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

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 (pCO(2)) at a resolution of 1 degrees x 1 degrees 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 pCO(2) data with a root mean square error (RMSE) of about 10 mu atm 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 mu atm for the entire Atlantic Ocean, which decreases to 16.3 mu atm for data south of 40 degrees N. We estimate a decadal mean uptake flux of -0.45 +/- 0.15 PgC yr(-1) for the Atlantic between 44 ffi S and 79 ffi N, representing the sum of a strong uptake north of 18 degrees N (-0.39 +/- 0.10 PgC yr(-1)), outgassing in the tropics (18 degrees S-18 degrees N, 0.11 +/- 0.07 PgC yr(-1)), and uptake in the subtropical/temperate South Atlantic south of 18 degrees S (-0.16 +/- 0.06 PgC 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 pCO(2) increased faster than t