Linking the North Atlantic Carbon Sink to Southern Ocean Winds

Under climate change, the increase in Southern Ocean winds is thought to upwell deep ocean carbon and increase atmospheric CO₂, resulting in a positive atmospheric CO₂ feedback due to changes in the local dynamics. Using idealised MITgcm simulations, we demonstrate that the increase in Southern Ocean winds can increase atmospheric CO₂ even further via a non-local and novel mechanism by which the chemistry of the North Atlantic sub-polar gyre is altered. The increase in Southern Ocean winds leads to a local increase in the upwelling of deep ocean nutrients and an intensification of the upper ocean meridional transport of nutrient towards low latitudes, resulting in higher biological productivity in the tropical Atlantic. The increase in tropical biological productivity alters the chemistry of sub-thermocline waters in the oxygen minimum zone. These waters are subsequently advected to the North Atlantic sub-polar gyre through interior ocean pathways, where the gyre’s capacity for absorbing atmospheric carbon is reduced. We show that the atmospheric CO₂ feedback induced by the increase in Southern Ocean winds due to non-local dynamical pathways is of the same order of magnitude as the feedback induced by the local Southern Ocean dynamics. As a result, the effect of Southern Ocean winds on atmospheric CO₂ is larger than previously thought. The results demonstrate the importance of circulation pathways within the ocean and show that an increase in biologic productivity in key regions such as the tropics can cause an increase in atmospheric CO₂ levels. While the results are presented using idealised MITgcm simulations, we show evidence that the same novel mechanism is present in complex CMIP5 models under 4xCO₂ forcing.