Multidecadal to Centennial Variability of the AMOC: HadCM3 and a Perturbed Physics Ensemble

Multidecadal to centennial variability of the Atlantic meridional overturning circulation (AMOC) is investigated in a multi-thousand-year simulation of the third version of the Hadley Centre Coupled Model (HadCM3) and in an ensemble of general circulation models (GCMs) based on HadCM3 with perturbed physics. Large changes in the AMOC in the standard HadCM3 are strongly related to salinity anomalies in the deep-water formation regions, with anomalies arriving via two pathways. The first is from a coupled feedback in the equatorial Atlantic Ocean, described previously by Vellinga and Wu, and the second is from variability in the Arctic Ocean, possibly driven by stochastic sea level pressure. The low-frequency variability of the AMOC in HadCM3 is well predicted from salinity anomalies from these two pathways. The sensitivity of these processes to model physics is investigated using a small ensemble based on HadCM3 where parameters relating to physical processes are varied. The AMOC responds consistently to the salinity anomalies in the ensemble members. However, 1) the timing of the response depends on the background climate state and 2) some ensemble members have significantly larger AMOC and salinity variability than in standard HadCM3 simulations. In this small ensemble, the presence and strength of multidecadal to centennial AMOC variability is associated with the variability of salinity exported from the Arctic, with little multidecadal to centennial variability of either in the coldest members. This demonstrates how the background climate state can alter the frequency and strength of AMOC variability and is a first step toward understanding how AMOC variability differs within a multimodel context.