Optimal evaluation of the surface ocean CO2 system in the northern North Atlantic using data from voluntary observing ships

Abstract

This work evaluates whether an accurate calculation of the entire CO2 system in the northern North Atlantic can be carried out using a combination of in situ fugacity of CO2 (fCO2) and ancillary data often measured on Voluntary Observing Ships (VOS), i.e., sea surface temperature (SST) and sea surface salinity (SSS), as well as nitrate (NO3−). Two approaches are tested: (I) determination of At from SSS and then calculating Ct from measured fCO2 and estimated At; and (II) determination of Ct from SSS, SST, and NO3− and then calculating At from measured fCO2 and estimated Ct. The optimal approach was found to be determination of At from SSS and then calculating Ct from measured fCO2 and estimated At. This allowed At to be determined with a mean bias of −1.8 µmol kg−1 and root mean square (rms) deviation 6.2 µmol kg−1 and then Ct to be calculated with a mean bias of −1.0 µmol kg−1 and standard error of calculation of 7.4 µmol kg−1, as validated using independent data sets.