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Alban Lazar, Gurvan Madec, and Pascale Delecluse

Abstract

Numerous numerical simulations of basin-scale ocean circulation display a vast interior downwelling and a companion intense western boundary layer upwelling at midlatitude below the thermocline. These features, related to the so-called Veronis effect, are poorly rationalized and depart strongly from the classical vision of the deep circulation where upwelling is considered to occur in the interior. Furthermore, they significantly alter results of ocean general circulation models (OGCMs) using horizontal Laplacian diffusion. Recently, some studies showed that the parameterization for mesoscale eddy effects formulated by Gent and McWilliams allows integral quantities like the streamfunction and meridional heat transport to be free of these undesired effects. In this paper, an idealized OGCM is used to validate an analytical rationalization of the processes at work and help understand the physics.

The results show that the features associated with the Veronis effect can be related quantitatively to three different width scales that characterize the baroclinic structure of the deep western boundary current. In addition, since one of these scales may be smaller than the Munk barotropic layer, usually considered to determine the minimum resolution and horizontal viscosity for numerical models, the authors recommend that it be taken into account. Regarding the introduction of the new parameterization, diagnostics in terms of heat balances underline some interesting similarities between local heat fluxes by eddy-induced velocities and horizontal diffusion at low and midlatitudes when a common large diffusivity (here 2000 m2 s−1) is used. The near-quasigeostrophic character of the flow explains these results. As a consequence, the response of the Eulerian-mean circulation is locally similar for runs using either of the two parameterizations. However, it is shown that the advective nature of the eddy-induced heat fluxes results in a very different effective circulation, which is the one felt by tracers.

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Friederike Pollmann, Fabien Roquet, and Gurvan Madec

Abstract

Large-scale overturning cells in the ocean typically combine an essentially horizontal surface branch and an interior branch below, where the circulation spans both horizontal and vertical scales. The aim of this study is to analyze the impact of this asymmetry between the two branches by “folding” a one-dimensional thermohaline loop, such that its lower part remains vertical while its upper part is folded down into the horizontal plane. It is found that both the transitory response and the distribution of thermohaline properties are modified significantly when the loop is folded. In some cases, velocity oscillations are induced during the spinup that were not seen in the unfolded case. This is because a circular loop allows for compensations between the density torques produced above and below the heat forcing level, while such compensations are not possible in the folded loop because of the horizontal direction of the surface circulation. Furthermore, the dynamical effects associated with nonlinearities of the equation of state are significantly altered by the folding. Cabbeling tends to decelerate the flow in the folded loop, instead of accelerating it as in the circular case, and can also act to dampen velocity oscillations. Thermobaricity also alters the loop circulation, although comparatively less.

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Fabien Roquet, Carl Wunsch, and Gurvan Madec

Abstract

Pathways of wind-power input into the ocean general circulation are analyzed using Ekman theory. Direct rates of wind work can be calculated through the wind stress acting on the surface geostrophic flow. However, because that energy is transported laterally in the Ekman layer, the injection into the geostrophic interior is actually controlled by Ekman pumping, with a pattern determined by the wind curl rather than the wind itself. Regions of power injection into the geostrophic interior are thus generally shifted poleward compared to regions of direct wind-power input, most notably in the Southern Ocean, where on average energy enters the interior 10° south of the Antarctic Circumpolar Current core. An interpretation of the wind-power input to the interior is proposed, expressed as a downward flux of pressure work. This energy flux is a measure of the work done by the Ekman pumping against the surface elevation pressure, helping to maintain the observed anomaly of sea surface height relative to the global-mean sea level.

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Daniele Iudicone, Gurvan Madec, and Trevor J. McDougall

Abstract

A new formulation is proposed for the evaluation of the dianeutral transport in the ocean. The method represents an extension of the classical diagnostic approach for estimating the water-mass formation from the buoyancy balance. The inclusion of internal sources such as the penetrative solar shortwave radiation (i.e., depth-dependent heat transfer) in the estimate of surface buoyancy fluxes has a significant impact in several oceanic regions, and the former simplified formulation can lead to a 100% error in the estimate of water-mass formation due to surface buoyancy fluxes. Furthermore, internal mixing can also be overestimated in inversions of in situ data when the shortwave radiation is not allowed to be penetrative.

The method examines the evolution equation of neutral density via the tendencies of potential temperature and salinity. The neutral density framework does not require the choice of a reference pressure and thus, unlike previous approaches that consider potential density, it is well suited for examining the whole open-ocean water column.

The methodology is easy to implement, particularly for ocean numerical models. The authors present here its application to a long simulation made with an ice–ocean global model, which allowed the method to be validated.

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Etienne Pauthenet, Fabien Roquet, Gurvan Madec, and David Nerini

Abstract

The thermohaline structure of the Southern Ocean is deeply influenced by the presence of the Antarctic Circumpolar Current (ACC), where water masses of the World Ocean are advected, transformed, and redistributed to the other basins. It remains a challenge to describe and visualize the complex 3D pattern of this circulation and its associated tracer distribution. Here, a simple framework is presented to analyze the Southern Ocean thermohaline structure. A functional principal component analysis (PCA) is applied to temperature θ and salinity S profiles to determine the main spatial patterns of their variations. Using the Southern Ocean State Estimate (SOSE), this study determines the vertical modes describing the Southern Ocean thermohaline structure between 5 and 2000 m. The first two modes explain 92% of the combined θS variance, thus providing a surprisingly good approximation of the thermohaline properties in the Southern Ocean. The first mode (72% of total variance) accurately describes the north–south property gradients. The second mode (20%) mostly describes salinity at 500 m in the region of Antarctic Intermediate Water formation. These two modes present circumpolar patterns that can be closely related with standard frontal definitions. By projecting any given hydrographic profile onto the SOSE-based modes, it is possible to determine its position relative to the fronts. The projection is successfully applied on the hydrographic profiles of the WOCE SR3 section. The Southern Ocean thermohaline decomposition provides an objective way to define water mass boundaries and their spatial variability and has useful application for comparing model output with observations.

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Bruno Blanke, Michel Arhan, Gurvan Madec, and Sophie Roche

Abstract

Monthly mean velocity fields from a global ocean general circulation model are used to study the main circulation patterns within the upper 1200 m of the equatorial Atlantic. Some recently developed Lagrangian techniques are used to picture and quantify the routes followed in the model by distinct water mass classes, defined by their initial temperature on model transatlantic sections at 10°S and 10°N. The qualitative description in terms of equatorial pathways of this warm component of the so-called global “conveyor belt” is found coherent with the most recent circulation schemes inferred from direct measurements. Diagnostics emphasize the crucial role of the western boundary current system and that of the equatorial subsurface jets in distributing the flow in the equatorial domain, both for northward-flowing and southward-recirculating warm water masses. As the model tracer fields are constrained to remain close to the observed climatology outside the equatorial strip, the circulation calculated by the model is also shown to fairly reproduce the intensity of the net northward interhemispheric warm water exchange as inferred from direct measurements, as well as known conversions of warm water masses within the tropical Atlantic.

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Gurvan Madec, Pascale Delecluse, Michel Crepon, and Michel Chartier

Abstract

Deep-water formation (DWF) in the northwestern Mediterranean Sea and the subsequent horizontal circulation are investigated in a rectangular basin with a three-dimensional primitive equation model. The basin is forced by constant climatological heat and salt fluxes. Convective motion is parameterized by a simple nonpenetrative convective adjustment process plus Richardson number–dependent vertical eddy viscosity and diffusivity. A homogeneous column of dense water is progressively formed in the forcing area. Meanders of 40-km wavelength develop at the periphery of the column. These features agree with observations. Energy studies show that the meanders are generated mainly through a baroclinic instability process. These meanders, and the associated cells of vertical motion, contribute to the process of DWF. They generate vertical advection, while the associated horizontal advection tends to restratify the surface water of the column, and thus to inhibit very deep convection. Just before the end of the forcing period, 80-km meanders appear, which create advection strong enough to erase the column within two weeks. The associated horizontal cyclonic circulation is of the same order of magnitude as that estimated from observations.

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Gurvan Madec, Pascale Delecluse, Michel Crépon, and François Lott

Abstract

The large-scale processes preconditioning the winter deep-water formation in the northwestern Mediterranean Sea are investigated with a primitive equation numerical model where convection is parameterized by a non-penetrative convective adjustment algorithm. The ocean is forced by momentum and buoyancy fluxes that have the gross features of mean winter forcing found in the MEDOC area. The wind-driven barotropic circulation appears to be a major ingredient of the preconditioning phase of deep-water formation. After three months, the ocean response is dominated by a strong barotropic cyclonic vortex located under the forcing area, which fits the Sverdrup balance away from the northern coast. In the vortex center, the whole water column remains trapped under the forcing area all winter. This trapping enables the thermohaline forcing to drive deep-water formation efficiently. Sensitivity studies show that, β effect and bottom topography play a paramount role and confirm that deep convection occurs only in areas that combine a strong surface thermohaline forcing and a weak barotropic advection so that water masses are submitted to the negative buoyancy fluxes for a much longer time. In particular, the impact of the Rhône Deep Sea Fan on the barotropic circulation dominates the β effect: the barotropic flow is constrained to follow the bathymetric contours and the cyclonic vortex is shifted southward so that the fluid above the fan remains quiescent. Hence, buoyancy fluxes trigger deep convection above the fan in agreement with observations. The selection of the area of deep-water formation through the defection of the barotropic circulation by the topography seems a more efficient mechanism than those associated with the wind- driven barotropic vortex. This is due to its permanency, while the latter may be too sensitive to time and space variations of the forcing.

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Fabien Roquet, Gurvan Madec, Laurent Brodeau, and J. Nycander

Abstract

There is a growing realization that the nonlinear nature of the equation of state has a deep impact on the global ocean circulation; however, the understanding of the global effects of these nonlinearities remains elusive. This is partly because of the complicated formulation of the seawater equation of state making it difficult to handle in theoretical studies. In this paper, a hierarchy of polynomial equations of state of increasing complexity, optimal in a least squares sense, is presented. These different simplified equations of state are then used to simulate the ocean circulation in a global 2°-resolution configuration. Comparisons between simulated ocean circulations confirm that nonlinear effects are of major importance, in particular influencing the circulation through determination of the static stability below the mixed layer, thus controlling rates of exchange between the atmosphere and the ocean interior. It is found that a simple polynomial equation of state, with a quadratic term in temperature (for cabbeling), a temperature–pressure product term (for thermobaricity), and a linear term in salinity, that is, only four tuning parameters, is enough to simulate a reasonably realistic global circulation. The best simulation is obtained when the simplified equation of state is forced to have an accurate thermal expansion coefficient near the freezing point, highlighting the importance of polar regions for the global stratification. It is argued that this simplified equation of state will be of great value for theoretical studies and pedagogical purposes.

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Daniele Iudicone, Sabrina Speich, Gurvan Madec, and Bruno Blanke

Abstract

Recent studies have proposed the Southern Ocean as the site of large water-mass transformations; other studies propose that this basin is among the main drivers for North Atlantic Deep Water (NADW) circulation. A modeling contribution toward understanding the role of this basin in the global thermohaline circulation can thus be of interest. In particular, key pathways and transformations associated with the thermohaline circulation in the Southern Ocean of an ice–ocean coupled model have been identified here through the extensive use of quantitative Lagrangian diagnostics. The model Southern Ocean is characterized by a shallow overturning circulation transforming 20 Sv (1 Sv ≡ 106 m3 s−1) of thermocline waters into mode waters and a deep overturning related to the formation of Antarctic Bottom Water. Mode and intermediate waters contribute to 80% of the upper branch of the overturning in the Atlantic Ocean north of 30°S. A net upwelling of 11.5 Sv of Circumpolar Deep Waters is simulated in the Southern Ocean. Antarctic Bottom Water upwells into deep layers in the Pacific basin, forming Circumpolar Deep Water and subsurface thermocline water. The Southern Ocean is a powerful consumer of NADW: about 40% of NADW net export was found to upwell in the Southern Ocean, and 40% is transformed into Antarctic Bottom Water. The upwelling occurs south of the Polar Front and mainly in the Indian and Pacific Ocean sectors. The transformation of NADW to lighter water occurs in two steps: vertical mixing at the base of the mixed layer first decreases the salinity of the deep water upwelling south of the Antarctic Circumpolar Current, followed by heat input by air–sea and diffusive fluxes to complete the transformation to mode and intermediate waters.

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