Advective Controls on the North Atlantic Anthropogenic Carbon Sink

Abstract

Though it is clear that the North Atlantic is the site of the highest storage of anthropogenic carbon (Cant) per area, it is uncertain whether the air‐sea Cant fluxes contributing to North Atlantic Cant storage occur in the subpolar gyre or upstream in the subtropical gyre. Using data and models, we show that air‐sea Cant uptake capacity is advected into the subpolar gyre along the same subsurface pathway as nutrients. This pathway is known as the nutrient stream. On the A22 section between Woods Hole and Bermuda, nutrient stream waters are the oldest in the upper 2,000 m and contain low Cant. These northward moving waters are sufficiently depleted in Cant such that they could sustain a subpolar air‐sea Cant flux of −0.19 Pg Cant year−1. The ocean hindcast model used here indicates that despite some subtropical re‐circulation, uptake capacity is transported into the subpolar gyre where it sustains subpolar air‐sea Cant uptake. With this model, we show that high‐ and low‐end estimates of subpolar air‐sea Cant flux are reconciled by accounting for a factor of two difference in their respective study areas. If half of the observed air‐sea Cant uptake capacity transport at A22 is ventilated in the subpolar region, it can fully support high‐end estimates of the subpolar air‐sea Cant sink (−0.09±0.01 Pg Cant year−1).