Florian Sévellec

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

AbstractVariations 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 [Ocean Parallelise (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 30° and 60°N. The westward propagation results from a competition among mean eastward zonal advection, equivalent anomalous westward advection caused by the mean meridional temperature gradient, and westw...

Bifurcation structure of thermohaline millennial oscillations

Abstract The question of the generation of millennial oscillations by internal ocean dynamics is studied through deliberate use of the simplest geometry and surface forcing, namely a hemispheric ocean with time-independent mixed boundary conditions (autonomous system). The lowest-order model that supports free oscillations has three horizontal and two vertical boxes. The essential ingredients permitting the existence of the oscillations are turbulent mixing and freshwater forcing. The finite amplitude oscillations share the advective–convective–diffusive characteristics of neighboring stable thermal and haline steady states. There are limits to the quantity of precipitation in polar regions for the existence of oscillatory states. When the freshwater forcing amplitude is increased, the system evolves from a stable thermal state through a global bifurcation to a finite amplitude limit cycle. The period of the limit cycle remains constant when freshwater is increased until at a second global bifurcation it ...

Optimal Surface Salinity Perturbations of the Meridional Overturning and Heat Transport in a Global Ocean General Circulation Model

Abstract Recent observations and modeling studies have stressed the influence of surface salinity perturbations on the North Atlantic circulation over the past few decades. As a step toward the estimation of the sensitivity of the thermohaline circulation to salinity anomalies, optimal initial surface salinity perturbations are computed and described for a realistic mean state of a global ocean general circulation model [Ocean Parallelise (OPA)]; optimality is defined successively with respect to the meridional overturning circulation intensity and the meridional heat transport maximum. Although the system is asymptotically stable, the nonnormality of the dynamics is able to produce a transient growth through an initial stimulation. Optimal perturbations are calculated subject to three constraints: the perturbation applies to surface salinity; the perturbation conserves the global salt content; and the perturbation is normalized, to remove the degeneracy in the linear maximization problem. Maximization us...

Optimal Surface Salinity Perturbations Influencing the Thermohaline Circulation

Optimal surface salinity perturbations influencing the meridional overturning circulation maximum are exhibited and interpreted on a stable steady state of a 2D latitude–depth ocean thermohaline circulation model. Despite the stability of the steady state, the nonnormality of the dynamics is able to create some transient growth and variability through stimulation by optimal perturbations. Two different measures are compared to obtain the optimum—one associated with the departure from steady state in terms of density, and the other with the overturning circulation intensity. It is found that such optimal analysis is measure dependent; hence, the latter measure is chosen for studying the following physical mechanisms. The response to the optimal initial sea surface salinity perturbation involves a transient growth mechanism leading to a maximum modification of the circulation intensity after 67 yr; the amplification is linked to the most weakly damped linear eigenmode, oscillating on a 150-yr period. Optimal constant surface salinity flux perturbations are also obtained, and confirm that a decrease in the freshwater flux amplitude enhances the circulation intensity. At last, looking for the optimal stochastic surface salinity flux perturbation, it is established that the variance of the circulation intensity is controlled by the weakly damped 150-yr oscillation. Two approaches are tested to consider extending such studies in more realistic 3D models. Explicit solutions (versus eigenvalue problems) are found for the overturning circulation measure (except for the stochastic optimal); a truncation method on a few leading eigenmodes usually provides the optimal perturbations for analyses on long time scales.

Nonnormal multidecadal response of the thermohaline circulation induced by optimal surface salinity perturbations

Optimal perturbations of sea surface salinity are obtained for an idealized North Atlantic basin using a 3D planetary geostrophic model—optimality is defined with respect to the intensity of the meridional overturning circulation. Both optimal initial and stochastic perturbations are computed in two experiments corresponding to two different formulations of the surface boundary conditions: the first experiment uses mixed boundary conditions (i.e., restoring surface temperature and prescribed freshwater flux), whereas the second experiment uses flux boundary conditions for both temperature and salinity. The latter reveals greater responses to both initial and stochastic perturbations that are related to the existence of a weakly damped oscillatory eigenmode of the Jacobian matrix, the optimal perturbations being closely related to its biorthogonal. The optimal initial perturbation induces a transient modification of the circulation after 24 yr. The spectral response to the optimal stochastic perturbation reveals a strong peak at 35 yr, corresponding to the period of this oscillatory eigenmode. This study provides an upper bound of the meridional overturning response at multidecadal time scales to freshwater flux perturbation: for typical amplitudes of Great Salinity Anomalies, initial perturbations can alter the circulation by 12.25 Sv (1 Sv [ 10 6 m 3 s 21 ; i.e., 12.5% of the mean circulation) at most; stochastic perturbations with amplitudes typical of the interannual variability of the freshwater flux in midlatitudes induce a circulation variability with a standard deviation of 1 Sv (i.e., 5.5% of the mean circulation) at most.

On the mechanism of centennial thermohaline oscillations

Centennial oscillations of the ocean thermohaline circulation are studied in a 2-D latitude-depth model under mixed boundary conditions (i.e. restoring surface temperature and prescribed freshwater flux). The oscillations are revealed through linear stability analysis of a steady state obtained in a single hemisphere configuration. A density variance budget is performed and helps determine the physical processes sustaining these oscillations: the restoring surface temperature appears as a source of density variance – this is a consequence of positively-correlated temperature and salinity anomalies. A minimal model, the Howard-Malkus loop oscillator, enables us to understand physically the oscillatory and growth mechanisms. The centennial oscillation is connected to the advection of salinity anomaly around the loop; it is also related to the salinity feedback on the overturning which reinforces anomalies through a change of residence time in the freshwater flux regions. Analytical solutions of this loop model show that these centennial oscillations exist in a specific parameter regime in terms of the freshwater flux amplitude F0: oscillations are damped if F0 is too weak, but if F0 is too large, the instability grows exponentially without oscillating – the latter regime is known as the positive salinity feedback. The robustness of these oscillations is then analyzed in more realistic bihemispheric configurations, some including a highly idealized Antarctic Circumpolar Current: oscillations are then always damped. These results are rationalized with the loop model, and compared to the oscillations found in general circulation models.

Model Bias Reduction and the Limits of Oceanic Decadal Predictability: Importance of the Deep Ocean

AbstractOcean general circulation models (GCMs), as part of comprehensive climate models, are extensively used for experimental decadal climate prediction. Understanding the limits of decadal ocean predictability is critical for making progress in these efforts. However, when forced with observed fields at the surface, ocean models develop biases in temperature and salinity. Here, the authors ask two complementary questions related to both decadal prediction and model bias: 1) Can the bias be temporarily reduced and the prediction improved by perturbing the initial conditions? 2) How fast will such initial perturbations grow? To answer these questions, the authors use a realistic ocean GCM and compute temperature and salinity perturbations that reduce the model bias most efficiently during a given time interval. The authors find that to reduce this bias, especially pronounced in the upper ocean above 1000 m, initial perturbations should be imposed in the deep ocean (specifically, in the Southern Ocean). O...

Excitation of SST anomalies in the eastern equatorial Pacific by oceanic optimal perturbations

A generalized stability analysis is used to explore the excitation of sea-surface temperature anoma- lies in the eastern equatorial Pacific by optimal initial perturbations in temperature and salinity within an ocean general circulation model. We find perturbations that can efficiently modify the SST of the Nino3 region with an approximately 9-month delay. The time interval between the end of March and mid-April is particularly favorable for these perturbations to cause subsequent changes in the eastern equatorial Pacific. This sensitivity is related to two critical factors: during the boreal spring Equinox the heat content of the western equatorial Pacific reaches its seasonal maximum, whereas the zonal winds along the equator relax. The optimal initial anomalies in temperature and salinity have a complex spatial structure extending between 20?S and 20?N. This large meridional extent of the anomalies allows for a strong focus- ing effect - signals from different locations reach the eastern equatorial Pacific simultaneously thus generating a strong transient warming in a relatively small region. Ocean adjustment to the optimal perturbations involves a basin-size Rossby wave that propagates westward and induces a large-scale anomalous eastward advection along the equator, which acts on the mean east-west temperature gra- dient and gradually warms SST in the east. At the same time, a continuum of Kelvin waves are being excited, which reduce the thermocline slope along the equator and deepen the thermocline in the east- ern equatorial Pacific. Thus, the Rossby wave and Kelvin waves work constructively to generate the transient warming of the Nino3 SST. An idealized model with two variables is formulated to illustrate these ideas and, in particular, show the importance of zonal advection feedback for the amplitude and duration of the transient warming. Ultimately, this study highlights the importance of non-normal dynamics for generating an efficient transient growth of SST anomalies in the tropical Pacific even in the absence of coupled ocean-atmosphere interactions.

Theoretical Investigation of the Atlantic Multidecadal Oscillation

A weakly damped mode of variability, corresponding to the oceanic signature of the Atlantic multidecadal oscillation (AMO) was found through the linear stability analysis of a realistic ocean general circulation model. A simpletwo-level model wasproposed torationalize bothits period and damping rate.This model isextended heretothree levelstoinvestigate how the modecan draw energy fromthe mean flow, as found in various ocean and coupled models. A linear stability analysis in this three-level model shows that the positive growth rate of the oscillatory mode depends on the zonally averaged isopycnal slope. This mode corresponds to a westward propagation of density anomalies in the pycnocline, typical of large-scale baroclinic Rossby waves. The most unstablemodecorrespondstothelargestscaleone(atleastforlowisopycnalslope).Themodecanbedescribed in four phases composing a full oscillation cycle: 1) basin-scale warming of the North Atlantic (AMO positive phase), 2) decrease in upper-ocean poleward transport [hence a reduction of the Atlantic meridional overturning circulation (AMOC)], 3) basin-scale cooling (negative AMO), and 4) AMOC intensification. A criterion is developed to test, in oceanic datasets or numerical models, whether this multidecadal oscillation is an unstable oceanic internal mode of variability or if it is stable and externally forced. Consistent with the classical theory of baroclinic instability, this criterion depends on the vertical structure of the mode. If the upper pycnoclinesignatureisinadvanceofthedeeper pycnoclinesignaturewith respecttothewestwardpropagation,the mode is unstable and could be described as an oceanic internal mode of variability.

Multidecadal Variability of the Overturning Circulation in Presence of Eddy Turbulence

At low-resolution, idealized ocean circulation models forced by prescribed differential surface heat fluxes show spontaneous multidecadal variability depending critically on eddy diffusivity coefficients. The existence of this critical threshold in the range of observational estimates legitimates some doubt on the relevance of such intrinsic oscillations in the real ocean. Through a series of numerical simulations with increasing resolution up to eddy-resolving ones (10 km) and various diapycnal diffusivities, this multidecadal variability proves a generic ubiquitous feature, at least in model versions with a flat bottom. The mean circulation largely changes in the process of refining the horizontal grid (along with the associated implicit viscosity and diffusivity), and the spatial structure of the variability is largely modified, but there is no clear influence of the resolution on the main oscillation period. The interdecadal variability appears even more robust to low vertical diffusivity and overturning when mesoscale eddies are resolved. The mechanism previously proposed for these oscillations, involving westward-propagating baroclinically unstable Rossby waves in the subpolar region and its feedback on the mean circulation, appears unaffected by mesoscale turbulence and is simply displaced following the polar front.