Preprints
https://doi.org/10.5194/bg-2021-304
https://doi.org/10.5194/bg-2021-304

  17 Nov 2021

17 Nov 2021

Review status: this preprint is currently under review for the journal BG.

Data-based estimates of interannual sea–air CO2 flux variations 1957–2020 and their relation to environmental drivers

Christian Rödenbeck1, Tim DeVries2, Judith Hauck3, Corinne Le Quéré4, and Ralph Keeling5 Christian Rödenbeck et al.
  • 1Max Planck Institute for Biogeochemistry, Jena, Germany
  • 2Department of Geography, University of California, Santa Barbara, USA
  • 3Alfred Wegener Institute Helmholtz Centre for Polar and Marine Research, Bremerhaven, Germany
  • 4School of Environmental Sciences, University of East Anglia, Norwich, UK
  • 5Scripps Institution of Oceanography, University of California, San Diego, USA

Abstract. This study considers year-to-year and decadal variations as well as secular trends of the sea–air CO2 flux over the 1957–2020 period, as constrained by the pCO2 measurements from the SOCAT data base. In a first step, we relate interannual anomalies in ocean-internal carbon sources and sinks to local interannual anomalies in sea surface temperature (SST), the temporal changes of SST (dSST/dt), and squared wind speed (u2), employing a multi-linear regression. In the tropical Pacific, we find interannual variability to be dominated by dSST/dt, as arising from variations in the upwelling of colder and more carbon-rich waters into the mixed layer. In the eastern upwelling zones as well as in circumpolar bands in the high latitudes of both hemispheres, we find sensitivity to wind speed, compatible with the entrainment of carbon-rich water during wind-driven deepening of the mixed layer and wind-driven upwelling. In the Southern Ocean, the secular increase in wind speed leads to a secular increase in the carbon source into the mixed layer, with an estimated reduction of the sink trend in the range 17 to 42 %. In a second step, we combined the result of the multi-linear regression and an explicitly interannual pCO2-based additive correction into a “hybrid” estimate of the sea–air CO2 flux over the period 1957–2020. As a pCO2 mapping method, it combines (a) the ability of a regression to bridge data gaps and extrapolate into the early decades almost void of pCO2 data based on process-related observables and (b) the ability of an autoregressive interpolation to follow signals even if not represented in the chosen set of explanatory variables. The “hybrid” estimate can be applied as ocean flux prior for atmospheric CO2 inversions covering the whole period of atmospheric CO2 data since 1957.

Christian Rödenbeck et al.

Status: open (until 29 Dec 2021)

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Christian Rödenbeck et al.

Christian Rödenbeck et al.

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Short summary
The ocean is an important part of the global carbon cycle, taking up about a quarter of the anthropgenic CO2 emitted by burning of fossil fuels, and thus slowing down climate change. However, the CO2 uptake by the ocean is, in turn, affected by variability and trends in climate. Here we use carbon measurements in the surface ocean to quantify the response of the oceanic CO2 exchange to environmental conditions, and discuss possible mechanisms underlying this response.
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