Locally and remotely forced subtropical AMOC variability: A matter of time scales

Mechanisms driving the North Atlantic Meridional Overturning Circulation (AMOC) variability at low-frequency are of central interest for accurate climate predictions. Although the subpolar gyre region has been identified as a preferred place for generating climate time scales signals, their southward propagation remains under consideration, complicating the interpretation of the observed time series provided by the RAPID-MOCHA-WBTS program. In this study, we aim at disentangling the respective contribution of the local atmospheric forcing from signals of remote origin for the subtropical low-frequency AMOC variability. We analyse for this a set of four ensembles of a regional (20^oS-55^oN), eddy-resolving (1/12^o) North Atlantic oceanic configuration, where surface forcing and open boundary conditions are alternatively permuted from fully varying (realistic) to yearly repeating signals. Their analysis reveals predominance of local, atmospherically forced signal at interannual time scales (2-10 years), while signals imposed by the boundaries are responsible for the decadal (10-30 years) part of the spectrum. Due to this marked time scale separation, we show that, although the intergyre region exhibits peculiarities, most of the subtropical AMOC variability can be understood as a linear superposition of these two signals. Finally, we find that the decadal scale, boundary forced AMOC variability has both northern and southern origin, although the former dominates over the latter at the site of the RAPID array (26.5^oN).