Circulation-driven variability of Atlantic anthropogenic carbon transports and uptake

Abstract

The ocean absorbs approximately a quarter of the carbon dioxide currently released to the atmosphere by human activities (Canth). A disproportionately large fraction accumulates in the North Atlantic due to the combined effects of transport by the Atlantic Meridional Overturning Circulation (AMOC) and air–sea exchange. However, discrepancies exist between modelled and observed estimates of the air–sea exchange due to unresolved ocean transport variability. Here we quantify the strength and variability of Canth transports across 26.5° N in the North Atlantic between 2004 and 2012 using circulation measurements from the RAPID mooring array and hydrographic observations. Over this period, decreasing circulation strength tended to decrease northward Canth transport, while increasing Canth concentrations (preferentially in the upper limb of the overturning circulation) tended to increase northward Canth transport. These two processes compensated each other over the 8.5-year period. While ocean transport and air–sea Canth fluxes are approximately equal in magnitude, the increasing accumulation rate of Canth in the North Atlantic combined with a stable ocean transport supply means we infer a growing contribution from air–sea Canth fluxes over the period. North Atlantic Canth accumulation is thus sensitive to AMOC strength, but growing atmospheric Canth uptake continues to significantly impact Canth transports. Slowdown of the Atlantic Meridional Overturning Circulation from 2004 to 2012 led to a decrease in its relative contribution to North Atlantic carbon accumulation, while the supply from air–sea fluxes increased, according to an analysis of ocean mooring circulation observations.