The magnitude and interannual variability of uptake of carbon dioxide (CO₂) into subtropical mode waters (STMW's) are poorly quantified. In the North Atlantic Ocean, STMW ventilates the shallow depths (~200-400 m) of the subtropical gyre, and interannual variability of STMW formation is primarily associated with climate variability (i.e. North Atlantic Oscillation, NAO). We have found that the long-term ocean CO₂ records in the western North Atlantic indicate a recent divergence in the rates of CO₂ uptake, with CO₂ increasing in STMW (Figure 1, blue symbol is TCO₂ or dissolved inorganic carbon) at a rate double that of the surface layer (Figure 1; red symbol Bates et al., 2002).

The non-conservative increase of CO₂ does not result from remineralization of organic matter or density variability, but rather, weak wintertime mixing and lack of STMW reventilation, which appear associated with an NAO positive phase. Since 1988, ~0.6-2.8 Pg (1015 g) of CO₂ has accumulated within the gyre STMW, representing a long-term oceanic sink of CO₂ (>10 years). The accumulation of CO₂ in STMW should continue until winters with stronger mixing (associated with a NAO negative phase) entrain STMW CO₂ into surface waters, ultimately releasing CO₂ to the atmosphere. Interannual variability in the uptake of CO₂ into STMW thus provides another factor and feedback controlling the global ocean uptake of CO₂.

In 2003, new CO₂ data has been collected along two repeat sections (~55°W and ~66°W) as part of the WOCE repeat hydrography program by the NOAA lab PMEL. We have also collected new data along a vertical section between Bermuda and Puerto Rico. Synthesis of these datasets in the next couple of years will provide additional understanding about the magnitude and variability of the oceanic CO₂ sink into subtropical mode water of the North Atlantic