On the temporal increase of anthropogenic CO2 in the subpolar North Atlantic

The subpolar North Atlantic (NA) plays a key role in the oceanic uptake of anthropogenic CO2. The availability of a historical high quality data set from the Transient Tracers in the Ocean North Atlantic Study (TTO-NAS) in 1981, together with data from recent studies in 1997 and 1999, makes it possible to assess the temporal increase of anthropogenic CO2 (CTant) in the region. We introduce an extension of a previous published empirical approach for estimating temporal increases of CTant, which is known as multiple linear regression approach (MLR). The method is based on a multiple linear-regression model employing hydrographic and chemical parameters. The accuracy of the extended MLR calculation (eMLR) proposed here is estimated to be ±3 μmol/kg for a parameterization based on potential temperature, total alkalinity, silicate, and phosphate. Calculated increases of CTant (ΔCTant(PO4)) for the time period 1981-1997 are 1-20 μmol/kg at depths greater than 100 m. The distribution corresponds well to silicate and CFC-12 distributions. Open ocean profiles show a relative minimum between 300 and 1000 m, which is not apparent in profiles of the total CTant concentration. The ΔCTant(PO4) inventory calculation for the northern NA region (40-65°N) yields a change in anthropogenic CO2 storage of 4.2 (±1) pg C over the 16-yr time period 1981-1997. This is equivalent to a mean annual CTant increase of 0.27 (±0.06) pg C/yr or more than 10% of the global ocean uptake for this period.