A hybrid statistical–dynamical framework for a coupled multidecadal NAO–AMV oscillation

The North Atlantic exhibits prominent multidecadal variability affecting climate impacts across Europe and the Arctic. Yet, separating internally generated variability from externally forced components remains challenging, especially when univariate indices are used and when models exhibit a low signal-to-noise ratio and biases in coupled feedbacks. Here we present a hybrid statistical–dynamical framework to identify and interpret a predictable coupled multidecadal mode linking the North Atlantic Oscillation (NAO), the Atlantic Meridional Overturning Circulation (AMOC), and Atlantic Multidecadal Variability (AMV), with implications for Arctic sea-ice variability.

We combine (i) long climate-model experiments (preindustrial control and transient forced integrations, including volcanic-only forcing) with (ii) subsampling/filtering using joint multivariate Singular Spectrum Analysis (MSSA) applied to a physically motivated set of fields spanning the stratosphere–troposphere–ocean system. Treating NAO/AMV as an inherently coupled multivariate process enables a clearer separation of signal from noise and isolates an oscillatory mode with a characteristic multidecadal timescale that emerges in unforced control conditions. External forcing primarily modulates the mode’s amplitude and apparent period rather than generating it: volcanic perturbations project efficiently onto ocean circulation and can intermittently excite the coupled state.

To interpret these results, we use a minimal conceptual model of a damped coupled NAO–AMV oscillator under periodic and stochastic forcing. The model demonstrates that multidecadal oscillations in unforced control simulations can be sustained by white-noise atmospheric variability, and clarifies how periodic forcing shape phase, period, and amplitude modulation.