Bernard Barnier

Thermal forcing for a global ocean circulation model using a three-year climatology of ECMWF analyses

A surface thermal boundary condition for a world ocean model is proposed. The formulation is based on previous methods which have used bulk formulas to define a model-dependent correction to the air-sea fluxes applied to the model. An estimate of the flux correction is calculated from a recent 3-year climatology of atmospheric surface fields provided by the 6-hour analyses performed at the European Center for Medium-range Weather Forecasts. The mean correction term and its seasonnal cycle are analysed and compared to similar climatological quantities.

Circulation characteristics in three eddy-permitting models of the North Atlantic

A systematic intercomparison of three realistic eddy-permitting models of the North Atlantic circulation has been performed. The models use different concepts for the discretization of the vertical coordinate, namely geopotential levels, isopycnal layers, terrain-following (sigma) coordinates, respectively. Although these models were integrated under nearly identical conditions, the resulting large-scale model circulations show substantial differences. The results demonstrate that the large-scale thermohaline circulation is very sensitive to the model representation of certain localised processes, in particular to the amount and water mass properties of the overflow across the Greenland–Scotland region, to the amount of mixing within a few hundred kilometers south of the sills, and to several other processes at small or sub-grid scales. The different behaviour of the three models can to a large extent be explained as a consequence of the different model representation of these processes.

An eddy-permitting model of the Atlantic circulation: Evaluating open boundary conditions

As part of the French CLIPPER project, an eddy permitting model of the Atlantic circulation has been run for 22 years. The domain has open boundaries at Drake passage and at 30°E, from Africa to Antarctica. The simulated mean circulation, as well as the eddy activity, is satisfactory for a 1/3° model resolution, and the meridional heat transport at 30°S is within the range estimated from observations. We use the “mixed” open boundary algorithm of Barnier et al. [1998], which has both a radiation condition and a relaxation to climatology. The climatological boundary forcing strongly constrains the solution in the whole domain. The model heat balance adjusts through the surface (heat flux retroaction term) more than the open boundaries. The radiation phase velocities calculated within the algorithm are analyzed. This shows, quite surprisingly, that both the eastern and western boundaries have a similar behavior, regardless of the preferred directions for advection (mainly eastward) and wave propagation (mainly westward). Our results confirm that open boundary algorithms behave differently according to the dynamics of the region considered. The passive boundary condition that Penduff et al. [2000] applied successfully in the north eastern Atlantic does not work in the present South Atlantic model. We emphasize the need for a careful prescription of the climatology at the open boundary, for which a new approach based on synoptic sections is implemented.

Mesoscale Eddies in the Labrador Sea and Their Contribution to Convection and Restratification

Abstract The cycle of open ocean deep convection in the Labrador Sea is studied in a realistic, high-resolution (4 km) regional model, embedded in a coarser (⅓°) North Atlantic setup. This configuration allows the simultaneous generation and evolution of three different eddy types that are distinguished by their source region, generation mechanism, and dynamics. Very energetic Irminger Rings (IRs) are generated by barotropic instability of the West Greenland and Irminger Currents (WGC/IC) off Cape Desolation and are characterized by a warm, salty subsurface core. They densely populate the basin north of 58°N, where their eddy kinetic energy (EKE) matches the signal observed by satellite altimetry. Significant levels of EKE are also found offshore of the West Greenland and Labrador coasts, where boundary current eddies (BCEs) are spawned by weakly energetic instabilities all along the boundary current system (BCS). Baroclinic instability of the steep isopycnal slopes that result from a deep convective over...

Sea Level Expression of Intrinsic and Forced Ocean Variabilities at Interannual Time Scales

AbstractThis paper evaluates in a realistic context the local contributions of direct atmospheric forcing and intrinsic oceanic processes on interannual sea level anomalies (SLAs). A ¼° global ocean–sea ice general circulation model, driven over 47 yr by the full range of atmospheric time scales, is quantitatively assessed against altimetry and shown to reproduce most observed features of the interannual SLA variability from 1993 to 2004. Comparing this simulation with a second driven only by the climatological annual cycle reveals that the intrinsic part of the total interannual SLA variance exceeds 40% over half of the open-ocean area and exceeds 80% over one-fifth of it. This intrinsic contribution is particularly strong in eddy-active regions (more than 70%–80% in the Southern Ocean and western boundary current extensions) as predicted by idealized studies, as well as within the 20°–35° latitude bands. The atmosphere directly forces most of the interannual SLA variance at low latitudes and in most mid...

A sigma-coordinate primitive equation model for studying the circulation in the South Atlantic. Part I: Model configuration with error estimates

This paper describes the configuration of a topography-following (sigma) coordinate, numerical ocean model for studying the circulation in the South Atlantic. An analysis is performed (i) to ensure that the model configuration does not introduce a numerical bias in the model solution and (ii) to give estimates of numerical errors. The model is the Semi-spectral Primitive Equation Model (SPEM) from Rutgers University (Haidvogel et al., 1991). Two important issues relating to the sigma-coordinate are investigated: the pressure gradient calculation and the diffusion of tracers. Errors in the pressure gradient calculation are investigated by simulating an ocean at rest, and the choice is made to reduce errors by smoothing the bathymetry. A smoothing criterion is derived that permits a limitation of the errors in the pressure gradient calculation to an acceptable level (i.e. maximum errors on velocities below a millimeter per second). It is applied to define the model bottom topography. Errors in the tracer fields, induced by a diffusion scheme operating along constant sigma surfaces, generates large unrealistic velocities (of the order of 10 cm/s). A rotation of the diffusion tensor into geopotential coordinates is proposed. Tests show that errors are then reduced to an insignificant level. The rotation of the diffusion tensor is therefore retained. The numerical treatment of the open boundaries and the flux conditions that yields the most realistic circulation is also described. Open boundary conditions are based on radiation conditions and relaxation to climatology. They appear to be numerically robust, and to be able to bring into the South Atlantic basin the necessary information from the outer oceans. A configuration of the SPEM model to study the large scale circulation in the South Atlantic is then obtained. Errors due to model configuration are shown to be small compared to the signal one wants to simulate, and their spatial pattern is known, which will facilitate the interpretation of the model simulations presented in following papers.

A Numerical Study on the Influence of the Mid-Atlantic Ridge on Nonlinear First-Mode Baroclinic Rossby Waves Generated by Seasonal Winds

Abstract A numerical model simulation investigates the influence of the mid-Atlantic ridge on nonlinear first-mode baroclinic Rossby waves generated by seasonal wind fluctuations. The North Atlantic is simulated by a square-box, two-layer quasi-geostrophic (QG) model. The bottom topography is ridge-like and compromises the QG approximation and the actual shape of the ridge. Sponge layers protect all boundaries except the eastern one from wave reflection and eliminate the build-up of the western boundary current. The model is forced by a purely fluctuating wind stress curl derived from the most significant EOFs of the FGGE winds. A flat bottom and a ridge experiment are compared. In both experiments the eastern boundary is an important source of annual-period baroclinic Rossby waves. Wave trains having a wavelength of about 1060 km and a westward phase speed around 3.4 cm s−1 propagate energy westward at 3 cm s−1. In the flat bottom experiment a source of directly wind-forced baroclinic waves of annual per...

Agulhas eddy fluxes in a 1/6° Atlantic model

A 1/6° resolution primitive equation model of the Atlantic circulation is analyzed in the Agulhas region. The model has a realistic level of eddy kinetic energy, and produces anticyclonic Agulhas rings as well as cyclonic structures. In the model as well as in the data, ring trajectories undergo a transition between a turbulent character in the Cape Basin and a steady propagation in the rest of the South Atlantic. The topography of the Walvis Ridge does not seem to play a part in generating this contrast in the model. The model shows that cyclones are primarily generated from the negative shear vorticity side of the Agulhas Current as it leaves the coast, and they are most of the time paired with anticyclones in dipolar or tripolar structures. Contribution of Agulhas rings to the transports has been estimated by two methods, either focussing on the amount of water trapped inside the eddies and carried with them, or as a perturbation to the time-mean flow. The second estimate always produces smaller mass fluxes than the first. Even so, the transient eddy flux (2 Sv of warm water over the Agulhas Ridge) is very large when compared to parameterizations of eddy fluxes used in low-resolution climate models.

On the dynamics of the Zapiola Anticyclone

Recent observations obtained in the South Atlantic suggest the existence of a strong anticyclonic flow positioned over a major bottom topographic feature known as the Zapiola Drift. Here a numerical simulation of the South Atlantic is described in which this anticyclone is reproduced, and the model is used to diagnose the dynamics maintaining this flow. With a mean barotropic transport of 140 Sv and bottom velocities of 10 cm s−1, the simulated Zapiola Anticyclone compares well with in situ observations. Furthermore, the model surface eddy kinetic energy shows a local minimum over the drift, in agreement with observations from TOPEX/POSEIDON. Numerical experiments show that the circulation feature is sensitive to the intensity of the eddy field and to the particular value of the bottom friction. Both of these tendencies are in agreement with a theoretical explanation of the Zapiola Anticyclone that has recently appeared elsewhere. Thus we argue that the anticyclone is maintained by eddy-driven potential vorticity fluxes accelerating flow within topographically closed, ambient potential vorticity contours. As far as we know, this is the first South Atlantic simulation to reproduce the Zapiola Anticyclone in a realistic fashion. The quantitative success of this experiment is attributed to the use of a topography following (sigma) coordinate in a spatially well resolved model.

Water Mass Transformation in the North Atlantic and Its Impact on the Meridional Circulation: Insights from an Ocean Model Forced by NCEP–NCAR Reanalysis Surface Fluxes

Decadal-scale climate variability in the North Atlantic thermohaline circulation is simulated using a sigma-coordinate primitive equation model, forced by NCEP–NCAR reanalysis surface forcing fields for the period from 1958 to 1997. Surface heat and freshwater flux are expressed in terms of surface thermal and haline density inputs, diagnosed by the model. Variability in surface density fluxes is closely correlated with the North Atlantic Oscillation and demonstrates differences with the original surface heat and freshwater fluxes. Leading modes of surface water mass transformation are considered in the T–S plane. They identify decadal-scale variability associated with the transformation of the Labrador Sea Waters and Subtropical Mode Waters. Analysis of the model responses to the surface forcing shows an immediate reaction of meridional heat transport to the wind stress curl, resulting in a decrease of meridional heat transport at 48°N and an increase in the subtropics. Delayed baroclinic responses to the surface heat forcing are identified at time lags of 3 and 7 yr. The 3-yr response is represented by an increase in the total meridional heat transport in subpolar latitudes and its simultaneous increase in the Tropics and midlatitudes. The 7-yr delayed response to the surface heat forcing is associated with the strengthening of meridional heat transport at all latitudes. However, 7-yr responses may be influenced by the self-correlation in the meridional heat transport and forcing function. Meridional overturning is largely responsible for the variability observed, demonstrating high correlation with meridional heat transport.