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

Gisle Nondal, Richard G. J. Bellerby, Are Olsenc, Truls Johannessena, Jón Olafssond

Limnol. Oceanogr. Methods 7:109-118 (2009) | DOI: 10.4319/lom.2009.7.109

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 A_t from SSS and then calculating C_t from measured fCO₂ and estimated A_t; and (II) determination of C_t from SSS, SST, and NO₃^- and then calculating A_t from measured fCO₂ and estimated C_t. The optimal approach was found to be determination of A_t from SSS and then calculating C_t from measured fCO₂ 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.