A neural network-based estimate of the seasonal to inter-annual variability of the Atlantic Ocean carbon sink

• Published in: Biogeosciences
• Main Authors: P. Landschützer, N. Gruber, D. C. E. Bakker, U. Schuster, S. Nakaoka, M. R. Payne, T. P. Sasse, J. Zeng
• Format: Article in Journal/Newspaper
• Language: English
• Published: Copernicus Publications 2013
• Subjects: Ecology; QH540-549.5; Life; QH501-531; Geology; QE1-996.5; North Atlantic
• Online Access: https://doi.org/10.5194/bg-10-7793-2013; https://doaj.org/article/38ad8a7a9465440796fa04007476c9b3

The Atlantic Ocean is one of the most important sinks for atmospheric carbon dioxide (CO 2 ), but this sink has been shown to vary substantially in time. Here we use surface ocean CO 2 observations to estimate this sink and the temporal variability from 1998 through 2007 in the Atlantic Ocean. We benefit from (i) a continuous improvement of the observations, i.e. the Surface Ocean CO 2 Atlas (SOCAT) v1.5 database and (ii) a newly developed technique to interpolate the observations in space and time. In particular, we use a two-step neural network approach to reconstruct basin-wide monthly maps of the sea surface partial pressure of CO 2 ( p CO 2 ) at a resolution of 1° × 1°. From those, we compute the air–sea CO 2 flux maps using a standard gas exchange parameterization and high-resolution wind speeds. The neural networks fit the observed p CO 2 data with a root mean square error (RMSE) of about 10 μatm and with almost no bias. A check against independent time-series data and new data from SOCAT v2 reveals a larger RMSE of 22.8 μatm for the entire Atlantic Ocean, which decreases to 16.3 μatm for data south of 40° N. We estimate a decadal mean uptake flux of −0.45 ± 0.15 Pg C yr −1 for the Atlantic between 44° S and 79° N, representing the sum of a strong uptake north of 18° N (−0.39 ± 0.10 Pg C yr −1 ), outgassing in the tropics (18° S–18° N, 0.11 ± 0.07 Pg C yr −1 ), and uptake in the subtropical/temperate South Atlantic south of 18° S (−0.16 ± 0.06 Pg C yr −1 ), consistent with recent studies. The strongest seasonal variability of the CO 2 flux occurs in the temperature-driven subtropical North Atlantic, with uptake in winter and outgassing in summer. The seasonal cycle is antiphased in the subpolar latitudes relative to the subtropics largely as a result of the biologically driven winter-to-summer drawdown of CO 2 . Over the 10 yr analysis period (1998 through 2007), sea surface p CO 2 increased faster than that of the atmosphere in large areas poleward of 40° N, while in other regions of the North Atlantic ...