Changes in column inventories of carbon and oxygen in the Atlantic Ocean

Abstract

Increasing concentrations of dissolved inorganic carbon (DIC) in the interior ocean are expected as a direct consequence of increasing concentrations of CO₂ in the atmosphere. This extra DIC is often referred to as anthropogenic carbon (C_ant), and its inventory, or increase rate, in the interior ocean has previously been estimated by a multitude of observational approaches. Each of these methods is associated with hard to test assumptions since C_ant cannot be directly observed. Results from a simpler concept with fewer assumptions applied to the Atlantic Ocean are reported on here using two large data collections of carbon relevant bottle data. The change in column inventory on decadal time scales, i.e. the storage rate, of DIC, respiration compensated DIC and oxygen is calculated for the Atlantic Ocean. We report storage rates and the confidence intervals of the mean trend at the 95% level (CI), reflecting the mean trend but not considering potential biasing effects of the spatial and temporal sampling. For the whole Atlantic Ocean the mean trends for DIC and oxygen are non-zero at the 95% confidence level: DIC: 0.86 (CI: 0.72–1.00) and oxygen: −0.24 (CI: −0.41–(−0.07)) mol m⁻² yr⁻¹. For oxygen, the whole Atlantic trend is dominated by the subpolar North Atlantic, whereas for other regions the O₂ trends are not significant. The storage rates are similar to changes found by other studies, although with large uncertainty. For the subpolar North Atlantic the storage rates show significant temporal and regional variation of all variables. This seems to be due to variations in the prevalence of subsurface water masses with different DIC and oxygen concentrations leading to sometimes different signs of storage rates for DIC compared to published C_ant estimates. This study suggest that accurate assessment of the uptake of CO₂ by the oceans will require accounting not only for processes that influence C_ant but also additional processes that modify CO₂ storage.