Interannual to multi-decadal variability of the AMOC and Atlantic SSTs in CMIP5: Is there a connection?

In this study we explore decadal to multi-decadal climate variability in the North Atlantic in the Coupled Model Intercomparison Project (CMIP5) dataset, which provides a unique opportunity to investigate climate variability across multiple models in simulations extending longer than 450 years. In particular, we focus on variations in the Atlantic Meridional Overturning Circulation (AMOC) and how they affect SSTs in the northern Atlantic. Are variations in the AMOC and SST decadal variability linked to each other? This question remains a subject of intensive research, since ocean memory should enhance potential predictability of decadal climate variations. As our results show, a significant number of coupled models exhibit a strong variability of the AMOC volume transport in the decadal to multi-decadal frequency band. However, the crucial characteristics of this variability vary greatly from one model to the next, including the typical oscillation period and amplitude, the spatio-temporal characteristics of the dominant modes, and their effect on temperature anomalies. For instance, the dominant period of AMOC variations ranges from 8 years (ACCESS1-0) to 80 years (CSIO-Mk3-6-0), while their magnitude expressed as standard deviation varies from 0.6 Sv (FIO-ESM) to 2 Sv (GFDL-CM3). In some models the AMOC variability is associated with the westward propagation of depth-integrated temperature anomalies, but in other cases no dominant propagation is observed. We investigate the causes of these differences and how these different modes influence SSTs in the northern Atlantic. One of the central conclusions of our study is that those models that show a strong decadal to multi-decadal internal mode related to ocean dynamics develop strong oscillations in SST (up to 0.8C) with the AMOC leading SST changes by 2 to 9 years. Other models however show little to no connection between the AMOC and SST variability. Finally, we discuss the relation of the Atlantic SST Dipole index to AMOC variations as seen in the CMIP5 dataset.