Abstract
Abstract. A key step in assessing the global carbon budget is the determination of the partial pressure of CO2 in seawater (pCO2 (sw)). Spatially complete observational fields of pCO2 (sw) are routinely produced for regional and global ocean carbon budget assessments by extrapolating sparse in situ measurements of pCO2 (sw) using satellite observations. As part of this process, satellite chlorophyll a (Chl a) is often used as a proxy for the biological drawdown or release of CO2. Chl a does not, however, quantify carbon fixed through photosynthesis and then respired, which is determined by net community production (NCP). In this study, pCO2 (sw) over the South Atlantic Ocean is estimated using a feed forward neural network (FNN) scheme and either satellite-derived NCP, net primary production (NPP) or Chl a to compare which biological proxy produces the most accurate fields of pCO2 (sw). Estimates of pCO2 (sw) using NCP, NPP or Chl a were similar, but NCP was more accurate for the Amazon Plume and upwelling regions, which were not fully reproduced when using Chl a or NPP. A perturbation analysis assessed the potential maximum reduction in pCO2 (sw) uncertainties that could be achieved by reducing the uncertainties in the satellite biological parameters. This illustrated further improvement using NCP compared to NPP or Chl a. Using NCP to estimate pCO2 (sw) showed that the South Atlantic Ocean is a CO2 source, whereas if no biological parameters are used in the FNN (following existing annual carbon assessments), this region appears to be a sink for CO2. These results highlight that using NCP improved the accuracy of estimating pCO2 (sw) and changes the South Atlantic Ocean from a CO2 sink to a source. Reducing the uncertainties in NCP derived from satellite parameters will ultimately improve our understanding and confidence in quantification of the global ocean as a CO2 sink.
Highlights
Since the industrial revolution, anthropogenic CO2 emissions have resulted in an increase in atmospheric CO2 concentrations (Friedlingstein et al, 2020; IPCC, 2013)
This showed that a reduction in pCO2 root mean square difference (RMSD) of 36 % was achieved by eliminating satellite net community production (NCP) uncertainties, 34 % was achieved by eliminating satellite net primary production (NPP) uncertainties and 19 % was achieved by eliminating satellite chlorophyll a (Chl a) uncertainties
These results were verified using in situ observations from the Atlantic Meridional Transect, which resulted in a 25 % improvement in pCO2 RMSD when the in situ NCP uncertainties were reduced to ∼ 0, compared to 7 % for Chl a and 13 % for NPP
Summary
Anthropogenic CO2 emissions have resulted in an increase in atmospheric CO2 concentrations (Friedlingstein et al, 2020; IPCC, 2013). Observational fields of the partial pressure of CO2 in seawater (pCO2 (sw)) are one of the key datasets needed to routinely assess the strength of the oceanic CO2 sink (Friedlingstein et al, 2020; Landschützer et al, 2014, 2020; Rödenbeck et al, 2015; Watson et al, 2020b). These methods are reliant on the extrapolation of sparse in situ observations of pCO2 (sw) using satellite observations of parameters which account for the variability of, and the controls on, pCO2 (sw) (Shutler et al, 2020). SST and salinity control pCO2 (sw) by changing the solubility of CO2 in seawater (Weiss, 1974), whilst biological processes such as photosynthesis and respiration contribute by modulating its concentration
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