Increasing anthropogenic CO2 emissions to the atmosphere are partially sequestered into the global oceans through the air-sea exchange of CO2 and its subsequent movement to depth, and this collective large-scale absorption is commonly referred to as the global ocean carbon sink. Quantifying this ocean carbon sink provides a key component for closing the global carbon budget which is used to inform and guide policy decisions. These estimates are typically accompanied by an uncertainty budget built by selecting what are perceived as critical uncertainty components based on selective experimentation. However, there is a growing realisation that these budgets are incomplete and may be underestimated, which limits their power as a constraint within global budgets. In this study, we present a methodology for quantifying spatially and temporally varying uncertainties in the air-sea CO2 flux calculations and data that allows an exhaustive assessment of all known sources of uncertainties, including decorrelation length scales between gridded measurements, and the approach follows standard uncertainty propagation methodologies. The resulting standard uncertainties are higher than previously suggested budgets, but the components are consistent with previous work, and they identify how the significance and importance of key uncertainty components change in space and time. For an exemplar method (the UEP-FNN-U method) the work identifies that we can currently estimate the annual ocean carbon sink to an accuracy of ±0.72PgCyr-1 (1 standard deviation uncertainty). Due to this method having been built on established uncertainty propagation and approaches, it appears applicable to all data-product assessments of the ocean carbon sink.

Measurements of the surface ocean fugacity of carbon dioxide (fCO2) provide an important constraint on the global ocean carbon sink, yet the gap filling products developed so far to cope with the sparse observations are relatively coarse (1°x1° by 1 month). Here, we overcome this limitation by using the newly developed surface Ocean Carbon dioxide Neural Network (OceanCarbNN) method to estimate surface ocean fCO2 and the associated air sea CO2 fluxes (FCO2) at a resolution of 8-daily by 0.25°x0.25° (8D) over the period 1982 through 2022. The method reconstructs fCO2 with accuracy like that of low-resolution methods (~19 µatm) but improves it in the coastal ocean. Although global ocean CO2 uptake differs little, the 8D product captures 15\% more variance in FCO2. Most of this increase comes from the better-represented subseasonal scale variability, which is largely driven by the better resolved variability of the winds, but also contributed to by the better resolved fCO2. The high-resolution fCO2 is also able to capture the signal of short-lived regional events such as coastal upwelling and hurricanes. For example, the 8D product reveals that fCO2 was at least 25 µatm lower in the wake of Hurricane Maria (2017), the result of a complex interplay between the decrease in temperature, the entrainment of carbon-rich waters, and an increase in primary production. By providing new insights into the role of higher frequency variations of the ocean carbon sink and the underlying processes, the 8D product fills an important gap.

Mesoscale eddies are abundant in the global oceans and known to affect marine biogeochemistry. Understanding their cumulative impact on the air-sea carbon dioxide (CO2) flux is likely important for quantifying the ocean carbon sink. Here, observations and Lagrangian tracking are used to estimate the air-sea CO2 flux of 67 long lived (i.e. > 1 year) mesoscale eddies in the South Atlantic Ocean over a 16 year period. We find that anticyclonic eddies originating from the Agulhas retroflection and cyclonic eddies originating from the Benguela upwelling act as net CO2 sinks over their lifetimes. In combination, the eddies significantly enhanced the CO2 sink into the South Atlantic Ocean by 0.08 ± 0.01%. Although this modification appears small, long lived eddies account for just ~0.4% of global ocean eddies and eddy activity is increasing; therefore, explicitly resolving eddy processes within all models used to assess the ocean carbon sink would appear critical.