Evidence of the AMOC interdecadal mode related to westward propagation of temperature anomalies in CMIP5 models.

Climate models show a broad diversity in the simulations of the Atlantic meridional overturning circulation (AMOC) with its leading modes of variability having different amplitudes, periods and driving mechanisms. Theoretical considerations and computations using ocean GCMs suggest that on interdecadal timescales this variability can be controlled by an internal weakly-damped oceanic mode associated with westward propagation of large-scale density anomalies in the North Atlantic Ocean. These anomalies are dominated by temperature with some compensation from salinity. The quadrature phases of this mode include the strengthening of the AMOC, followed a quarter-period later by the development of a broad warm temperature anomaly in the northern Atlantic extending to about 1000 m, then followed by the weakening of the AMOC, and then the upper-ocean cooling. Here, we investigate whether this mode is present in the simulations of Coupled Model Intercomparison Project 5 (CMIP5). Out of the 25 models investigated, we find that more than half of the models exhibit variability consistent with this mode. Some of the relevant modal features includes statistically significant spectral peaks in the band between 15 and 35 years, the westward propagation of density anomalies in the $$40^{\circ }$$ N- $$60^{\circ }$$ N latitudinal band, which sets the period of the mode, the existence of the distinct quadrature phases of the AMOC variability, and the predominant effect of temperature on density anomalies. [ABSTRACT FROM AUTHOR]