The influence of a weakening of the Atlantic meridional overturning circulation on ENSO

The influences of a substantial weakening of the Atlantic Meridional Overturning Circulation (AMOC) on the tropical Pacific climate mean state, the annual cycle and ENSO variability are studied using five different Coupled General Circulation Models. In the CGCMs a substantial weakening of the AMOC is induced by adding freshwater flux forcing in the northern North Atlantic. In response, the well-known surface temperature dipole in the low-latitude Atlantic establishes, which re-organizes the large-scale tropical atmospheric circulation by increasing the northeasterly trade winds. This leads to a southward shift of the Intertropical Convergence Zone in the tropical Atlantic and also the eastern tropical Pacific. Due to evaporative fluxes, mixing and changes in Ekman divergence a meridional temperature anomaly is generated in the northeastern tropical Pacific which leads to the development of a meridionally symmetric thermal background state. This in turn triggers a substantial weakening of the annual cycle in the eastern equatorial Pacific and a subsequent intensification of ENSO variability due to the nonlinear frequency-entrainment mechanism. The analysis suggests that the tropical Atlantic SSTs can easily influence the large-scale atmospheric circulation and hence tropical eastern Pacific climate. Furthermore, we conclude that the existence of the present-day tropical Pacific cold tongue complex and the annual cycle in the eastern equatorial Pacific are partly controlled by the strength of the AMOC. The results may have important consequences for the interpretation of global multidecadal variability and paleo-proxy data.