An exploration into the origin of the Atlantic Multidecadal Variability through the North Atlantic Oscillation and the Atlantic Meridional Mode

Recently there has been intense debate about the relative roles of external forcing, stochastic atmospheric variability, and ocean dynamics (e.g., the AMOC) in the Atlantic Multidecadal Variability (AMV). We consider the portion of the AMV that can be explained by dynamical modes of variability, including the North Atlantic Oscillation (NAO) and the Atlantic Meridional Mode (AMM). The relationship between the tripole SST anomalies forced by the NAO and the AMV has been intensely studied. However, analysis of the AMM's contribution to the AMV has received little attention. It is compelling to consider the AMM and its contribution to the AMV, since it is the leading mode of tropical Atlantic SST variability. We construct a statistical model, which will test the amount of SST variance in the North Atlantic that can be explained by the NAO and the AMM. The variance that the model is unable to explain will indicate the extent to which other processes, including ocean dynamics, forces the AMV. We then separate North Atlantic SST into three regions: the subpolar, subtropical, and tropical regions, and determine the degree to which the NAO and the AMM explain SST variability in each region. We show that that the NAO and the AMM explain most SST variability in the tropical and subtropical branch, but cannot explain most of the variability in the subpolar region. These results suggest that ocean dynamics make a substantial contribution to the AMV in the subpolar North Atlantic.