Evidence of the AMOC Interdecadal Mode Related to Westward Propagation of Temperature Anomalies in Climate Models.

Climate GCMs show a broad variety of Atlantic meridional overturning circulation (AMOC) variability with the leading modes exhibiting different amplitudes, periods and potential driving mechanisms. Theoretical considerations and computations using ocean GCMs suggest that on interdecadal timescales this variability can be controlled by an internal damped oceanic mode associated with westward propagation of depth-integrated density anomalies in the North Atlantic Ocean. These density anomalies are dominated by temperature with some compensation from salinity. The quadrature phases of this mode correspond to the strengthening of the AMOC, followed by the development of a broad warm temperature anomaly in the northern Atlantic extending to 500m or deeper, then by a weakening of the AMOC, and then a broad cooling of the northern Atlantic. Here, we investigate whether this mode operates in the models of Coupled Model Intercomparison Project 5 (CMIP5). Out of the 26 models investigated, we find that more than half of the models exhibit variability consistent with this mode (e.g. GFDL-CM3 and GFDL-ESM2M). Some of the relevant modal features includes statistically significant spectral peaks in the band between 15 and 35 years, the westward propagation of upper-ocean density anomalies in the 40^o-60^oN latitudinal band, which controls the period of the mode, the existence of the well-defined quadrature phases of the AMOC variability with alternating volume transport and ocean heat content anomalies, and the predominant effect of temperature on density anomalies with a small compensation from salinity. Further, we show that typically the models that have this mode exhibit the strongest effect of AMOC variations on SST in the North Atlantic.