A seamless ensemble-based reconstruction of surface ocean pCO2 and air–sea CO2 fluxes over the global coastal and open oceans

• Published in: Biogeosciences
• Main Authors: Chau, Thi Tuyet Trang, Gehlen, Marion, Chevallier, Frédéric
• Format: Text
• Language: English
• Published: 2022
• Subjects: Arctic; Indian; Pacific; Southern Ocean
• Online Access: https://doi.org/10.5194/bg-19-1087-2022; https://bg.copernicus.org/articles/19/1087/2022/

We have estimated global air–sea CO 2 fluxes ( fg CO 2 ) from the open ocean to coastal seas. Fluxes and associated uncertainty are computed from an ensemble-based reconstruction of CO 2 sea surface partial pressure ( p CO 2 ) maps trained with gridded data from the Surface Ocean CO 2 Atlas v2020 database. The ensemble mean (which is the best estimate provided by the approach) fits independent data well, and a broad agreement between the spatial distribution of model–data differences and the ensemble standard deviation (which is our model uncertainty estimate) is seen. Ensemble-based uncertainty estimates are denoted by ±1 σ . The space–time-varying uncertainty fields identify oceanic regions where improvements in data reconstruction and extensions of the observational network are needed. Poor reconstructions of p CO 2 are primarily found over the coasts and/or in regions with sparse observations, while fg CO 2 estimates with the largest uncertainty are observed over the open Southern Ocean (44 ∘ S southward), the subpolar regions, the Indian Ocean gyre, and upwelling systems. Our estimate of the global net sink for the period 1985–2019 is 1.643±0.125 PgC yr −1 including 0.150±0.010 PgC yr −1 for the coastal net sink. Among the ocean basins, the Subtropical Pacific (18–49 ∘ N) and the Subpolar Atlantic (49–76 ∘ N) appear to be the strongest CO 2 sinks for the open ocean and the coastal ocean, respectively. Based on mean flux density per unit area, the most intense CO 2 drawdown is, however, observed over the Arctic (76 ∘ N poleward) followed by the Subpolar Atlantic and Subtropical Pacific for both open-ocean and coastal sectors. Reconstruction results also show significant changes in the global annual integral of all open- and coastal-ocean CO 2 fluxes with a growth rate of <math xmlns="http://www.w3.org/1998/Math/MathML" id="M26" display="inline" overflow="scroll" dspmath="mathml"><mrow><mo>+</mo><mn mathvariant="normal">0.062</mn><mo>±</mo><mn mathvariant="normal">0.006</mn></mrow></math> <svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="76pt" height="10pt" class="svg-formula" dspmath="mathimg" md5hash="cc04dd43dbc4d8e7219a1e12606bad01"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="bg-19-1087-2022-ie00001.svg" width="76pt" height="10pt" src="bg-19-1087-2022-ie00001.png"/></svg:svg> PgC yr −2 and a temporal standard deviation of 0.526±0.022 PgC yr −1 over the 35-year period. The link between the large interannual to multi-year variations of the global net sink and the El Niño–Southern Oscillation climate variability is reconfirmed.