Spatiotemporal Patterns of Chaos in the Atlantic Overturning Circulation

Abstract

Examining an ensemble of high‐resolution ((1/12)°) North Atlantic ocean simulations, we provide new insights into the partitioning of the Atlantic Meridional Overturning Circulation (AMOC) variability between forced and intrinsic at low‐frequency (2–30 years). We highlight the existence of a basin‐scale intrinsic mode that shares similarities with the atmospherically forced signal. The RAPID‐MOCHA‐WBTS array is found to be part of this mode, such that we ascribe about 0.9 Sv (50% in our configuration) of its interannual variability as intrinsic. At decadal time scales, intrinsic variability is rather small (∼0.2 Sv) compared to the recently observed 2‐ to 3‐Sv AMOC downturn. This downturn is thus unlikely to be induced by locally generated intrinsic ocean dynamics. We interpret this intrinsic variability as “chaotic,” that is, somewhat unpredictable, providing an estimation of the quantitative accuracy of AMOC variability within eddy‐resolving numerical models.

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

“…We use the regional North Atlantic configuration of the Massachusetts Institute of Technology general circulation model (MITgcm; Marshall et al 1997) described in Jamet et al (2019b). It extends from 208S to 558N with a horizontal resolution of 1/128 and 46 layers in the vertical, ranging from 6 m at the surface to 250 m at depth.…”

“…The realistic ensemble (referred to as ORAR hereafter, for open boundary conditions real and atmosphere real) uses the full spectrum of open boundary conditions and surface forcing. This ensemble represents the reference test case associated with realistic conditions, which has been used by Jamet et al (2019b) to separate forced and intrinsic AMOC variability. Results from the three other ensembles are compared to this reference experiment.…”

“…The yearly repeating atmospheric forcing follows a ''normal'' year strategy (Large and Yeager 2004). This choice emerged from the recognition that, when using CheapAML, transient atmospheric winds need to be accounted for to simulate a realistic oceanic mean state (Jamet et al 2019a). These are absent from climatological atmospheric conditions.…”

“…As nicely reviewed by Biastoch et al (2008a), mechanisms involved in the southward propagation of signals within the DWBC include a rapid exit of newly generated deep water masses out of the subpolar gyre and a fast equatorward communication through coastal Kelvin waves (Kawase 1987;Johnson and Marshall 2002;Deshayes and Frankignoul 2005;Hodson and Sutton 2012). Those southward traveling coastally trapped density anomalies thus lead to a zonal gradient across the North Atlantic basin, pacing an AMOC variability through geostrophic adjustment (Hirschi and Marotzke 2007;Cabanes et al 2008;Tulloch and Marshall 2012;Buckley et al 2012;Jamet et al 2016).…”

“…To explicitly resolve the oceanic mesoscale dynamics (important for many oceanic processes, and in particular involved in the evolution of water mass properties in the DWBC downstream of Grand Bank; Bower and Hunt 2000;Lozier 2010), we have performed these simulations at eddy-resolving (1/128) horizontal resolution. With such a resolution, a significant fraction of the AMOC variability is expected to be intrinsic, that is, driven by processes other than the forcing and with a random phase (Hirschi et al 2013;Grégorio et al 2015;Leroux et al 2018;Jamet et al 2019b). We have thus carried out these simulations with an ensemble strategy.…”