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

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 (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 (NO3-). 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 NO3- 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 mu mol kg(-1) and root mean square (rms) deviation 6.2 mu mol kg(-1) and then C-t to be calculated with a mean bias of -1.0 mu mol kg(-1) and standard error of calculation of 7.4 mu mol kg(-1), as validated using independent data sets.