Climate response to Atlantic meridional energy transport variations

The climate impacts of fluctuations in the Atlantic meridional overturning circulation (AMOC) are studied using an atmosphere-ocean general circulation model (AOGCM). In two experiments, the baroclinic component of the North Atlantic Ocean currents is modified online to reproduce typical strong and weak AMOC conditions found in a preindustrial control simulation using the same model. These experiments are compared with slab ocean model (SOM) experiments that use heat flux corrections from the coupled model in the Atlantic Ocean. The main impacts of a strong AMOC include widespread warming in the Northern Hemisphere and a northward shift of the intertropical convergence zone (ITCZ). SOM experiments show similar atmospheric responses to AMOC-related heat flux anomalies, but with much larger impacts in the tropics. The atmospheric changes are driven by an anomalous cross-equatorial Hadley circulation transporting energy southward and moisture and heat northward. In the AOGCM, changes in the Indo-Pacific Ocean circulation and heat transport, driven by the wind stress associated with the abnormal Hadley cell, damp the atmospheric response. In the SOM simulations, the lack of Indo-Pacific transport and of ocean heat storage leads to larger atmospheric changes, that are further amplified by a positive tropical low cloud feedback.