The OceanFlux Greenhouse Gases methodology for deriving a sea surface climatology of CO2 fugacity in support of air--sea gas flux studies.

Climatologies, or long-term averages, of essential climate variables are useful for evaluating models and providing a baseline for studying anomalies. The Surface Ocean CO₂ Atlas (SOCAT) has made millions of global underway sea surface measurements of CO₂ publicly available, all in a uniform format and presented as fugacity, fCO₂. As fCO₂ is highly sensitive to temperature, the measurements are only valid for the instantaneous sea surface temperature (SST) that is measured concurrently with the in-water CO₂ measurement. To create a climatology of fCO₂ data suitable for calculating air-sea CO₂ fluxes, it is therefore desirable to calculate fCO₂ valid for a more consistent and averaged SST. This paper presents the OceanFlux Greenhouse Gases methodology for creating such a climatology. We recomputed SOCAT's fCO₂ values for their respective measurement month and year using monthly composite SST data on a 1° x 1° grid from satellite Earth observation and then extrapolated the resulting fCO2 values to reference year 2010. The data were then spatially interpolated onto a 1° x 1° grid of the global oceans to produce 12 monthly fCO₂ distributions for 2010, including the prediction errors of fCO₂ produced by the spatial interpolation technique. The partial pressure of CO2 (pCO2) is also provided for those who prefer to use pCO₂. The CO₂ concentration difference between ocean and atmosphere is the thermodynamic driving force of the air-sea CO₂ flux, and hence the presented fCO₂ distributions can be used in air-sea gas flux calculations together with climatologies of other climate variables. [ABSTRACT FROM AUTHOR]