Locally and Remotely Forced Subtropical AMOC Variability: A Matter of Time Scales

Abstract

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-scale signals, their southward propagation remains under consideration, complicating the interpretation of the observed time series provided by the Rapid Climate Change–Meridional Overturning Circulation and Heatflux Array–Western Boundary Time Series (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 analyze for this a set of four ensembles of a regional (20°S–55°N), eddy-resolving (1/12°) 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 the predominance of local, atmospherically forced signal at interannual time scales (2–10 years), whereas 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 origins, although the former dominates over the latter, including at the site of the RAPID array (26.5°N).

Imprint of chaotic ocean variability on transports in the southwestern Pacific at interannual timescales

“…Published by Copernicus Publications on behalf of the European Geosciences Union. energy level (Qiu and Chen, 2004;Qiu et al, 2009), strongly sheared mean currents and vigorous western boundary currents; the presence of islands favors the generation of instabilities and mesoscale eddies (Qiu et al, 2009;Keppler et al, 2018;Rieck et al, 2018;Travis and Qiu, 2017).…”

“…Mesoscale eddies, whose evolution is largely chaotic, are ubiquitous in the region south of about 20 • S (Keppler et al, 2018), and the current variability at intraseasonal timescales is much larger than at interannual timescales (Qiu and Chen, 2004;Qiu et al, 2009;Cravatte et al, 2015). Sérazin et al (2015) also show that interannual intrinsic variability is important in this region, suggesting a nonlinear upscaling from mesoscale fluctuations towards low frequencies: nonlinear dynamics, including baroclinic instability, scale interactions and rectification (e.g., Sérazin et al 2018;Zanna et al, 2018), may be strong enough to eventually generate intrinsic interannual variability (Belmadani et al, 2017;Davis et al, 2014).…”

An Ensemble‐Based Eddy and Spectral Analysis, With Application to the Gulf Stream

“…3 of 23 0.25° resolution. More recently, Jamet et al (2019Jamet et al ( , 2020) analyze a regional North Atlantic ensemble consisting of 60 members in various configurations at an eddying 1/12°, and Aoki et al (2020) discuss an 80-member 1/36° eddy-resolving regional Kuroshio model. It is clear that oceanography is still in the early stages of exploiting ensemble methodologies, particularly at resolutions adequate to reliably host mesoscale eddy dynamics.…”

“…More recently, Jamet et al. (2019, 2020) analyze a regional North Atlantic ensemble consisting of 60 members in various configurations at an eddying 1/12°, and Aoki et al. (2020) discuss an 80‐member 1/36° eddy‐resolving regional Kuroshio model.…”

Past to Future South Atlantic Overturning Circulation: upper-ocean pathways and low-frequency variability

“…Nevertheless, besides being sensitive to local-freshwater and global-heat perturbations, overturning strength can also be influenced by variations in farther upstream circulation processes and wind-driven gyre exchanges (Wood et al, 2019;Evans et al, 2017;Jamet et al, 2020). Both changes in the amount of contributions from the Indian 'warm water' or the Pacific 'cold water' routes (Gordon, 1985, 1986and Rintoul, 1991, respectively) -the two contrasting main sources for the near-surface upper limb flow of the Atlantic Meridional Overturning Circulation (AMOC) -, or changes in how these water masses are circulated and transformed within the South Atlantic itself, might influence deepwater convection and therefore 5.1 Introductory remarks affect overturning variability and strength (Garzoli and Matano, 2011;Biastoch et al, 2008;Leroux et al, 2018;Speich et al, 2007).…”