Multi-decadal uptake of carbon dioxide into subtropical mode water of the North Atlantic Ocean

Abstract. Natural climate variability impacts the multi-decadal uptake of anthropogenic carbon dioxide (C_ant) into the North Atlantic Ocean subpolar and subtropical gyres. Previous studies have shown that there is significant uptake of CO₂ into subtropical mode water (STMW) of the North Atlantic. STMW forms south of the Gulf Stream in winter and constitutes the dominant upper-ocean water mass in the subtropical gyre of the North Atlantic Ocean. Observations at the Bermuda Atlantic Time-series Study (BATS) site near Bermuda show an increase in dissolved inorganic carbon (DIC) of +1.51 ± 0.08 μmol kg⁻¹ yr⁻¹ between 1988 and 2011, but also an increase in ocean acidification indicators such as pH at rates (−0.0022 ± 0.0002 yr⁻¹) higher than the surface ocean (Bates et al., 2012). It is estimated that the sink of CO₂ into STMW was 0.985 ± 0.018 Pg C (Pg = 10¹⁵ g C) between 1988 and 2011 (70 ± 1.8% of which is due to uptake of C_ant). The sink of CO₂ into the STMW is 20% of the CO₂ uptake in the North Atlantic Ocean between 14°–50° N (Takahashi et al., 2009). However, the STMW sink of CO₂ was strongly coupled to the North Atlantic Oscillation (NAO), with large uptake of CO₂ into STMW during the 1990s during a predominantly NAO positive phase. In contrast, uptake of CO₂ into STMW was much reduced in the 2000s during the NAO neutral/negative phase. Thus, NAO induced variability of the STMW CO₂ sink is important when evaluating multi-decadal changes in North Atlantic Ocean CO₂ sinks.