Models of ocean : Wich ocean ?

Volume XII ISBN : 978-1-4020-3981-

Modélisation numérique pour l'océanographie physique.

International audienc

Evaluating eddy mixing coefficients from eddy-resolving ocean models: A case study.

International audienceA numerical model of a baroclinically unstable jet in a zonally periodic channel is used to analyze mesoscale eddy fluxes and their relationship with the gradients of the mean flow. The quasi-geostrophic approximation proves the best way to calculate potential vorticity fluxes in the primitive equation model. Away from the surface layers, eddy fluxes of potential density are consistent with advection by eddy-induced velocities v* and w* as suggested by Gent et al. (1995). Eddies mix potential vorticity along isopycnals, so that v* is related to the gradients of potential vorticity rather than potential density as implicitly assumed by Gent et al. The mixing coefficient for potential vorticity, associated with the advective component of the eddy fluxes, is found to be similar to the mixing coefficient of tracer anomalies on isopycnals. Both show a maximum at mid-depth below the jet core. The present calculations support the analysis of Treguier, Held and Larichev (1997) and encourage further attempts to derive a parameterization based on true mixing of potential vorticity

Remotely forced biweekly deep oscillations on the continental slope of the Gulf of Guinea.

International audienceCurrent meter measurements on the continental slope of the Gulf of Guinea (at 7°20′S and 1300 m depth) have revealed biweekly oscillations of the currents, bottom intensified and oriented along the bathymetry. We develop a three-dimensional primitive equation model of the Gulf of Guinea to study the oscillations and their forcing mechanism. The high resolution (1/12°) regional model reproduces remarkably well the main characteristics of the deep currents on the continental slope. Experiments with different forcings demonstrate that the biweekly variability at 1300 m depth is remotely forced by equatorial winds. Deep Yanai waves generated by the wind propagate eastward along the equator. Upon reaching the African coast, the energy propagates poleward in both directions as coastal trapped waves. The selection of the dominant biweekly period is explained by the absence of equatorial waves with westward group velocities in that frequency band. Contrary to a previous hypothesis involving tidal forcing, our interpretation is coherent with the significant interannual variability of the biweekly energy

Seasonal fluctuations in the deep central equatorial Atlantic Ocean: a data–model comparison.

International audienceLow-frequency current fluctuations in the deep central equatorial Atlantic are analyzed using current meter measurements recorded from November 1992 to November 1994. Current meters were located at about 14°W of longitude and 1° of latitude on both sides of the equator between 1,700 m depth and the ocean bottom. At all sampling depths, the velocity fluctuations are dominantly zonal and symmetrical with respect to the equator. At 1,700 and 2,000 m, the flow is dominated by annual period fluctuations, at 3,000 m, the velocity field amplitude presents a minimum, and at 3,750 and 3,950 m, the flow is modulated by annual and semiannual period variability. The annual signal exhibits an apparent upward phase propagation. When considering the phase and the amplitude of the seasonal fluctuations, the data compare well with the outputs of a realistic numerical simulation of the Atlantic Ocean. Together with a previous analysis of the model simulations, this supports the idea that the observed annual fluctuations are due to wind-forced vertically propagating Kelvin and Rossby waves. Data and model do not provide deciding evidences of the presence of semiannual equatorial waves deeper than 3,500 m depth in the central equatorial Atlantic Ocean

Model-inferred upper ocean circulation in the eastern tropics of the North Atlantic.

International audienceThe seasonal regime associated with the eastern part of the cyclonic tropical gyre in the North Atlantic is studied using a high-resolution model. The model reproduces a vertical water mass transition (between South and North Atlantic Central Water) previously observed near σ0=26.8, which approximately coincides with the base of the North Equatorial Undercurrent (NEUC). The same density may be taken as the lower limit of the large-scale cyclonic flow. The gyre southern flank is formed of the North Equatorial Countercurrent (NECC) in the near-surface layer, and of the NEUC and deep NECC (when this current is developed) in the Central Water. Its poleward limb is made up from the Ekman drift of the trade winds, a northeastward extension of the NEUC/deep NECC, and reinforced by an extension of the Guinea undercurrent along the African continental slope. Model experiments with two different wind forcings were carried out to estimate the transports across 12°N, an approximate gyre central latitude. The time-averaged transport for densities lower than σ1=32.3 and to the east of 35°W is around 4 Sv northward, about one-third of the basin-wide net value which measures the Meridional Overturning Circulation. The near-surface layer accounts for about 3 Sv, and the Central Water for more than 1 Sv, in keeping with the presence of South Atlantic Central Water in the tropical eastern basin. The eastern basin near-surface flow is lowest in summer, when the trade winds nearly vanish at this latitude. In this season the NECC, though having entered its phase of intensification, leaves the region of positive wind stress curl, and temporarily does not act as the tropical gyre southern limb. At the subsurface, most of the NEUC folds up westward at not, vert, similar10°N in winter, thus forming an inner circulation of the basin-wide gyre, previously reported from observations. The cyclonic flow associated with the Guinea Dome is another inner circulation, which the model found to be permanent below the surface down to 400–500 m, but masked in the Ekman layer, except in summer

Impact of sub-mesoscale physics on production and subduction of phytoplankton in an oligotrophic regime.

International audienceUsing a protocol of numerical experiments where horizontal resolution is progressively increased, we show that small-scale (or sub-mesoscale) physics has a strong impact on both mesoscale physics and phytoplankton production/subduction. Mesoscale and sub-mesoscale physics result from the nonlinear equilibration of an unstable baroclinic jet. The biogeochemical context is oligotrophy. The explicitly resolved sub-mesoscales, at least smaller than one fifth of the internal Rossby radius of deformation, reinforce the mesoscale eddy field and contribute to double the primary production and phytoplankton subduction budgets. This enhancement is due to the reinforced mesoscale physics and is also achieved by the small-scale frontal dynamics. This sub-mesoscale physics is associated with density and vorticity gradients around and between the eddies. It triggers a significant small-scale nutrient injection in the surface layers, leading to a phytoplankton field mostly dominated by fine spatial structures. It is believed that, depending on wind forcings, this scenario should work alternately with that of Abraham (1998) which invokes horizontal stirring of nutrient injected at large scales. Results also reveal a strong relationship between new production and negative vorticity, in the absence of wind forcing and during the period of formation of the eddies

Aliasing inertial oscillations in a 1/6° Atlantic circulation model: impact on the mean meridional heat transport

A 1/6° model simulation of the Atlantic ocean forced with daily fluxes from ECMWF (re-analysis 1979–1993 and analysis 1994–1999) has been carried out within the Clipper project. A storage strategy which filters out inertial oscillations is defined: five-day mean fields are continuously stored at five-day intervals. It is shown that aliasing errors on the monthly mean meridional heat transport (MHT, a second-order moment) are negligible in that case. These errors are of the order of 0.8 PW in the tropics in the case of a sampling strategy based on instantaneous fields stored every five days, even in the case where step-like variations in the forcing are avoided by an interpolation of the daily wind stress to the model time step. It is also shown that aliasing errors on the annual mean MHT can be as large as 0.2 PW in the tropics in the case of sub-sampling with instantaneous fields

Numerical study of the annual and semi-annual fluctuations in the deep equatorial Atlantic Ocean

International audienceUsing idealized and realistic high-resolution primitive equation models, we explore the annual and semi-annual period fluctuations in the deep equatorial Atlantic Ocean. The deep seasonal variability derives its energy from the equatorial zonal wind stress component and the wind energy reaches the deep layers by means of vertically propagating Kelvin and Rossby waves. The first meridional mode (l=1) annual Rossby wave reaches the western boundary while the other Rossby waves directly propagate toward the bottom. A deep Kelvin wave is generated by the reflection of the incoming l=1 annual Rossby wave at the western boundary. Due to the complexity of the surface layers in the non-linear simulations, one cannot separate the waves generated at the boundaries and the waves directly forced by the wind. Below the thermocline, the waves characteristics agree well with the linear theory when considering the Kelvin and the l=1 Rossby wave but discrepancies occur for the l=3 and 5 Rossby waves. Non-linearities explain that minima reached by the zonal velocity anomaly is larger, in absolute value, than maxima. An important result of that study concerns the enhancement of the semi-annual cycle compared to the annual signal in the equatorial Atlantic Ocean. A resonance phenomenon occurs even when realistic coastline geometry, dissipation and non-linear terms are taken into account. Along with the wind-stress amplitude, the forcing period and the basin width, the horizontal dissipation controls the amplitude of the deep seasonal fluctuations. The waves are damped during their propagation when the lateral dissipation coefficient is set to a high value (νh=2000 m2 s−1). In an “inviscid” regime (when the waves reach the other side of the basin before being damped), the amplitude of the deep reponse is not much influenced by the parameterization chosen. Finally, bottom reflections are not necessary for the set up of the deep response and the Mid-Atlantic Ridge does not change the nature of the deep signal