A short-term sink for atmospheric CO2 in subtropical mode water of the North Atlantic Ocean

Large-scale features of ocean circulation, such as deep water formation in the northern North Atlantic Ocean1, are known to regulate the long-term physical uptake of CO2 from the atmosphere by moving CO2-laden surface waters into the deep ocean. But the importance of CO2 uptake into water masses that ventilate shallower ocean depths, such as subtropical mode waters2 of the subtropical gyres, are poorly quantified. Here we report that, between 1988 and 2001, dissolved CO2 concentrations in subtropical mode waters of the North Atlantic have increased at a rate twice that expected from these waters keeping in equilibrium with increasing atmospheric CO2. This accounts for an extra ∼0.4–2.8 Pg C (1 Pg = 1015 g) over this period (that is, about 0.03–0.24 Pg C yr-1), equivalent to ∼3–10% of the current net annual ocean uptake of CO2 (ref. 3). We suggest that the lack of strong winter mixing events, to greater than 300 m in depth, in recent decades is responsible for this accumulation, which would otherwise disturb the mode water layer and liberate accumulated CO2 back to the atmosphere. However, future climate variability (which influences subtropical mode water formation1,4,5,6,7,8) and changes in the North Atlantic Oscillation9 (leading to a return of deep winter mixing events) may reduce CO2 accumulation in subtropical mode waters. We therefore conclude that, although CO2 uptake by subtropical mode waters in the North Atlantic—and possibly elsewhere—does not always represent a long-term CO2 sink, the phenomenon is likely to contribute substantially to interannual variability in oceanic CO2 uptake3.