Optimal evaluation of the surface ocean CO 2 system in the northern North Atlantic using data from voluntary observing ships

This work evaluates whether an accurate calculation of the entire CO 2 system in the northern North Atlantic can be carried out using a combination of in situ fugacity of CO 2 (fCO 2 ) 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 (NO 3 − ). Two approaches are tested: (I) determination of A t from SSS and then calculating C t from measured fCO 2 and estimated A t ; and (II) determination of C t from SSS, SST, and NO 3 − and then calculating A t from measured fCO 2 and estimated C t . The optimal approach was found to be determination of A t from SSS and then calculating C t from measured fCO 2 and estimated A t . This allowed A t 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 C t 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.