An Assessment of CO2 Storage and Sea-Air Fluxes for the Atlantic Ocean and Mediterranean Sea Between 1985 and 2018

As part of the second phase of the Regional Carbon Cycle Assessment and Processes project (RECCAP2), we present an assessment of the carbon cycle of the Atlantic Ocean, including the Mediterranean Sea, between 1985 and 2018 using global ocean biogeochemical models (GOBMs) and estimates based on surface ocean carbon dioxide (CO₂) partial pressure (pCO₂ products) and ocean interior dissolved inorganic carbon observations. Estimates of the basin-wide long-term mean net annual CO₂ uptake based on GOBMs and pCO₂ products are in reasonable agreement (−0.47 ± 0.15 PgC yr⁻¹ and −0.36 ± 0.06 PgC yr⁻¹, respectively), with the higher uptake in the GOBM-based estimates likely being a consequence of a deficit in the representation of natural outgassing of land derived carbon. In the GOBMs, the CO₂ uptake increases with time at rates close to what one would expect from the atmospheric CO₂ increase, but pCO₂ products estimate a rate twice as fast. The largest disagreement in the CO₂ flux between GOBMs and pCO₂ products is found north of 50°N, coinciding with the largest disagreement in the seasonal cycle and interannual variability. The mean accumulation rate of anthropogenic CO₂ (C_ant) over 1994–2007 in the Atlantic Ocean is 0.52 ± 0.11 PgC yr⁻¹ according to the GOBMs, 28% ± 20% lower than that derived from observations. Around 70% of this C_ant is taken up from the atmosphere, while the remainder is imported from the Southern Ocean through lateral transport.