Bates¹, N.R., A.C Pequignet¹, R.J. Johnson¹ and N. Gruber²

¹Bermuda Biological Station For Research, In., Ferry Reach, Bermuda, GE01, Tel: 441-297-1880, E-mail: nick@bbsr.edu, ²University of California at Los Angeles, Los Angeles, California

Changes in the oceanic sink of CO₂ in subtropical mode water of the North Atlantic Ocean

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). 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 at a rate double that of the surface layer. The 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, ~0.6-1.7 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 deep winter mixing entrains STMW CO₂ into surface waters, shifting mode water to a short-term sink for CO₂ (<3 years). Interannual variability in the uptake of CO₂ into STMW thus provides another factor and feedback controlling the global ocean uptake of CO₂.