New insights into fCO₂ variability in the tropical eastern Pacific Ocean using SMOS SSS

Abstract. Complex oceanic circulation and air–sea interaction make the eastern tropical Pacific Ocean (ETPO) a highly variable source of CO₂ to the atmosphere. Although the scientific community have amassed 70 000 surface fugacities of carbon dioxide (fCO₂) data points within the ETPO region over the past 25 years, the spatial and temporal resolution of this data set is insufficient to fully quantify the seasonal to interannual variability of the region, a region where fCO₂ has been observed to fluctuate by > 300 μatm.

Upwelling and rainfall events dominate the surface physical and chemical characteristics of the ETPO, with both yielding unique signatures in sea surface temperature and salinity. Thus, we explore the potential of using a statistical description of fCO₂ within sea-surface salinity–temperature space. These SSS/SST relationships are based on in situ surface ocean CO₂ atlas (SOCAT) data collected within the ETPO. This statistical description is then applied to high-resolution (0.25°) Soil Moisture and Ocean Salinity (SMOS) sea surface salinity (SSS) and Operational Sea Surface Temperature and Sea Ice Analysis (OSTIA) sea surface temperature (SST) in order to compute regional fCO₂. As a result, we are able to resolve fCO₂ at sufficiently high resolution to elucidate the influence that various physical processes have on the fCO₂ of the surface ETPO.

Normalised (to 2014) oceanic fCO₂ between July 2010 and June 2014 within the entire ETPO was 39 (±10.7) μatm supersaturated with respect to 2014 atmospheric partial pressures, and featured a CO₂ outgassing of 1.51 (±0.41) mmol m⁻² d⁻¹. Values of fCO₂ within the ETPO were found to be broadly split between the Gulf of Panama region and the rest of the tropical eastern Pacific Ocean. The northwest, central and offshore regions were supersaturated, with wintertime wind-jet-driven upwelling found to constitute the first-order control on fCO₂ values. This contrasts with the southeastern/Gulf of Panama region, where heavy rainfall combined with rapid stratification of the upper water column act to dilute dissolved inorganic carbon, and yield fCO₂ values undersaturated with respect to atmospheric fugacities of CO₂.