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

Abstract. The Atlantic Ocean is one of the most important sinks for atmospheric carbon dioxide (CO₂), but this sink has been shown to vary substantially in time. Here we use surface ocean CO₂ 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 CO₂ 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 CO₂ (pCO₂) at a resolution of 1° × 1°. From those, we compute the air–sea CO₂ flux maps using a standard gas exchange parameterization and high-resolution wind speeds. The neural networks fit the observed pCO₂ 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 CO₂ 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 CO₂. Over the 10 yr analysis period (1998 through 2007), sea surface pCO₂ increased faster than that of the atmosphere in large areas poleward of 40° N, while in other regions of the North Atlantic the sea surface pCO₂ increased at a slower rate, resulting in a barely changing Atlantic carbon sink north of the Equator (−0.01 ± 0.02 Pg C yr⁻¹ decade⁻¹). Surface ocean pCO₂ increased at a slower rate relative to atmospheric CO₂ over most of the Atlantic south of the Equator, leading to a substantial trend toward a stronger CO₂ sink for the entire South Atlantic (−0.14 ± 0.02 Pg C yr⁻¹ decade⁻¹). In contrast to the 10 yr trends, the Atlantic Ocean carbon sink varies relatively little on inter-annual timescales (±0.04 Pg C yr⁻¹; 1 σ).