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Rick Lumpkin, Anne-Marie Treguier, and Kevin Speer

Abstract

Eddy time and length scales are calculated from surface drifter and subsurface float observations in the northern Atlantic Ocean. Outside the energetic Gulf Stream, subsurface timescales are relatively constant at depths from 700 m to 2000 m. Length scale and the characteristic eddy speed decrease with increasing depth below 700 m, but length scale stays relatively constant in the upper several hundred meters of the Gulf Stream. It is suggested that this behavior is due to the Lagrangian sampling of the mesoscale field, in limits set by the Eulerian eddy scales and the eddy kinetic energy. In high-energy regions of the surface and near-surface North Atlantic, the eddy field is in the “frozen field” Lagrangian sampling regime for which the Lagrangian and Eulerian length scales are proportional. However, throughout much of the deep ocean interior, the eddy field may be in the “fixed float” regime for which the Lagrangian and Eulerian timescales are nearly equal. This does not necessarily imply that the deep interior is nearly linear, as fixed-float sampling is possible in a flow field of O(1) nonlinearity.

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Yves Du Penhoat and Anne Marie Treguier

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The linear response of the tropical Atlantic ocean to climatological winds is calculated in a continuously stratified model and compared with observations. All the characteristic features of the dynamic topography are reproduced at the proper 1ocations including the series of zonally oriented ridges and troughs, the equatorial “pivot” zone and the seasonal variations of slope along the equator. Linear theory fails to explain seasonal variations of the South Equatorial Current and of the Equatorial Undercurrent, but it compares favorably for the major zonal currents off the equator (the North Equatorial Countercurrent, the Guinea Current and Undercurrent).

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Pascal Rivière, Anne Marie Treguier, and Patrice Klein

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Bottom friction is an important sink of energy in the ocean. Indeed, high-resolution ocean models need bottom friction to achieve a satisfactory kinetic energy level at equilibrium. However, bottom friction has also subtle and discriminating effects on the different energy transfers and therefore on the 3D structure of the flow, some of which have to be clarified. In this study, those effects on an unstable baroclinic jet are reexamined using a primitive equation model. As in previous studies using quasigeostrophic models, it was found that bottom friction strongly affects the barotropic mode whereas the baroclinic modes are weakly changed. The new result is that bottom friction yields a significant space-scale selection. Analysis of the dynamics reveals strong agreement with previous quasigeostrophic studies at the mesoscale in the interior but differences in the eddy field at small scales close to the surface. A rationalization of these results is proposed by a comparison with preceding atmospheric studies. It is shown that the “barotropic governor” of James and Gray is not active in ocean simulations and that the scale selection induced by bottom friction is primarily induced by nonlinear interactions in the three-dimensional structure of the eddy field.

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Anne Marie Treguier and Bach Lien Hua

Abstract

The quasi-geostrophic response to stochastic wind fluctuations is calculated using a doubly periodic nonlinear model, with a vertical resolution of three modes in most cases. The influence of various parameters on the response is investigated: space and time scale of the forcing, stratification, bottom friction and β-effect. One aim of this study is to understand the influence of nonlinear transfer, and therefore, most simulations are situated in a parameter range where nonlinearities are important. The model “pseudodispersion relation” clearly shows two regimes: a linear regime made of resonant Rossby waves for the barotropic large scales and a nonlinear regime that is dynamically similar, for example, to quasi-geostrophic turbulence forced by baroclinic instability. The forcing time scale and β-effect have little influence on the response. The most important parameter is found to be the ratio R = κ1/K min of the largest forced wavelength to the wavelength of the first baroclinic mode Rossby radius. When R grows, the amount of energy in the linear regime grows, and the kinetic energy becomes essentially barotrophic (currents are then depth-independent). For our model, R must be of order 5 in order to obtain a realistic vertical structure, while observations show that R is larger than 10. From this discrepancy we conclude that other physical mechanisms have to be taken into account to reproduce the vertical structure of the oceanic response, although our results confirm that wind fluctuations can effectively generate eddy energy in the ocean.

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Maria Valdivieso Da Costa, Herlé Mercier, and Anne Marie Treguier

Abstract

An eddy-resolving primitive equation general circulation model is used to estimate water-mass subduction rates in the North Atlantic Ocean subtropical gyre. The diagnostics are based on the instantaneous kinematic approach, which allows the calculation of the annual rate of water-mass subduction at a given density range, following isopycnal outcrop positions over the annual cycle. It is shown that water-mass subduction is effected rapidly (∼1–2 months) as the mixed layer depth decreases in spring, consistent with Stommel’s hypothesis, and occurs mostly over the area of deep late-winter mixed layers (≥150 m) across the central North Atlantic in the density range 26 ≤ σ ≤ 27.2. Annual subduction rates O(100–200 m yr–1) are found south and east of the Gulf Stream extension in the density range of subtropical mode waters from roughly 26.2 to 26.6. In the northeastern part of the subtropical gyre, annual subduction rates are somewhat larger, O(250 m yr–1), from a density of about 26.9 east of the North Atlantic Current to 27.4 (upper cutoff in this study). The overall basin-integrated subduction rate for subtropical mode waters (26.2 ≤ σ ≤ 26.6) is about 12.2 Sv (Sv ≡ 106 m3 s−1), comparable to the total formation rate inferred from the surface density forcing applied in the model of roughly 11 Sv in this density range. In contrast, basin-integrated rates for denser central water (26.8 ≤ σ ≤ 27.2) provide a vanishingly small net subduction. In this range, eddy correlations (<30 days) between the surface outcrop area and the local subduction rate counteract the net subduction by the mean flow (deduced from monthly averaged model fields). Comparison with estimates of the annual subduction rate based on the annual mean velocity and late-winter mixed layer properties alone, as is usual in climatological and coarse-resolution model analyses, indicates a mismatch of at least 8 Sv in the density range where the model forms subtropical mode water. This mismatch is primarily due to time-varying mixed layer processes rather than small-scale mixing not resolved explicitly by the model. Our diagnostics based on the instantaneous kinematic approach provide a more complete picture of the water-mass formation process than diagnostics based only on air–sea flux or late-winter mixed layer model data. They reveal the crucial importance of both the seasonal mixed layer cycle and mesoscale eddies to the overall formation rate and provide thus a valuable tool for the analysis of water-mass formation rates in eddy-resolving numerical simulations at basin scale.

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Cédric Legal, Patrice Klein, Anne-Marie Treguier, and Jerome Paillet

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A high-resolution survey was conducted as part of the 2001 Programme Ocean Multidisciplinaire Meso Echelle (POMME 2) experiment in a region of the northeast Atlantic Ocean characterized by a large number of strongly interacting mesoscale eddies. The survey was located between mesoscale eddies in an area where the horizontal stirring processes were dominant. Diagnosis, using SeaSoar data combined with the analysis of altimeter data, reveals an energetic vertical velocity field involving elongated thin structures with alternate signs and amplitude up to 20 m day−1. The 3D dynamics involved in the appearance of these vertical motions is the restoration of the thermal wind balance within the small-scale density filaments that are elongated by the stirring processes. These experimental results reinforce the conclusions of previous numerical studies pointing out the necessity to explicitly include the effects of the filamentation process in ocean models.

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Clément Vic, Henrick Berger, Anne-Marie Tréguier, and Xavier Couvelard

Abstract

The Congo River has the second largest rate of flow in the world and is mainly responsible for the broad tongue of low-salinity water that is observed in the Gulf of Guinea. Despite their importance, near-equatorial river plumes have not been studied as thoroughly as midlatitude plumes and their dynamics remain unclear. Using both theory and idealized numerical experiments that reproduce the major characteristics of the region, the authors have investigated the dynamics of the Congo River plume and examine its sensitivity to different forcing mechanisms. It is found that near-equatorial plumes are more likely to be surface trapped than midlatitude plumes, and the importance of the β effect in describing the strong offshore extent of the low-salinity tongue during most of the year is demonstrated. It is shown that the buoyant plume constrained by the geomorphology is subject to the β pulling of nonlinear structures and wavelike equatorial dynamics. The wind is found to strengthen the intrinsic buoyancy-driven dynamics and impede the development of the coastal southward current, in coherence with observations.

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Nicolas Barrier, Christophe Cassou, Julie Deshayes, and Anne-Marie Treguier

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A new framework is proposed for investigating the atmospheric forcing of North Atlantic Ocean circulation. Instead of using classical modes of variability, such as the North Atlantic Oscillation (NAO) or the east Atlantic pattern, the weather regimes paradigm was used. Using this framework helped avoid problems associated with the assumptions of orthogonality and symmetry that are particular to modal analysis and known to be unsuitable for the NAO. Using ocean-only historical and sensitivity experiments, the impacts of the four winter weather regimes on horizontal and overturning circulations were investigated. The results suggest that the Atlantic Ridge (AR), negative NAO (NAO), and positive NAO (NAO+) regimes induce a fast (monthly-to-interannual time scales) adjustment of the gyres via topographic Sverdrup dynamics and of the meridional overturning circulation via anomalous Ekman transport. The wind anomalies associated with the Scandinavian blocking regime (SBL) are ineffective in driving a fast wind-driven oceanic adjustment. The response of both gyre and overturning circulations to persistent regime conditions was also estimated. AR causes a strong, wind-driven reduction in the strengths of the subtropical and subpolar gyres, while NAO+ causes a strengthening of the subtropical gyre via wind stress curl anomalies and of the subpolar gyre via heat flux anomalies. NAO induces a southward shift of the gyres through the southward displacement of the wind stress curl. The SBL is found to impact the subpolar gyre only via anomalous heat fluxes. The overturning circulation is shown to spin up following persistent SBL and NAO+ and to spin down following persistent AR and NAO conditions. These responses are driven by changes in deep water formation in the Labrador Sea.

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Anne Marie Treguier, Pierre Mathiot, Tim Graham, Dan Copsey, Camille Lique, and Jean Sterlin

Abstract

The Nordic seas are a gateway to the Arctic Ocean, where Atlantic water undergoes a strong cooling during its transit. Here we investigate the heat balance of these regions in the high-resolution Met Office Global Coupled Model GC3 with a 1/12° grid. The GC3 model reproduces the contrasted ice conditions and ocean heat loss between the eastern and western regions of the Nordic seas. In the west (Greenland and Iceland seas), the heat loss experienced by the ocean is stronger than the atmospheric heat gain, because of the cooling by ice melt. The latter is a major contribution to the heat loss over the path of the East Greenland Current and west of Svalbard. In the model, surface fluxes balance the convergence of heat in each of the eastern and western regions. The net east–west heat exchange, integrated from Fram Strait to Iceland, is relatively small: the westward heat transport of the Return Atlantic Current over Knipovich Ridge balances the eastward heat transport by the East Icelandic Current. Time fluctuations, including eddies, are a significant contribution to the net heat transports. The eddy flux represents about 20% of the total heat transport in Denmark Strait and across Knipovich Ridge. The coupled ocean–atmosphere–ice model may overestimate the heat imported from the Atlantic and exported to the Arctic by 10% or 15%. This confirms the tendency toward higher northward heat transports as model resolution is refined, which will impact scenarios of future climate.

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Matthew D. Thomas, Anne-Marie Tréguier, Bruno Blanke, Julie Deshayes, and Aurore Voldoire

Abstract

Large differences in the Atlantic meridional overturning circulation (AMOC) exhibited between the available ocean models pose problems as to how they can be interpreted for climate policy. A novel Lagrangian methodology has been developed for use with ocean models that enables a decomposition of the AMOC according to its source waters of subduction from the mixed layer of different geographical regions. The method is described here and used to decompose the AMOC of the Centre National de Recherches Météorologiques (CNRM) ocean model, which is approximately 4.5 Sv (1 Sv = 106 m3 s−1) too weak at 26°N, compared to observations. Contributions from mixed layer subduction to the peak AMOC at 26°N in the model are dominated by the Labrador Sea, which contributes 7.51 Sv; but contributions from the Nordic seas, the Irminger Sea, and the Rockall basin are also important. These waters mostly originate where deep mixed layers border the topographic slopes of the Subpolar Gyre and Nordic seas. The too-weak model AMOC can be explained by weak model representations of the overflow and of Irminger Sea subduction. These are offset by the large Labrador Sea component, which is likely to be too strong as a result of unrealistically distributed and too-deep mixed layers near the shelf.

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