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, <i>f</i>_CO₂. As <i>f</i>_CO₂ 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 <i>f</i>_CO₂ data suitable for calculating air–sea CO₂ fluxes, it is therefore desirable to calculate <i>f</i>_CO₂ 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 <i>f</i>_CO₂ values for their respective measurement month and year using monthly composite SST data on a 1° × 1° grid from satellite Earth observation and then extrapolated the resulting <i>f</i>_CO₂ values to reference year 2010. The data were then spatially interpolated onto a 1° × 1° grid of the global oceans to produce 12 monthly <i>f</i>_CO₂ distributions for 2010, including the prediction errors of <i>f</i>_CO₂ produced by the spatial interpolation technique. The partial pressure of CO₂ (<i>p</i>_CO₂) is also provided for those who prefer to use <i>p</i>_CO₂. The CO₂ concentration difference between ocean and atmosphere is the thermodynamic driving force of the air–sea CO₂ flux, and hence the presented <i>f</i>_CO₂ distributions can be used in air–sea gas flux calculations together with climatologies of other climate variables