Meridional reorganizations of marine and terrestrial productivity during Heinrich events

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 preindustrial 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 paleoproxy 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 preindustrial 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.