The Role of Vertical Mixing in Atlantic Multidecadal Variability Amplitude

There is a vigorous debate about whether Atlantic Multidecadal Variability (AMV) is driven by changes in ocean circulation or external forcing. Generally, CMIP5 fully-coupled climate models faithfully reproduce the phasing of the observed AMV over the 20^th century. However, these same models underestimate the amplitude of the AMV index, relative to observations. Using a large ensemble climate model hierarchy with CESM, we show that when the ocean model is replaced by a slab-ocean the AMV index amplitude is too large. We hypothesize that vertical mixing to depths well below the mixed layer is the dominant mechanism responsible for the difference in variance between the two model configurations. We test our hypothesis in both the CMIP5 Archive and Multi-Model Large Ensemble Archive and compare results in the Atlantic to other measures of SST variability.