Reducing Uncertainties in Anthropogenic Carbon Budget Projections for the Subpolar North Atlantic

The Subpolar North Atlantic (SPNA) is one of the most important carbon sinks and therefore a key region when it comes to projecting the anthropogenic carbon uptake. The effectiveness of the SPNA carbon sink is driven by the physical pump, which is associated with deep water formation in the Labrador Sea and coupled to the Atlantic Meridional Overturning Circulation (AMOC). Recent observational studies have shown that the ventilation of Labrador Sea Water and the AMOC are sensitive to climate change, and that climate-induced circulation changes affect the uptake and storage rate of anthropogenic carbon in the SPNA. Future climate change is expected to increasingly alter the ventilation and hence the SPNA's anthropogenic carbon budget. The IPCC-AR5 report relies on CMIP5 Earth System Models as state of the art modeling tools for projections of future climate change. Yet, especially on the sub-regional scale, the performance of these models in representing the observed physical-biogeochemical interactions is not well assessed and the accuracy of the projected climate properties is therefore questionable. In order to assess the ability of CMIP5 models to project the SPNA's future carbon fluxes and to identify possible drivers of a reduced carbon uptake, we compare the present-day anthropogenic carbon uptake, transports, and storage as simulated by CMIP5 models with corresponding observation-based estimates to calculate model uncertainties for each quantity. Based on these uncertainties, we utilize the SPNA's carbon budget as benchmark for future projections and form a new CMIP5 model suite containing only models that are able to accurately reproduce the observation-based anthropogenic carbon budget. Finally, we analyze the future carbon budget and its dominant drivers with our revised CMIP5 model suite and thereby reduce uncertainties in anthropogenic carbon budget projections for the SPNA.