Derivation of seawater <i>p</i>CO<sub>2</sub> from net community production identifies the South Atlantic Ocean as a CO<sub>2</sub> source
oleh: D. J. Ford, D. J. Ford, G. H. Tilstone, J. D. Shutler, V. Kitidis
| Format: | Article |
|---|---|
| Diterbitkan: | Copernicus Publications 2022-01-01 |
Deskripsi
<p>A key step in assessing the global carbon budget is the determination of the partial pressure of <span class="inline-formula">CO<sub>2</sub></span> in seawater (<span class="inline-formula"><i>p</i>CO<sub>2 (sw)</sub></span>). Spatially complete observational fields of <span class="inline-formula"><i>p</i>CO<sub>2 (sw)</sub></span> are routinely produced for regional and global ocean carbon budget assessments by extrapolating sparse in situ measurements of <span class="inline-formula"><i>p</i>CO<sub>2 (sw)</sub></span> using satellite observations. As part of this process, satellite chlorophyll <span class="inline-formula"><i>a</i></span> (Chl <span class="inline-formula"><i>a</i></span>) is often used as a proxy for the biological drawdown or release of <span class="inline-formula">CO<sub>2</sub></span>. Chl <span class="inline-formula"><i>a</i></span> does not, however, quantify carbon fixed through photosynthesis and then respired, which is determined by net community production (NCP).</p> <p>In this study, <span class="inline-formula"><i>p</i>CO<sub>2 (sw)</sub></span> 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 <span class="inline-formula"><i>a</i></span> to compare which biological proxy produces the most accurate fields of <span class="inline-formula"><i>p</i>CO<sub>2 (sw)</sub></span>. Estimates of <span class="inline-formula"><i>p</i>CO<sub>2 (sw)</sub></span> using NCP, NPP or Chl <span class="inline-formula"><i>a</i></span> were similar, but NCP was more accurate for the Amazon Plume and upwelling regions, which were not fully reproduced when using Chl <span class="inline-formula"><i>a</i></span> or NPP. A perturbation analysis assessed the potential maximum reduction in <span class="inline-formula"><i>p</i>CO<sub>2 (sw)</sub></span> 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 <span class="inline-formula"><i>a</i></span>. Using NCP to estimate <span class="inline-formula"><i>p</i>CO<sub>2 (sw)</sub></span> showed that the South Atlantic Ocean is a <span class="inline-formula">CO<sub>2</sub></span> source, whereas if no biological parameters are used in the FNN (following existing annual carbon assessments), this region appears to be a sink for <span class="inline-formula">CO<sub>2</sub></span>. These results highlight that using NCP improved the accuracy of estimating <span class="inline-formula"><i>p</i>CO<sub>2 (sw)</sub></span> and changes the South Atlantic Ocean from a <span class="inline-formula">CO<sub>2</sub></span> 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 <span class="inline-formula">CO<sub>2</sub></span> sink.</p>