Search Results

You are looking at 1 - 10 of 11 items for

  • Author or Editor: A. M. Treguier x
  • Refine by Access: All Content x
Clear All Modify Search
P. Klein and A. M. Treguier

Abstract

The dynamics of the mixed layer in the presence of an embedded geostrophic jet has been investigated using a simple 1½-layer model and a two-dimensional primitive equation model. The jet vorticity induces a spatial variability of the wind-driven inertial motions that can have some important consequences on the mixed-layer dynamics. With a steady wind stress parallel to the front, the main effect is the generation of steady upwellings and downwellings due to the divergence of the mean Ekman drift (as reported by Niiler). With a cross-front wind, however, a dramatic exponential amplification of the inertial oscillations caused by an inertial resonance mechanism is found: this mechanism can increase the inertial waves amplitude by a factor up to 10 within ten inertial periods. Competition between this resonance mechanism and the dispersion mechanisms (mainly the horizontal and vertical propagation of inertial waves) that can limit its effects has been assessed. A consequence of horizontal propagation is that energetic waves can propagate well away from the jet while continuing to absorb energy from the wind. Downward propagation disperses this energy to a depth of at least 500 m in a few days.

Full access
P. Klein and A. M. Treguier

Abstract

No abstract available

Full access
A. M. Treguier and R. L. Panetta

Abstract

The presence of multiple zonal fronts in the Antarctic Circumpolar Current (ACC), a phenomenon described as zonation, has been confirmed by many observations suggesting that the fronts are circumpolar in extent. In the context of quasigeostrophic turbulence models, Panetta has shown that fronts associated with long-lived zonal jets develop spontaneously in very wide baroclinically unstable regions (of width large compared with the Rossby radius of deformation). The present paper examines the relevance of this mechanism to the zonation of the ACC, using a quasigeostrophic wind-forced channel model. Multiple jets appear when the forcing scale is wide enough. and they have horizontal scales and a strength that compares well with observations. However, in the presence of large-scale random topography, a different dynamical regime emerges in which the flow structure depends on the topographically controlled stationary eddies.

Full access
A. M. Treguier and J. C. McWilliams

Abstract

Topographic influences are examined in an eddy-resolving model of oceanic channel flow forced by steady zonal winds. With small explicit lateral friction, transient eddies generated by the baroclinic instability of the mean flow transfer momentum downward to the bottom layer. In the flat-bottom case, bottom friction is the only efficient sink of eastward momentum. When bottom topography is present, the topographic form stress can replace the bottom friction sink in the momentum budget, and a large decrease of the zonal transport results. Large wale topography (of the scale of the forcing) provides the largest form stress. Topographic effects decay with height as suggested by the Prandit scaling, and therefore only topographic scales larger than the Rossby radius can affect the whole water column. In that case, the interfaces are deformed by standing eddies on topographic length scales, and standing eddies replace transient eddies in transferring momentum downward. The bottom-layer mean streamfunction tends to be correlated with the topography as in inviscid solutions. Because of this, only a small part of the flow (the larger scales) contributes to the domain-averaged momentum sink. On smaller scales, the topographic form stress is anticorrelated with the Reynolds stress and has no net effect on the transport. The energy level of the transients is less affected by the topography than is the mean energy. With topography, the space scale of the transients decreases and their time scale increases, and the ratio of potential and kinetic energies is higher.

Full access
M. Arhan, A. M. Treguier, B. Bourlès, and S. Michel

Abstract

Ten-year-long output series from a general circulation model forced by daily realistic winds are used to analyze the annual cycle of the Equatorial Undercurrent (EUC) in the Atlantic Ocean. Two well-defined transport maxima are found: One, present during boreal summer and autumn in the central part of the basin, is generally recognized and regarded as a near-equilibrium response to the equatorial easterly trades that culminate in this period. Another one, most pronounced near the western boundary, occurs in April–May when the trades relax. This second maximum is less patent in the observations, but concomitant signals in previously published analyses of the North Brazil Current and surface velocity seasonal variations might be indirect manifestations of its reality. Because this intensification appears at periods when the boundary between the tropical and equatorial gyres nears the equator, the authors relate its existence to wind stress curl variations at subequatorial latitudes. A link between the interannual variability of the spring transport maximum and that of the low-latitude wind stress curl is, indeed, found in the model. This diagnostic approach suggests that two different dynamical regimes shape up the EUC seasonal cycle: in summer and autumn, local forcing by the equatorial zonal wind component and main supply from the ocean interior; in winter and spring, remote forcing by the low-latitude rotational wind component and supply from the western boundary currents.

Full access
A. M. Treguier, C. Lique, J. Deshayes, and J. M. Molines

Abstract

Correlations between temperature and velocity fluctuations are a significant contribution to the North Atlantic meridional heat transport, especially at the northern boundary of the subtropical gyre. In satellite observations and in a numerical model at ⅞° resolution, a localized pattern of positive eddy heat flux is found northwest of the Gulf Stream, downstream of its separation at Cape Hatteras. It is confined to the upper 500 m. A simple kinematic model of a meandering jet can explain the surface eddy flux, taking into account a spatial shift between the maximum velocity of the jet and the maximum cross-jet temperature gradient. In the Gulf Stream such a spatial shift results from the nonlinear temperature profile and the vertical tilting of the velocity profile with depth. The numerical model suggests that the meandering of the Gulf Stream could account, at least in part, for the large eddy heat transport (of order 0.3 PW) near 36°N in the North Atlantic and for its compensation by the mean flow.

Full access
A. M. Treguier, I. M. Held, and V. D. Larichev

Abstract

A parameterization of mesoscale eddy fluxes in the ocean should be consistent with the fact that the ocean interior is nearly adiabatic. Gent and McWilliams have described a framework in which this can be approximated in z-coordinate primitive equation models by incorporating the effects of eddies on the buoyancy field through an eddy-induced velocity. It is also natural to base a parameterization on the simple picture of the mixing of potential vorticity in the interior and the mixing of buoyancy at the surface. The authors discuss the various constraints imposed by these two requirements and attempt to clarify the appropriate boundary conditions on the eddy-induced velocities at the surface. Quasigeostrophic theory is used as a guide to the simplest way of satisfying these constraints.

Full access
V. O. Ivchenko, A. M. Treguier, and S. E. Best

Abstract

An energy analysis of the Fine Resolution Antarctic Model (FRAM) reveals the instability processes in the model. The main source of time-mean kinetic energy is the wind stress and the main sink is transfer to mean potential energy. The wind forcing thus helps maintain the density structure. Transient motions result from internal instabilities of the flow rather than seasonal variations of the forcing.

Baroclinic instability is found to be an important mechanism in FRAM. The highest values of available potential energy are found in the western boundary regions as well as in the Antarctic Circumpolar Current (ACC) region. All subregions with predominantly zonal flow are found to be baroclinically unstable. The observed deficit of eddy kinetic energy in FRAM occurs as a result of the high lateral friction, which decreases the growth rates of the most unstable waves. This high friction is required for the numerical stability of the model and can only be made smaller by using a finer horizontal resolution. A grid spacing of at least 10–15 km would be required to resolve the most unstable waves in the southern part of the domain.

Barotropic instability is also found to be important for the total domain balance. The inverse transfer (that is, transfer from eddy to mean kinetic energy) does not occur anywhere, except in very localized tight jets in the ACC.

The open boundary condition at the northern edge of the model domain does not represent a significant source or sink of eddy variability. However, a large exchange between internal and external mode energies is found to occur. It is still unclear how these boundary conditions affect the dynamics of adjacent regions.

Full access
A. M. Treguier, N. G. Hogg, M. Maltrud, K. Speer, and V. Thierry

Abstract

Recent data from a deployment of Lagrangian floats in the Brazil Basin of the South Atlantic reveal a swift western boundary current and predominantly zonal flow in the interior at a depth of about 2500 m. Dynamical mechanisms for the deep interior flow are considered using two high-resolution models, a global and a regional one, together with a suite of sensitivity studies at low resolution. Outside the western boundary region, model energy levels are similar to observations. The models are able to reproduce, at somewhat reduced strength depending on resolution, much of the meridional structure of the observed deep zonal flows. Several candidates for generating such flows are examined, including nonlinear rectification, baroclinic instability, and thermohaline and wind forcing. A primary mechanism for the deep flow in the models is the response to the wind stress, as recently argued to be the case for a model of the Pacific Ocean. However, thermohaline forcing is significant, especially where density contrasts between basins generate strong currents in deep passages. The deep thermohaline flow appears to be linked to the depth of the midocean ridge. Baroclinic instability of the mean meridional flow, which is alone capable of generating nearly zonal currents of the observed scale, is a possible additional forcing but is not essential in the models investigated here. The meridional scale of the zonal flows in the models is extremely dependent on the horizontal resolution and horizontal mixing.

Full access
A. M. Treguier, J. Le Sommer, J. M. Molines, and B. de Cuevas

Abstract

The authors evaluate the response of the Southern Ocean to the variability and multidecadal trend of the southern annular mode (SAM) from 1972 to 2001 in a global eddy-permitting model of the DRAKKAR project. The transport of the Antarctic Circumpolar Current (ACC) is correlated with the SAM at interannual time scales but exhibits a drift because of the thermodynamic adjustment of the model (the ACC transport decreases because of a low renewal rate of dense waters around Antarctica). The interannual variability of the eddy kinetic energy (EKE) and the ACC transport are uncorrelated, but the EKE decreases like the ACC transport over the three decades, even though meridional eddy fluxes of heat and buoyancy remain stable. The contribution of oceanic eddies to meridional transports is an important issue because a growth of the poleward eddy transport could, in theory, oppose the increase of the mean overturning circulation forced by the SAM. In the authors’ model, the total meridional circulation at 50°S is well correlated with the SAM index (and the Ekman transport) at interannual time scales, and both increase over three decades between 1972 and 2001. However, given the long-term drift, no SAM-linked trend in the eddy contribution to the meridional overturning circulation is detectable. The increase of the meridional overturning is due to the time-mean component and is compensated by an increased buoyancy gain at the surface. The authors emphasize that the meridional circulation does not vary in a simple relationship with the zonal circulation. The model solution points out that the zonal circulation and the eddy kinetic energy are governed by different mechanisms according to the time scale considered (interannual or decadal).

Full access