| Summary: | The fugacity of CO2 (fCO2) has been measured underway during the voyages of a merchant ship sailing from France to Brazil since 2008. High fCO2 values are observed in 2010, between approximately 8 degrees S and 8 degrees N, and are particularly pronounced in boreal spring. These high values are explained by the anomalous situation occurring in the tropical Atlantic in 2010 after the 2009 El Nino in the Pacific. The weakening of the trade winds during the El Nino event contributes to an increase of sea surface temperatures in the tropical Atlantic and a northward shift of the intertropical convergence zone. The anomalous position of the intertropical convergence zone is accompanied by reduced precipitation in boreal spring and hence a higher than usual sea surface salinity. The year 2010 is also characterized by a strong positive Atlantic multidecadal oscillation index and a negative North Atlantic oscillation index that contribute to the tropical Atlantic warming. Positive anomalies of both surface temperature and salinity cause an increase of surface CO2, leading to a stronger outgassing of CO2 in 2010 compared with 2009 and 2011. The main factor responsible for the CO2 anomalies is the Pacific El Nino teleconnection. The Atlantic multidecadal oscillation index also contributes in the northern hemisphere, and the role of the North Atlantic oscillation is negligible.
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