Trends of anthropogenic CO₂ storage in North Atlantic water masses

Abstract. A high-quality inorganic carbon system database, spanning over three decades (1981–2006) and comprising of 13 cruises, has allowed the applying of the φC°_T method and coming up with estimates of the anthropogenic CO₂ (C_ant) stored in the main water masses of the North Atlantic. In the studied region, strong convective processes convey surface properties, like C_ant, into deeper ocean layers and grants this region an added oceanographic interest from the point of view of air-sea CO₂ exchanges. Generally, a tendency for decreasing C_ant storage rates towards the deep layers has been observed. In the Iberian Basin, the North Atlantic Deep Water has low C_ant concentrations and negligible storage rates, while the North Atlantic Central Water in the upper layers shows the largest C_ant values and the largest annual increase of its average concentration (1.13 ± 0.14 μmol kg⁻¹ yr⁻¹). This unmatched rate of change in the C_ant concentration of the warm upper limb of the Meridional Overturning Circulation decreases towards the Irminger basin (0.68 ± 0.06 μmol kg⁻¹ yr⁻¹) due to the lowering of the buffering capacity. The mid and deep waters in the Irminger Sea show rather similar C_ant concentration rates of increase (between 0.33 and 0.45 μmol kg⁻¹ yr⁻¹), whereas in the Iceland basin these layers seem to have been less affected by C_ant. Overall, the C_ant storage rates in the North Atlantic subpolar gyre during the first half of the 1990s, when a high North Atlantic Oscillation (NAO) phase was dominant, are ~48% higher than during the 1997–2006 low NAO phase that followed. This result suggests that a net decrease in the strength of the North Atlantic sink of atmospheric CO₂ has taken place during the present decade. The changes in deep-water ventilation are the main driving processes causing this weakening of the North Atlantic CO₂ sink.