Pascale Bouruet-Aubertot

Cirene: Air—Sea Interactions in the Seychelles—Chagos Thermocline Ridge Region

The Vasco-Cirene program explores how strong air-sea interactions promoted by the shallow thermocline and high sea surface temperature in the Seychelles-Chagos thermocline ridge results in marked variability at synoptic, intraseasonal, and interannual time scales. The Cirene oceanographic cruise collected oceanic, atmospheric, and air-sea flux observations in this region in January–February 2007. The contemporaneous Vasco field experiment complemented these measurements with balloon deployments from the Seychelles. Cirene also contributed to the development of the Indian Ocean observing system via deployment of a mooring and 12 Argo profilers. Unusual conditions prevailed in the Indian Ocean during January and February 2007, following the Indian Ocean dipole climate anomaly of late 2006. Cirene measurements show that the Seychelles-Chagos thermocline ridge had higher-than-usual heat content with subsurface anomalies up to 7°C. The ocean surface was warmer and fresher than average, and unusual eastward cur...

Impact of the spatial distribution of the atmospheric forcing on water mass formation in the Mediterranean Sea

The impact of the atmospheric forcing on the winter ocean convection in the Mediterranean Sea was studied with a high-resolution ocean general circulation model. The major areas of focus are the Levantine basin, the Aegean-Cretan Sea, the Adriatic Sea, and the Gulf of Lion. Two companion simulations differing by the horizontal resolution of the atmospheric forcing were compared. The first simulation (MED16-ERA40) was forced by air-sea fields from ERA40, which is the ECMWF reanalysis. The second simulation (MED16-ECMWF) was forced by the ECMWF-analyzed surface fields that have a horizontal resolution twice as high as those of ERA40. The analysis of the standard deviations of the atmospheric fields shows that increasing the resolution of the atmospheric forcing leads in all regions to a better channeling of the winds by mountains and to the generation of atmospheric mesoscale patterns. Comparing the companion ocean simulation results with available observations in the Adriatic Sea and in the Gulf of Lion shows that MED16-ECMWF is more realistic than MED16-ERA40. In the eastern Mediterranean, although deep water formation occurs in the two experiments, the depth reached by the convection is deeper in MED16-ECMWF. In the Gulf of Lion, deep water formation occurs only in MED16-ECMWF. This larger sensitivity of the western Mediterranean convection to the forcing resolution is investigated by running a set of sensitivity experiments to analyze the impact of different time-space resolutions of the forcing on the intense winter convection event in winter 1998-1999. The sensitivity to the forcing appears to be mainly related to the effect of wind channeling by the land orography, which can only be reproduced in atmospheric models of sufficient resolution. Thus, well-positioned patterns of enhanced wind stress and ocean surface heat loss are able to maintain a vigorous gyre circulation favoring efficient preconditioning of the area at the beginning of winter and to drive realistic buoyancy loss and mixing responsible for strong convection at the end of winter.

Breaking of standing internal gravity waves through two-dimensional instabilities

The evolution of an internal gravity wave is investigated by direct numerical computations. We consider the case of a standing wave confined in a bounded (square) domain, a case which can be directly compared with laboratory experiments. A pseudo-spectral method with symmetries is used. We are interested in the inertial dynamics occurring in the limit of large Reynolds numbers, so a fairly high spatial resolution is used (129 2 or 257 2 ), but the computations are limited to a two-dimensional vertical plane. We observe that breaking eventually occurs, whatever the wave amplitude: the energy begins to decrease after a given time because of irreversible transfers of energy towards the dissipative scales. The life time of the coherent wave, before energy dissipation, is found to be proportional to the inverse of the amplitude squared, and we explain this law by a simple theoretical model. The wave breaking itself is preceded by a slow transfer of energy to secondary waves by a mechanism of resonant interactions, and we compare the results with the classical theory of this phenomenon: good agreement is obtained for moderate amplitudes. The nature of the events leading to wave breaking depends on the wave frequency (i.e. on the direction of the wave vector); most of the analysis is restricted to the case of fairly high frequencies. The maximum growth rate of the inviscid wave instability occurs in the limit of high wavenumbers. We observe that a well-organized secondary plane wave packet is excited. Its frequency is half the frequency of the primary wave, corresponding to an excitation by a parametric instability. The mechanism of selection of this remarkable structure, in the limit of small viscosities, is discussed. Once this secondary wave packet has reached a high amplitude, density overturning occurs, as well as unstable shear layers, leading to a rapid transfer of energy towards dissipative scales. Therefore the condition of strong wave steepness leading to wave breaking is locally attained by the development of a single small-scale parametric instability, rather than a cascade of wave interactions. This fact may be important for modelling the dynamics of an internal wave field

Stratified turbulence produced by internal wave breaking: two-dimensional numerical experiments

Abstract In an earlier study (Bouruet-Aubertot et al., J. Fluid Mech., 285: 265–301, 1995) the evolution of a large-scale, high-frequency, standing internal gravity wave was investigated by means of two-dimensional numerical computations. It was shown that breaking eventually occurs, whatever the wave amplitude. The instability mechanisms leading to breaking were analysed in detail. In the present paper, we study the strongly nonlinear regime resulting from wave breaking. Buoyancy and velocity variance spectra display a wavenumber range scaling as 0.2N2ky−3 and 0.1N2ky−3 respectively (N being the Brunt-Vaisala frequency and ky the vertical wavenumber). The buoyancy variance spectrum has the same power law and level as in a laboratory experiment performed on the same subject by Benielli and Sommeria (Dyn. Atmos. Oceans, 23: 335–343, 1996), and is very close to fine-scale oceanic temperature spectra. The buoyancy flux spectrum appears to be counter-gradient at small scales, confirming results from previously published numerical simulations. We propose a physical interpretation for this small-scale behaviour. Finally, the spectrum of the buoyancy flux is compared with the kinetic energy and buoyancy variance spectra, to quantify its role in energy transfers. This point is essential for deciding between the theories proposed to explain the ky spectral range, which is also observed in the stratosphere.

Hydrography and flow in the Lucky Strike segment of the Mid-Atlantic Ridge

The Lucky Strike segment between 37 and 38N on the Mid-Atlantic Ridge is the focus of the international MoMAR program to monitor seafloor-spreading processes. During the GRAVILUCK cruise in August 2006, hydrographic, velocity and light-scattering data were collected in the rift valley at Lucky Strike in order to investigate the regional dynamics and to provide background information for the monitoring effort. The survey observations reveal a remarkably simple dynamical setting dominated by persistent northward flow transporting approximate to 0.2 Sv of water along the rift valley. Approximately half of this transport must upwell within a deep basin that occupies the northern half of the segment. In the comparatively shallow segment center, the along-valley transport takes place in two parallel, hydraulically controlled overflows on both sides of an active volcano that rises from the rift-valley floor. Within the better sampled of these overflows instantaneous velocities recorded during the Survey were northward more than 95% of the time and occasionally exceeded 20 cm s(-1). Similar to other laterally confined overflows in the deep ocean, the cross-sill density gradients are characterized by a lower layer with streamwise decreasing densities and an upper layer where the densities increase along the path of the flow. This vertical density-gradient dipole is the signature of the buoyancy flux associated with high levels of diapycnal mixing near the sill. Overall, the hydrography and dynamics in the rift valley of the Lucky Strike segment are highly reminiscent of many ridge-flank canyons in the western South Atlantic, where mean along-axial advection of density is balanced by vigorous diapycnal mixing. There is circumstantial evidence from historic hydrographic data Suggesting that northward flow below approximate to 1800 m in the rift valley in the MoMAR region is persistent on time scales of years to decades and that it extends more than 200 km to the south...

Nonlinear adjustment of density fronts. Part 1. The Rossby scenario and the experimental reality

This study deals with the nonlinear cyclo-geostrophic adjustment of a circular density front in a two-layer fluid. Laboratory experiments have been performed to investigate the dynamical evolution of a fixed volume of buoyant water, initially confined within a bottomless cylinder, which is quickly released in a dense rotating fluid. This configuration corresponds to a rapid input of potential energy in a geostrophic fluid layer and reproduces some dynamical processes which occur during oceanic upwelling or stratospheric warming events. We focus our efforts on the visualization techniques in order to have simultaneous and independent measurements of both the horizontal velocity field and the vertical density field. We thus obtained, for the first time, quantitative measurements of the potential vorticity and the flow balance after a geostrophic adjustment process. The density profile of the mean adjusted state observed in the experiment is in good agreement with the prediction of the standard adjustment theory based on Lagrangian conservation of potential vorticity except in the frontal region. There, strong three-dimensional motions (plume structures, shocks and rapid transient instabilities) take place during the early stage of adjustment. These transient three-dimensional motions could dissipate up to 50% of the initial energy of the system, especially when the size of the initial density anomaly is close to or larger than the deformation radius. Therefore, it significantly changes the velocity and the energy budget predicted by the standard Rossby adjustment. Both the kinetic energy of the mean adjusted state and the energy transferred to inertia-gravity wave modes are reduced by these transient dissipative processes.

Impact of penetrative solar radiation on the diagnosis of water mass transformation in the Mediterranean Sea

[1] We applied a revised diagnosis of water mass formation and mixing to a 1/8° resolution ocean model of the Mediterranean Sea. The diagnosis method used and presented by Iudicone et al. (2008) is similar to that developed by Walin (1982) and applied to the Mediterranean Sea by Tziperman and Speer (1994), to which we added a penetrative solar radiation. Both the prognostic model and the diagnostic method were in agreement with respect to the solar flux parameterization. Major changes were observed in the yearly budget of water mass transformation when the penetrative solar radiation is taken into account in the diagnosis. Annual estimates of water mass formation rates were decreased by a factor of two, with values within the range [-3.7 Sv, 1.5 Sv] compared to [-6 Sv, 3 Sv]. This decrease resulted from a lower seasonal variation when penetrative solar radiation was included. This can be explained by the fact that the solar radiation flux acted over a wider range of seawater density leading to lower net values over a given density interval. The major impact of the penetrative solar radiation occurred during spring and summer. Newly formed dense water was then transformed into lighter water with a rate reaching a value about 50% of that of the water mass formation rate in winter. Another consequence was that mixing processes which counteract formation rate in yearly budget of water mass formation rates, were overestimated. We showed that, in spring and summer, about a third of the transformation took place below the surface layer.

The influence of the coast on the dynamics of upwelling fronts: Part I. Laboratory experiments

We describe laboratory experiments on the instability and later evolution of a front in a two-layer rotating fluid. In particular, we focus on the influence of a nearby boundary on instability growth and eddy formation. The front is produced through the adjustment of a buoyant fluid initially confined within a bottomless cylinder. Typically a front in quasi-cyclostrophic balance establishes after two rotation periods, after which it becomes unstable. Measurements of the velocity and vorticity fields at the surface are made which provide detailed information on the evolution of the front as the instability grows to finite amplitude. We focus on the time evolution of the vorticity and distinguish between the cyclonic and anticyclonic components. The spatial averages of the cyclonic and anticyclonic vorticity first grow exponentially. This growth saturates when eddies form and are advected across the front. The growth rate depends upon two nondimensional parameters: the width W of the upwelling region in units of the internal radius of deformation and the depth ratio δ between the two layers. Measurements of the growth rates for the average of the cyclonic and anticyclonic vorticity are compared to the values inferred from a simplified model for baroclinic instability. A good agreement is obtained when the front develops far from the boundary (i.e. W >> 1). However, the agreement is only qualitative when the front is near the boundary (i.e. W ∼ 1). We find that, as W decreases, the growth of cyclonic eddies consisting of dense-coastal-water is enhanced compared to that of anticyclonic vorticity consisting of buoyant-off-shore-water. This crucial effect of the boundary with respect to the instability of the front has significant impact on exchanges across the front.

Mixing in weakly turbulent stably stratified flows

We analyse mixing in three weakly turbulent stably stratified flows, namely low amplitude breaking internal gravity waves, stably stratified homogeneous decaying turbulence and a stably stratified unstable shear layer. We use the method proposed by Winter et al. [J. Fluid Mech. 289 (1995) 115], which provides an exact expression of the diffusive flux of density responsible for mixing. When the three flows have stabilized and organized into quasi-horizontal layers, we show that the diffusion coefficient behaves linearly as a function of a dynamical parameter that characterizes the largest scales of the flow (a Froude number squared or the inverse of a Richardson number) and we provide a simple expression of this linear law.

Temperature Measurements from Surface Drifters

Abstract The accuracy of temperature measurements from drifters is first examined for 16 drifters (manufactured either by Metocean Data Systems or by Pacific Gyre) deployed with two temperature sensors in the tropical or North Atlantic Ocean. One of these sensors is the SST thermistor commonly used on Surface Velocity Program (SVP) drifters since the late 1980s; whereas the other sensor is a platinum temperature probe associated with a Seabird conductivity cell. The authors find (for 19 separate deployments) an average positive offset of the SST thermistor measurements in 17 out of 19 cases, exceeding 0.1°C in five instances. Among the five drifters that were at sea for a year or more, two present a large trend in this offset (0.10° and −0.10°C yr−1); and in two other cases, there is a clear annual cycle of the offset, suggesting a dependency on temperature. Offsets in 9 out of 12 drifters with sea time longer than 4 months present a negative trend, but the average trend is not significantly different fro...