Temporal Variability of North Atlantic Carbon Fluxes and their Sensitivity to the Meridional Overturning Circulation

The North Atlantic plays a critical role in the global carbon cycle both as a region of substantial air-sea carbon dioxide uptake and as a location for the transfer of CO₂ to depth on climatically-important timescales. While the magnitude of surface fluxes is relatively well constrained, our understanding of the processes that drive variability in ocean-atmosphere exchange and subsequent subsurface carbon accumulation is not as well defined.

Here we present observation-derived high-resolution estimates of short-term 10-day meridional ocean carbon transport variability across the subtropical North Atlantic for 2004-2012. Substantial seasonal, sub-annual and interannual transport variability is observed that is highly sensitive to the strength of the Atlantic Meridional Overturning Circulation. While the recently identified multi-year decrease in AMOC strength similarly impacts carbon transports, its full effect is masked by the northwards transport of increasing surface CO₂ levels.

A 30% slowdown in the meridional circulation in 2009-2010 and the anomalous effects it had on the transport, storage and divergence of heat and freshwater in the subtropical and subpolar gyres and local wind regimes are investigated for their impact on local air-sea CO₂ fluxes. Temperature and salt content anomalies identified in each gyre are found to drive (subtropics) or hinder (subpolar) additional carbon uptake from the atmosphere by affecting the physical solubility pump for CO₂. Additionally their simultaneous effect on mixed layer depth and the vertical supply of nutrients to the surface is shown to magnify the CO₂ flux observed by driving anomalous primary production rates.