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

[摘要] The Atlantic Ocean is one of the most important sinks for atmospheric carbondioxide (CO₂), but this sink has been shown to vary substantially intime. Here we use surface ocean CO₂ observations to estimate this sinkand the temporal variability from 1998 through 2007 in the Atlantic Ocean. Webenefit from (i) a continuous improvement of the observations, i.e. theSurface Ocean CO₂ Atlas (SOCAT) v1.5 database and (ii) a newly developedtechnique to interpolate the observations in space and time. In particular,we use a two-step neural network approach to reconstruct basin-wide monthlymaps of the sea surface partial pressure of CO₂ (pCO₂) at aresolution of 1° × 1°. From those, we compute theair–sea CO₂ flux maps using a standard gas exchange parameterization andhigh-resolution wind speeds. The neural networks fit the observed pCO₂data with a root mean square error (RMSE) of about 10 μatm and withalmost no bias. A check against independent time-series data and new datafrom SOCAT v2 reveals a larger RMSE of 22.8 μatm for the entireAtlantic Ocean, which decreases to 16.3 μatm for data south of40° N. We estimate a decadal mean uptake flux of−0.45 ± 0.15 Pg C yr⁻¹ for the Atlantic between44° S and 79° N, representing the sum of a strong uptakenorth of 18° N (−0.39 ± 0.10 Pg C yr⁻¹), outgassingin the tropics (18° S–18° N,0.11 ± 0.07 Pg C yr⁻¹), and uptake in the subtropical/temperateSouth Atlantic south of 18° S(−0.16 ± 0.06 Pg C yr⁻¹), consistent with recent studies. Thestrongest seasonal variability of the CO₂ flux occurs in thetemperature-driven subtropical North Atlantic, with uptake in winter andoutgassing in summer. The seasonal cycle is antiphased in the subpolarlatitudes relative to the subtropics largely as a result of the biologicallydriven winter-to-summer drawdown of CO₂. Over the 10 yr analysis period(1998 through 2007), sea surface pCO₂ increased faster than that ofthe atmosphere in large areas poleward of 40° N, while in otherregions of the North Atlantic the sea surface pCO₂ increased at aslower rate, resulting in a barely changing Atlantic carbon sink north of theEquator (−0.01 ± 0.02 Pg C yr⁻¹ decade⁻¹). Surfaceocean pCO₂ increased at a slower rate relative to atmospheric CO₂over most of the Atlantic south of the Equator, leading to a substantialtrend toward a stronger CO₂ sink for the entire South Atlantic(−0.14 ± 0.02 Pg C yr⁻¹ decade⁻¹). In contrast to the10 yr trends, the Atlantic Ocean carbon sink varies relatively little oninter-annual timescales (±0.04 Pg C yr⁻¹; 1 σ).

[学科分类] 地球化学与岩石