scholarly journals An Adjoint Analysis of the Meridional Overturning Circulation in an Ocean Model

2006 ◽  
Vol 19 (15) ◽  
pp. 3732-3750 ◽  
Author(s):  
Véronique Bugnion ◽  
Chris Hill ◽  
Peter H. Stone
Keyword(s):  
Boundary Conditions ◽  
Wind Stress ◽  
Ocean Model ◽  
Meridional Overturning Circulation ◽  
Wind Forcing ◽  
Surface Boundary ◽  
Diapycnal Mixing ◽  
Sensitivity Pattern ◽  
Flux Boundary Conditions ◽  
Meridional Overturning

Abstract Using the adjoint of a fully three-dimensional primitive equation ocean model in an idealized geometry, spatial variations in the sensitivity to surface boundary forcing of the meridional overturning circulation’s strength are studied. Steady-state sensitivities to diapycnal mixing, wind stress, freshwater, and heat forcing are examined. Three different, commonly used, boundary-forcing scenarios are studied, both with and without wind forcing. Almost identical circulation is achieved in each scenario, but the sensitivity patterns show major (quantitative and qualitative) differences. Sensitivities to surface forcing and diapycnal mixing are substantially larger under mixed boundary conditions, in which fluxes of freshwater and heat are supplemented by a temperature relaxation term or under flux boundary conditions, in which climatological fluxes alone drive the circulation, than under restoring boundary conditions. The sensitivity pattern to diapycnal mixing, which peaks in the Tropics is similar both with and without wind forcing. Wind does, however, increase the sensitivity to diapycnal mixing in the regions of Ekman upwelling and decreases it in the regions of Ekman downwelling. Wind stress in the Southern Oceans plays a crucial role in restoring boundary conditions, but the effect is largely absent under mixed or flux boundary conditions. The results highlight how critical a careful formulation of the surface forcing terms is to ensuring a proper response to changes in forcing in ocean models.

An Adjoint Analysis of the Meridional Overturning Circulation in a Hybrid Coupled Model

2006 ◽  
Vol 19 (15) ◽  
pp. 3751-3767 ◽  
Author(s):  
Véronique Bugnion ◽  
Chris Hill ◽  
Peter H. Stone
Keyword(s):  
Boundary Conditions ◽  
Wind Stress ◽  
Ocean Circulation ◽  
General Circulation ◽  
Meridional Overturning Circulation ◽  
Global Ocean ◽  
Model Parameters ◽  
Overturning Circulation ◽  
Near Surface ◽  
Meridional Overturning

Abstract Multicentury sensitivities in a realistic geometry global ocean general circulation model are analyzed using an adjoint technique. This paper takes advantage of the adjoint model’s ability to generate maps of the sensitivity of a diagnostic (i.e., the meridional overturning’s strength) to all model parameters. This property of adjoints is used to review several theories, which have been elaborated to explain the strength of the North Atlantic’s meridional overturning. This paper demonstrates the profound impact of boundary conditions in permitting or suppressing mechanisms within a realistic model of the contemporary ocean circulation. For example, the so-called Drake Passage Effect in which wind stress in the Southern Ocean acts as the main driver of the overturning’s strength, is shown to be an artifact of boundary conditions that restore the ocean’s surface temperature and salinity toward prescribed climatologies. Advective transports from the Indian and Pacific basins play an important role in setting the strength of the overturning circulation under “mixed” boundary conditions, in which a flux of freshwater is specified at the ocean’s surface. The most “realistic” regime couples an atmospheric energy and moisture balance model to the ocean. In this configuration, inspection of the global maps of sensitivity to wind stress and diapycnal mixing suggests a significant role for near-surface Ekman processes in the Tropics. Buoyancy also plays an important role in setting the overturning’s strength, through direct thermal forcing near the sites of convection, or through the advection of salinity anomalies in the Atlantic basin.

scholarly journals The Dependence of Global Ocean Modeling on Background Diapycnal Mixing

2014 ◽  
Vol 2014 ◽  
pp. 1-7
Author(s):  
Zengan Deng
Keyword(s):  
Surface Temperature ◽  
Cold Water ◽  
Ocean Model ◽  
Meridional Overturning Circulation ◽  
Ocean Modeling ◽  
Global Ocean ◽  
Diapycnal Mixing ◽  
Hybrid Coordinate Ocean Model ◽  
Order Of Magnitude ◽  
Meridional Overturning

The Argo-derived background diapycnal mixing (BDM) proposed by Deng et al. (in publish) is introduced to and applied in Hybrid Coordinate Ocean Model (HYCOM). Sensitive experiments are carried out using HYCOM to detect the responses of ocean surface temperature and Meridional Overturning Circulation (MOC) to BDM in a global context. Preliminary results show that utilizing a constant BDM, with the same order of magnitude as the realistic one, may cause significant deviation in temperature and MOC. It is found that the dependence of surface temperature and MOC on BDM is prominent. Surface temperature is decreased with the increase of BDM, because diapycnal mixing can promote the deep cold water return to the upper ocean. Comparing to the control run, more striking MOC changes can be caused by the larger variation in BDM.

scholarly journals Adjoint Sensitivity of an Ocean General Circulation Model to Bottom Topography

2007 ◽  
Vol 37 (2) ◽  
pp. 377-393 ◽  
Author(s):  
Martin Losch ◽  
Patrick Heimbach
Keyword(s):  
Atlantic Ocean ◽  
Boundary Conditions ◽  
Bottom Topography ◽  
Circulation Model ◽  
Drake Passage ◽  
Meridional Overturning Circulation ◽  
Surface Boundary ◽  
Overturning Circulation ◽  
Surface Boundary Conditions ◽  
Meridional Overturning

Abstract Bottom topography, or more generally the geometry of the ocean basins, is an important ingredient in numerical ocean modeling. With the help of an adjoint model, it is shown that scalar diagnostics or objective functions in a coarse-resolution model, such as the transport through Drake Passage, the strength of the Atlantic Ocean meridional overturning circulation, the Deacon cell, and the meridional heat transport across 32°S, are sensitive to bottom topography as much as they are to surface boundary conditions. For example, adjoint topography sensitivities of the transport through Drake Passage are large in choke-point areas such as the Crozet–Kerguélen Plateau and south of New Zealand; the Atlantic meridional overturning circulation is sensitive to topography in the western boundary region of the North Atlantic Ocean and along the Scotland–Iceland Ridge. Many sensitivities are connected to steep topography and can be interpreted in terms of bottom form stress, that is, the product of bottom pressure and topography gradient. The adjoint sensitivities are found to agree with direct perturbation methods with deviations smaller than 30% for significant perturbations on time scales of 100 yr, so that the assumption of quasi linearity that is implicit in the adjoint method holds. The horizontal resolution of the numerical model affects the sensitivities to bottom topography, but large-scale patterns and the overall impact of changes in topography appear to be robust. The relative impact of changes in topography and surface boundary conditions on the model circulation is estimated by multiplying the adjoint sensitivities with assumed uncertainties. If the uncertainties are correlated in space, changing the surface boundary conditions has a larger impact on the scalar diagnostics than topography does, but the effects can locally be on the same order of magnitude if uncorrelated uncertainties are assumed. In either case, bottom topography variations within their prior uncertainties affect the solution of an ocean circulation model. To this extent, including topography in the control vector can be expected to compensate for identifiable model errors and, thus, to improve the solutions of estimation problems.

scholarly journals The Global Overturning Circulation

2019 ◽  
Vol 11 (1) ◽  
pp. 249-270 ◽  
Author(s):  
Paola Cessi
Keyword(s):  
Wind Stress ◽  
Conceptual Models ◽  
Meridional Overturning Circulation ◽  
Diapycnal Mixing ◽  
Overturning Circulation ◽  
Meridional Overturning ◽  
The Antarctic ◽  
Global Overturning Circulation

In this article, I use the Estimating the Circulation and Climate of the Ocean version 4 (ECCO4) reanalysis to estimate the residual meridional overturning circulation, zonally averaged, over the separate Atlantic and Indo-Pacific sectors. The abyssal component of this estimate differs quantitatively from previously published estimates that use comparable observations, indicating that this component is still undersampled. I also review recent conceptual models of the oceanic meridional overturning circulation and of the mid-depth and abyssal stratification. These theories show that dynamics in the Antarctic circumpolar region are essential in determining the deep and abyssal stratification. In addition, they show that a mid-depth cell consistent with observational estimates is powered by the wind stress in the Antarctic circumpolar region, while the abyssal cell relies on interior diapycnal mixing, which is bottom intensified.

scholarly journals The Importance of Wind and Buoyancy Forcing for the Boundary Density Variations and the Geostrophic Component of the AMOC at 26°N

2014 ◽  
Vol 44 (9) ◽  
pp. 2387-2408 ◽  
Author(s):  
Irene Polo ◽  
Jon Robson ◽  
Rowan Sutton ◽  
Magdalena Alonso Balmaseda
Keyword(s):  
Density Gradient ◽  
Time Scales ◽  
Wind Stress ◽  
Coastal Upwelling ◽  
Ocean Model ◽  
Meridional Overturning Circulation ◽  
Wind Forcing ◽  
Western Boundary ◽  
Decadal Time Scale ◽  
Buoyancy Forcing

Abstract It is widely thought that changes in both the surface buoyancy fluxes and wind stress drive variability in the Atlantic meridional overturning circulation (AMOC), but that they drive variability on different time scales. For example, wind forcing dominates short-term variability through its effects on Ekman currents and coastal upwelling, whereas buoyancy forcing is important for longer time scales (multiannual and decadal). However, the role of the wind forcing on multiannual to decadal time scales is less clear. Here the authors present an analysis of simulations with the Nucleus for European Modelling of the Ocean (NEMO) ocean model with the aim of explaining the important drivers of the zonal density gradient at 26°N, which is directly related to the AMOC. In the experiments, only one of either the wind stress or the buoyancy forcing is allowed to vary in time, whereas the other remains at its seasonally varying climatology. On subannual time scales, variations in the density gradient, and in the AMOC minus Ekman, are driven largely by local wind-forced coastal upwelling at both the western and eastern boundaries. On decadal time scales, buoyancy forcing related to the North Atlantic Oscillation dominates variability in the AMOC. Interestingly, however, it is found that wind forcing also plays a role at longer time scales, primarily impacting the interannual variability through the excitation of Rossby waves in the central Atlantic, which propagate westward to interact with the western boundary, but also by modulating the decadal time-scale response to buoyancy forcing.

scholarly journals Optimal Initial Excitations of Decadal Modification of the Atlantic Meridional Overturning Circulation under the Prescribed Heat and Freshwater Flux Boundary Conditions

2016 ◽  
Vol 46 (7) ◽  
pp. 2029-2047 ◽  
Author(s):  
Ziqing Zu ◽  
Mu Mu ◽  
Henk A. Dijkstra
Keyword(s):  
Boundary Conditions ◽  
Initial Perturbation ◽  
Atlantic Meridional Overturning Circulation ◽  
Meridional Overturning Circulation ◽  
Sea Surface ◽  
Freshwater Flux ◽  
Overturning Circulation ◽  
Optimal Perturbation ◽  
Flux Boundary Conditions ◽  
Meridional Overturning

AbstractWithin a three-dimensional ocean circulation model, the nonlinear optimal initial perturbations (NOIP) of sea surface salinity (SSS) and sea surface temperature (SST) to excite variability in the Atlantic meridional overturning circulation (AMOC) were obtained under prescribed heat and freshwater flux boundary conditions, using the conditional nonlinear optimal perturbation (CNOP) method. After 10 years, the optimal SSS and SST perturbations lead to reductions of the AMOC by 3.6 and 2.5 Sv (1 Sv = 106 m3 s−1), respectively, followed by multidecadal oscillations with a period of about 50 years. During the first 30 years, nonlinear processes have an important influence on the AMOC strength: convection strengthens the AMOC during years 0–2, zonal density advection promotes the slowdown of the AMOC during years 7–20, and meridional density advection inhibits the slowdown of meridional velocities in the upper ocean during years 5–18. The linear optimal initial perturbation (LOIP) was also computed using the first singular vector (FSV) method. For SSS perturbations with an amplitude of 0.5 psu, the LOIP will cause an underestimation of the amplitude of the multidecadal AMOC variability by about 1 Sv, compared to that induced by the NOIP. This underestimation will become more significant as the amplitudes of SSS perturbations increase.

scholarly journals Nonnormal Multidecadal Response of the Thermohaline Circulation Induced by Optimal Surface Salinity Perturbations

2009 ◽  
Vol 39 (4) ◽  
pp. 852-872 ◽  
Author(s):  
Florian Sévellec ◽  
Thierry Huck ◽  
Mahdi Ben Jelloul ◽  
Jérôme Vialard
Keyword(s):  
Boundary Conditions ◽  
Spectral Response ◽  
Meridional Overturning Circulation ◽  
Freshwater Flux ◽  
Surface Boundary ◽  
Surface Salinity ◽  
Stochastic Perturbations ◽  
Meridional Overturning ◽  
The Mean ◽  
Mean Circulation

Abstract 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 +2.25 Sv (1 Sv ≡ 106 m3 s−1; 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.

scholarly journals The Dependence of Southern Ocean Meridional Overturning on Wind Stress

2011 ◽  
Vol 41 (12) ◽  
pp. 2261-2278 ◽  
Author(s):  
Ryan Abernathey ◽  
John Marshall ◽  
David Ferreira
Keyword(s):  
Heat Flux ◽  
Wind Stress ◽  
Mechanical Energy ◽  
Surface Wind ◽  
Zonal Flow ◽  
Eddy Diffusivity ◽  
Meridional Overturning Circulation ◽  
Surface Boundary ◽  
Surface Wind Stress ◽  
Meridional Overturning

Abstract An eddy-resolving numerical model of a zonal flow, meant to resemble the Antarctic Circumpolar Current, is described and analyzed using the framework of J. Marshall and T. Radko. In addition to wind and buoyancy forcing at the surface, the model contains a sponge layer at the northern boundary that permits a residual meridional overturning circulation (MOC) to exist at depth. The strength of the residual MOC is diagnosed for different strengths of surface wind stress. It is found that the eddy circulation largely compensates for the changes in Ekman circulation. The extent of the compensation and thus the sensitivity of the MOC to the winds depend on the surface boundary condition. A fixed-heat-flux surface boundary severely limits the ability of the MOC to change. An interactive heat flux leads to greater sensitivity. To explain the MOC sensitivity to the wind strength under the interactive heat flux, transformed Eulerian-mean theory is applied, in which the eddy diffusivity plays a central role in determining the eddy response. A scaling theory for the eddy diffusivity, based on the mechanical energy balance, is developed and tested; the average magnitude of the diffusivity is found to be proportional to the square root of the wind stress. The MOC sensitivity to the winds based on this scaling is compared with the true sensitivity diagnosed from the experiments.

The Zonal Dimension of the Indian Ocean Meridional Overturning Circulation

2008 ◽  
Vol 38 (2) ◽  
pp. 359-379 ◽  
Author(s):  
Sybren S. Drijfhout ◽  
Alberto C. Naveira Garabato
Keyword(s):  
Indian Ocean ◽  
Three Dimensional ◽  
Ocean Model ◽  
Meridional Overturning Circulation ◽  
Dimensional Structure ◽  
Asymmetric Distribution ◽  
Diapycnal Mixing ◽  
Overturning Circulation ◽  
Meridional Overturning ◽  
Zonal Asymmetry

Abstract The three-dimensional structure of the meridional overturning circulation (MOC) in the deep Indian Ocean is investigated with an eddy-permitting ocean model. The amplitude of the modeled deep Indian Ocean MOC is 5.6 Sv (1 Sv ≡ 106 m3 s−1), a broadly realistic but somewhat weak overturning. Although the model parameterization of diapycnal mixing is inaccurate, the model’s short spinup allows the effective diapycnal velocity (the sum of model drift and the explicitly modeled diapycnal velocity) to resemble the true, real-ocean diapycnal velocity. For this reason, the model is able to recover the broad zonal asymmetry in the turbulent buoyancy flux that is suggested by observations. The model features a substantial deep, depth-reversing zonal circulation of nearly 50% of the MOC. The existence of this circulation, brought about by the zonally asymmetric distribution of diapycnal mixing, implies a much slower ventilation of the deep Indian Ocean (by a factor of 5–6) than would be in place without zonal interbasin exchanges. It is concluded that the zonal asymmetry in the distribution of diapycnal mixing must have a major impact on the deep Indian Ocean’s capacity to store and transform climatically significant physical and biogeochemical tracers.

scholarly journals Eastern-Boundary Contribution to the Residual and Meridional Overturning Circulations

2010 ◽  
Vol 40 (9) ◽  
pp. 2075-2090 ◽  
Author(s):  
Paola Cessi ◽  
Christopher L. Wolfe ◽  
Bonnie C. Ludka
Keyword(s):  
Boundary Conditions ◽  
Wind Stress ◽  
Surface Wind ◽  
Meridional Overturning Circulation ◽  
Special Focus ◽  
Residual Flow ◽  
Effective Boundary ◽  
Meridional Overturning ◽  
Eddy Fluxes ◽  
Effective Boundary Conditions

Abstract A model of the thermocline linearized around a specified stratification and the barotropic linear wind-driven Stommel solution is constructed. The forcings are both mechanical (the surface wind stress) and thermodynamical (the surface buoyancy boundary condition). The effects of diapycnal diffusivity and of eddy fluxes of buoyancy, parameterized in terms of the large-scale buoyancy gradient, are included. The eddy fluxes of buoyancy are especially important near the boundaries where they mediate the transport in and out of the narrow ageostrophic down-/upwelling layers. The dynamics of these narrow layers can be replaced by effective boundary conditions on the geostrophically balanced flow. The effective boundary conditions state that the residual flow normal to the effective coast vanishes. The separate Eulerian and eddy-induced components may be nonzero. This formulation conserves the total mass and the total buoyancy while permitting an exchange between the Eulerian and eddy transport of buoyancy within the down-/upwelling layers. In turn, this exchange allows buoyancy gradients along all solid boundaries, including the eastern one. A special focus is on the buoyancy along the eastern and western walls since east–west buoyancy difference determines the meridional overturning circulation. The inclusion of advection of buoyancy by the barotropic flow allows a meaningful distinction between the meridional and the residual overturning circulations while retaining the simplicity of a linear model. The residual flow in both meridional and zonal directions reveals how the subsurface buoyancy distribution is established and, in particular, how the meridional buoyancy gradient is reversed at depth. In turn, the horizontal buoyancy gradient maintains stacked counterrotating cells in the meridional and residual overturning circulations. Quantitative scaling arguments are given for each of these cells, which show how the buoyancy forcing, the wind stress, and the diapycnal and eddy diffusivities, as well as the other imposed parameters, affect the strength of the overturn.

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