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

N R Bates

doi:10.5194/bg-9-2649-2012

Abstract

Abstract. Natural climate variability impacts the multi-decadal uptake of anthropogenic carbon dioxide (Cant) 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 Cant). 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.

Highlights

The North Atlantic Ocean contributes approximately 25 % to the global ocean sink of anthropogenic carbon dioxide (CO2) from the atmosphere (Takahashi et al, 2002, 2009)
The observations at Bermuda Atlantic Time-series Study (BATS) indicate that the phase of the North Atlantic Oscillation (NAO) is important for determining the long-term and short-term sinks of CO2 into subtropical mode water (STMW)
The reduced potential for uptake and long-term storage of CO2 in STMW during the 2000s likely contributed to the observed decline in the North Atlantic Ocean CO2 sink during this decade (Schuster and Watson, 2007; Watson et al, 2009)

Summary

Introduction

The North Atlantic Ocean contributes approximately 25 % to the global ocean sink of anthropogenic carbon dioxide (CO2) from the atmosphere (Takahashi et al, 2002, 2009). In the subtropical gyre of the North Atlantic Ocean, the formation of subtropical mode water (STMW) or Eighteen Degree Water (EDW; Worthington, 1959, 1976; Hanawa and Talley, 2001) in winter just south of the Gulf Stream (Marshall et al, 2009; Fig. 1a) may contribute substantively to variability in the CO2 sink in the North Atlantic (Gruber et al, 2002; Bates et al, 2002; Gruber, 2009) These studies indicate that the STMW absorbed 0.030 to 0.240 Pg C yr−1 in the 1990s (Gruber et al, 2002; Bates et al, 2002), while model studies suggest that the wintertime replenishment of CO2 to the storage of anthropogenic CO2 in STMW varies by approximately 0.100 Pg C yr−1, depending on the state of the NAO (Levine et al, 2011).

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Conclusion