| Summary: | To study the response of the global carbon cycle to a weakening of the Atlantic Meridional Overturning Circulation (AMOC) a series of freshwater perturbation experiments is conducted both, under pre-industrial and glacial conditions using the earth system model of intermediate complexity LOVECLIM. A shutdown of the AMOC leads to substantial cooling of the North Atlantic, a weak warming of the southern hemisphere, intensification of the northeasterly trade winds and a southward shift of the Intertropical Convergence Zone (ITCZ). Trade wind anomalies change upwelling in the tropical oceans and hence marine productivity. Furthermore, hydrological changes associated with a southward displacement of the ITCZ, lead to a reduction of terrestrial carbon stocks mainly in northern Africa and northern South America, in agreement with paleo-proxy data. In the freshwater perturbation experiments, the ocean acts as a sink of CO2, primarily through increased solubility. The net atmospheric CO2 anomaly induced by a shutdown of the AMOC amounts to about +15 ppmv and --10 ppmv for pre-industrial and glacial conditions, respectively. This background state dependence can be explained by the fact that the glacial climate is drier and the terrestrial vegetation therefore releases a smaller amount of carbon to the atmosphere. This study demonstrates that the net CO2 response to large-scale ocean circulation changes has significant contributions both, from the terrestrial and marine carbon cycle.
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