The Leading, Interdecadal Eigenmode of the Atlantic Meridional Overturning Circulation in a Realistic Ocean Model

Abstract

Variations in the strength of the Atlantic meridional overturning circulation (AMOC) are a major potential source of decadal and longer climate variability in the Atlantic. This study analyzes continuous integrations of tangent linear and adjoint versions of an ocean general circulation model [Océan Parallélisé (OPA)] and rigorously shows the existence of a weakly damped oscillatory eigenmode of the AMOC centered in the North Atlantic Ocean and controlled solely by linearized ocean dynamics. In this particular GCM, the mode period is roughly 24 years, its e-folding decay time scale is 40 years, and it is the least-damped oscillatory mode in the system. Its mechanism is related to the westward propagation of large-scale temperature anomalies in the northern Atlantic in the latitudinal band between 308 and 608N. The westward propagation results from a competition among mean eastward zonal advection, equivalent anomalous westward advection caused by the mean meridional temperature gradient, and westward propagation typical of long baroclinic Rossby waves. The zonal structure of temperature anomalies alternates between a dipole (corresponding to an anomalous AMOC) and anomalies of one sign (yielding no changes in the AMOC). Further, it is shown that the system is nonnormal, which implies that the structure of the least-damped eigenmode of the tangent linear model is different from that of the adjoint model. The ''adjoint'' mode describes the sensitivity of the system (i.e., it gives the most efficient patterns for exciting the leading eigenmode). An idealized model is formulated to highlight the role of the background meridional temperature gradient in the North Atlantic for the mode mechanism and the system nonnormality.

Oceanic control of multidecadal variability in an idealized coupled GCM

“…However, using observational data, Frankcombe et al (2008) observed the signature of large-scale SST and Sea Surface Height anomalies, propagating westward across the North Atlantic ocean. In addition, Sévellec and Fedorov (2013) show that, in a 2° global configuration of the OPA (Océan PArallélisé) model with realistic topography, the least damped mode of variability of the tangent adjoint linear model remains a potential candidate to explain the MOC multidecadal variability (Ortega et al 2015). This mode is characterized by large-scale temperature anomalies that propagate westward across the subpolar gyre, associated with long baroclinic Rossby waves .…”

“…This mechanism has been shown to be robust to the coupling to a variety of idealized atmospheric models, like energy balance models (Fanning and Weaver 1998;Huck et al 2001) or zonally-averaged statistical-dynamical atmosphere (Arzel et al 2007). This variability was also identified in realistic geometry ocean models, forced by fixed surface fluxes (Sévellec and Fedorov 2013), or coupled to an atmospheric energy balance model (Arzel et al 2012), but with a damped character due to a variety of processes. Introducing a 3D dynamical atmosphere, Buckley et al (2012) recently explored the multidecadal variability arising in two coupled model configurations with simplified flat bottom and bowl oceanic geometry.…”

“…In flat bottom configuration, stochastic atmospheric variability was shown to be unnecessary to the existence of the variability. When the flat bottom approximation is relaxed, and idealized (Winton 1997) or realistic (Sévellec and Fedorov 2013) bottom topography added to the ocean model, the intrinsic oceanic variability may require an extra source of energy to be maintained. The atmosphere is a potential candidate to energize the oceanic variability, as shown in many studies (e.g.…”

Spatial Patterns of Sea Level Variability Associated with Natural Internal Climate Modes

“…While most observational and modeling studies suggest that the bidecadal oscillations can be largely explained by internal ocean dynamics related to the AMOC and AMO (Huck and Vallis 2001;Von der Heydt and Dijkstra 2007;Danabasoglu 2008;Frankcombe and Dijkstra 2009;Frankcombe et al 2010), others argue that they involve westward-propagating baroclinic Rossby waves of large-scale temperature and sea level anomalies (e.g., Sevellec and Fedorov 2013;Vianna and Menezes 2013). Sevellec and Fedorov (2013) demonstrated that the zonal structures of temperature anomalies alternate between a dipole (corresponding to AMOC variability) and one sign pattern (no AMOC variability). Consistent with this result, Vianna and Menezes (2013) showed that the leading CEOF mode of the bidecadal sea level signals is associated with the AMOC variability, whereas the second CEOF mode has distinguishable westward-propagating thermal Rossby waves and is not apparently related to AMOC change.…”

Dynamical attribution of oceanic prediction uncertainty in the North Atlantic: application to the design of optimal monitoring systems

“…This re-normalization factor is thus a way to go from a global initial uncertainty to the local one. This last expression allows us to write the predictive power as: where 2 ∞ = 2 (∞) = 2 sto (∞), since lim t→∞ 2 ini (t) = 0 (i.e., the system is asymptotically stable, Sévellec and Fedorov 2013). This leads to the property that This gives a lower and upper theoretical bounds to the predictive power.…”