement the international CO2 observing system in undersampled regions (transect between Durban, Marion Is. and Crozet Is.).

These observations will allow to better understand the evolution of air-sea CO2 fluxes and ocean acidification in response to anthropogenic CO2 emissions and climate change in key regions, notably in the (under-sampled) Antarctic zone, including the phytoplankton bloom associated with the Kerguelen Plateau. These observations will be rapidly available through the SOCAT, GLODAP and GOA-ON data syntheses, and they will contribute to the French ANR program SWINGS (GEOTRACES cruise in January/February 2021 coupled with the OISO cruise).