Abstract
New estimates of pCO2 from profiling floats deployed by the Southern Ocean Carbon and Climate Observations and Modeling (SOCCOM) project have demonstrated the importance of wintertime outgassing south of the Polar Front, challenging the accepted magnitude of Southern Ocean carbon uptake (Gray et al., 2018, https://doi:10.1029/2018GL078013). Here, we put 3.5 years of SOCCOM observations into broader context with the global surface carbon dioxide database (Surface Ocean CO2 Atlas, SOCAT) by using the two interpolation methods currently used to assess the ocean models in the Global Carbon Budget (Le Quéré et al., 2018, https://doi:10.5194/essd-10-2141-2018) to create a ship‐only, a float‐weighted, and a combined estimate of Southern Ocean carbon fluxes (<35°S). In our ship‐only estimate, we calculate a mean uptake of −1.14 ± 0.19 Pg C/yr for 2015–2017, consistent with prior studies. The float‐weighted estimate yields a significantly lower Southern Ocean uptake of −0.35 ± 0.19 Pg C/yr. Subsampling of high‐resolution ocean biogeochemical process models indicates that some of the differences between float and ship‐only estimates of the Southern Ocean carbon flux can be explained by spatial and temporal sampling differences. The combined ship and float estimate minimizes the root‐mean‐square pCO2 difference between the mapped product and both data sets, giving a new Southern Ocean uptake of −0.75 ± 0.22 Pg C/yr, though with uncertainties that overlap the ship‐only estimate. An atmospheric inversion reveals that a shift of this magnitude in the contemporary Southern Ocean carbon flux must be compensated for by ocean or land sinks within the Southern Hemisphere.
Highlights
The ocean has absorbed approximately 25% of anthropogenic carbon emissions released during the industrial era (Le Quéré et al, 2018)
Present spatial assessments estimate that of the 2.6 ± 0.5 Pg C/yr of anthropogenic carbon dioxide dissolved in the ocean each year, over 40% enters in the Southern Ocean
While the biggest total regional differences between the Surface Ocean CO2 Atlas v6 (SOCAT)‐only and SOCCOM‐weighted products occurs in the Antarctic‐Southern Zone (ASZ), in broad agreement with the analysis of Gray et al (2018), we find significant changes in all regions other than the Subtropical Zone (STZ), and all toward a reduction in the Southern Ocean carbon sink
Summary
The ocean has absorbed approximately 25% of anthropogenic carbon emissions released during the industrial era (Le Quéré et al, 2018). While global climate models can be used to determine ocean and land carbon uptake (Le Quéré et al, 2018), atmospheric inversion estimates provide an independent, observation‐based method of partitioning contemporary terrestrial and ocean carbon fluxes (Peylin et al, 2013; Resplandy et al, 2018). Observation‐based global air‐sea carbon flux estimates are traditionally determined from measurements of the partial pressure of carbon dioxide (pCO2) from ships or moorings. These measurements are interpolated in time and space to produce global estimates of the pCO2 difference from the atmosphere and calculations of air‐sea flux (e.g., Landschützer et al, 2014; Rödenbeck et al, 2015; Takahashi et al, 2009). Integrated air‐sea flux estimates can be determined from changes in oceanic carbon content, though infrequent repeat hydrographic sampling leaves these estimates unable to resolve short‐term variations in the air‐sea flux (Gruber et al, 2009; Gruber et al, 2019)
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