Circulation-driven variability of Atlantic anthropogenic carbon transports and uptake

摘要

The ocean absorbs approximately a quarter of the carbon dioxide currently released to the atmosphere by human activities (C-anth). 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 C-anth transports across 26.5 degrees 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 C-anth transport, while increasing C-anth concentrations (preferentially in the upper limb of the overturning circulation) tended to increase northward C-anth transport. These two processes compensated each other over the 8.5-year period. While ocean transport and air-sea C-anth fluxes are approximately equal in magnitude, the increasing accumulation rate of C-anth in the North Atlantic combined with a stable ocean transport supply means we infer a growing contribution from air-sea C-anth fluxes over the period. North Atlantic C-anth accumulation is thus sensitive to AMOC strength, but growing atmospheric C-anth uptake continues to significantly impact C-anth transports.