Intrinsic and Atmospherically Forced Variability of the AMOC: Insights from a Large-Ensemble Ocean Hindcast

Abstract

This study investigates the origin and features of interannual–decadal Atlantic meridional overturning circulation (AMOC) variability from several ocean simulations, including a large (50 member) ensemble of global, eddy-permitting (1/4°) ocean–sea ice hindcasts. After an initial stochastic perturbation, each member is driven by the same realistic atmospheric forcing over 1960–2015. The magnitude, spatiotemporal scales, and patterns of both the atmospherically forced and intrinsic–chaotic interannual AMOC variability are then characterized from the ensemble mean and ensemble spread, respectively. The analysis of the ensemble-mean variability shows that the AMOC fluctuations north of 40°N are largely driven by the atmospheric variability, which forces meridionally coherent fluctuations reaching decadal time scales. The amplitude of the intrinsic interannual AMOC variability never exceeds the atmospherically forced contribution in the Atlantic basin, but it reaches up to 100% of the latter around 35°S and 60% in the Northern Hemisphere midlatitudes. The intrinsic AMOC variability exhibits a large-scale meridional coherence, especially south of 25°N. An EOF analysis over the basin shows two large-scale leading modes that together explain 60% of the interannual intrinsic variability. The first mode is likely excited by intrinsic oceanic processes at the southern end of the basin and affects latitudes up to 40°N; the second mode is mostly restricted to, and excited within, the Northern Hemisphere midlatitudes. These features of the intrinsic, chaotic variability (intensity, patterns, and random phase) are barely sensitive to the atmospheric evolution, and they strongly resemble the “pure intrinsic” interannual AMOC variability that emerges in climatological simulations under repeated seasonal-cycle forcing. These results raise questions about the attribution of observed and simulated AMOC signals and about the possible impact of intrinsic signals on the atmosphere.

Contributions of Atmospheric Forcing and Chaotic Ocean Variability to Regional Sea Level Trends Over 1993–2015

“…In other words, the use of a reanalyzed forcing (driving an ensemble) instead of a climatological forcing (driving one simulation) allows the separation of forced and chaotic signals, and shows that the latter is barely affected by atmospheric fluctuations. This substantial insensitivity was also reported for the Atlantic Meridional Overturning Circulation interannual variability (Leroux et al, ).…”

“…This uncertainty is defined as where N represents the total number of members (50), <T SLA > represents the ensemble mean sea level trend and T SLAi the i th‐member sea level trend. More details are given in Leroux et al ().…”

Spatiotemporal Patterns of Chaos in the Atlantic Overturning Circulation

“…This time series represents the signal that is common to all members and is assumed to originate from the external forcing, either from the surface or through the open boundaries. We interpret the ensemble mean as the forced signal and define its temporal variance 2 F following Leroux et al (2018):…”

“…Since only initial conditions differ between each realization, the residual of each member with respect to the ensemble mean is, by construction, due to ocean dynamics sensitive to the initial conditions. We interpreted this residual signal as the intrinsic variability and define its variance 2 I following Leroux et al (2018):…”

On the Chaotic Variability of Deep Convection in the Mediterranean Sea

“…The turbulent nature of the flow implies that a large fraction of its variability is unpredictable (Lorenz, ). This oceanic “weather” has only recently been characterized in mesoscale‐eddying oceanic general circulation models either by imposing a nominal year atmospheric forcing or by performing ensemble simulations with a full atmospheric forcing, both methods giving very similar results (Leroux et al, ). This chaotic variability, which we will name Intrinsic Ocean Variability (IOV), was found to play a major role in regulating the temporal variability of ocean characteristics such as its large‐scale overturning (Grégorio et al, ; Penduff et al, ) and horizontal (Penduff et al, ; Sérazin et al, , ; Venaille et al, ) circulation, as well as its heat content (Penduff et al, ; Sérazin et al, ).…”