Derivation of seawater p CO 2 from net community production identifies the South Atlantic Ocean as a CO 2 source

[摘要] A key step in assessing the global carbon budget is the determination of the partial pressure of CO 2 in seawater( p CO 2 (sw) ). Spatially complete observational fields of p CO 2 (sw) are routinely produced for regional andglobal ocean carbon budget assessments by extrapolating sparse in situ measurements of p CO 2 (sw) using satelliteobservations. As part of this process, satellite chlorophyll  a (Chl  a ) is often used as a proxy for the biological drawdown or release of CO 2 . Chl  a does not, however, quantify carbon fixed through photosynthesis and then respired, which is determined by net communityproduction (NCP). In this study, p CO 2 (sw) over the South Atlantic Ocean is estimated using a feed forward neural network (FNN) scheme and eithersatellite-derived NCP, net primary production (NPP) or Chl  a to compare which biological proxy produces the most accurate fields of p CO 2 (sw) . Estimates of p CO 2 (sw) using NCP, NPP or Chl  a were similar, but NCP was more accurate for theAmazon Plume and upwelling regions, which were not fully reproduced when using Chl  a or NPP. A perturbation analysis assessed the potentialmaximum reduction in p CO 2 (sw) uncertainties that could be achieved by reducing the uncertainties in the satellite biologicalparameters. This illustrated further improvement using NCP compared to NPP or Chl  a . Using NCP to estimate p CO 2 (sw) showedthat the South Atlantic Ocean is a CO 2 source, whereas if no biological parameters are used in the FNN (following existing annual carbonassessments), this region appears to be a sink for CO 2 . These results highlight that using NCP improved the accuracy of estimating p CO 2 (sw) and changes the South Atlantic Ocean from a CO 2 sink to a source. Reducing the uncertainties in NCP derivedfrom satellite parameters will ultimately improve our understanding and confidence in quantification of the global ocean as a CO 2 sink.

[学科分类] 大气科学