Forced and Chaotic AMOC Variability at Various Timescales : Insights from OGCM Ensemble Simulations

Recent eddying (1/4° and 1/12°) OGCM simulations have demonstrated that non-linear ocean dynamics spontaneously generate a chaotic variability that reaches up to multi-decadal timescales. This eddy-induced Low-Frequency Intrinsic Variability (LFIV) dominates the atmospherically-forced near-surface variability (SSH, SST) in eddy-active regions, but also has a substantial imprint on the AMOC weekly-to-multidecadal variability up to the scale of the entire Atlantic. We first present the spatio-temporal features and magnitude of this meridionally-coherent AMOC LFIV, and its signature at the main observational sites (RAPID, SAMOC, OVIDE, OSNAP).

This phenomenon is further investigated from regional and global ensembles of fully-forced 1/4° OGCM simulations driven by the same forcing, after a slight perturbation of their initial states. The atmospherically-forced (ensemble mean) and chaotic (ensemble spread) AMOC variabilities at various timescales are compared to each other and to the RAPID observational reference. The time-varying intensity of the chaotic low-frequency AMOC variability is shown to equal its deterministic counterpart in key regions. More generally, these ensembles of eddying ocean simulations provide new insight into the constraint exerted by the atmosphere on the multi-scale oceanic variability.