Cirene : air-sea iInteractions in the Seychelles-Chagos thermocline ridge region

Author Posting. © American Meteorological Society, 2009. This article is posted here by permission of American Meteorological Society for personal use, not for redistribution. The definitive version was published in Bulletin of the American Meteorological Society 90 (2009): 1337-1350, doi:10.1175/2008BAMS2499.1.The Vasco—Cirene program ex-plores how strong air—sea inter-actions 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 oceano-graphic cruise collected oceanic, atmospheric, and air—sea flux observations in this region in Jan-uary—February 2007. The contem-poraneous Vasco field experiment complemented these measure-ments with balloon deployments from the Seychelles. Cirene also contributed to the development of the Indian Ocean observing system via deployment of a moor-ing and 12 Argo profilers. Unusual conditions prevailed in the Indian Ocean during Janu-ary 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 currents prevailed down to 800 m. These anomalous conditions had a major impact on tuna fishing in early 2007. Our dataset also sampled the genesis and maturation of Tropical Cyclone Dora, including high surface temperatures and a strong diurnal cycle before the cyclone, followed by a 1.5°C cool-ing over 10 days. Balloonborne instruments sampled the surface and boundary layer dynamics of Dora. We observed small-scale structures like dry-air layers in the atmosphere and diurnal warm layers in the near-surface ocean. The Cirene data will quantify the impact of these finescale features on the upper-ocean heat budget and atmospheric deep convection.CNES funded the Vasco part of the experiment; INSU funded the Cirene part. R/V Suroît is an Ifremer ship. The contributions from ODU, WHOI, and FOI (Sweden) are supported by the National Science Foundation under Grant Number 0525657. The participation of the University of Miami group was funded though NASA (NNG04HZ33C). PMEL participation was supported through NOAA’s Office of Climate Observation

Characterization of turbulence from a fine-scale parameterization and microstructure measurements in the Mediterranean Sea during the BOUM experiment

One of the main purposes of the BOUM experiment was to find evidence of the possible impact of submesoscale dynamics on biogeochemical cycles. To this aim physical as well as biogeochemical data were collected along a zonal transect through the western and eastern basins of the Mediterranean sea. Along this transect 3 day fixed point stations were performed within anticyclonic eddies during which microstructure measurements of the temperature gradient were collected over the top 100m of the water column. We focus here on the characterization of turbulent mixing

Ondes internes générées par le forçage atmosphérique dans l'océan indien subtropical (modélisation et mesures in situ)

Le déferlement des ondes internes est un des principaux mécanismes de mélange turbulent dans l océan et joue un rôle clé sur la dynamique à plus grande échelle. L objectif de ce travail était de proposer une amélioration de la représentation du mélange turbulent dans les modèles de circulation générale.La première partie porte sur l'analyse des mesures in situ de la campagne CIRENE dans l'océan Indien subtropical. Le passage d'un cyclone a conduit à la génération d'une onde de fréquence proche-inertielle dont nous avons pu suivre la propagation en profondeur.La seconde partie est consacrée à l'analyse des simulations forcées et couplées du modèle NEMO. Dans tous les cas, on observe la génération d'ondes internes dans la bande de fréquence proche-inertielle suite à des coups de vent, cependant leur propagation est limitée en profondeur. Cette atténuation est attribuée à une trop forte valeur de la dissipation déterminée par le schéma TKE. Une paramétrisation spécifique déduite de paramétrisations fine-échelle est proposée.PARIS-BIUSJ-Sci.Terre recherche (751052114) / SudocSudocFranceF

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

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

Processus de mélange turbulent au niveau de la dorsale Médio-Atlantique

Dans l océan, le mélange turbulent, bien qu ayant lieu à petite échelle, a un impact important sur la circulation thermohaline et le transport de chaleur. Le mélange turbulent le plus intense est observé dans les régions de topographie accidentée, telles que les dorsales océaniques. La thèse porte sur la caractérisation des différents processus dynamiques responsables du mélange turbulent au niveau du site Lucky Strike, situé à 37N sur la dorsale Médio Atlantique. Le site se compose de deux bassins semi-isolés du reste de l'océan et localisés au sein de la vallée de la dorsale. Ces deux bassins sont reliés entre eux par un passage au milieu duquel se trouve un volcan sous marin, créant ainsi deux passages étroits à ses côtés. Différents processus dynamiques à l'origine du mélange turbulent ont été identifiés à partir de mesures in situ. Au niveau du seuil, les données de deux mouillages montrent que la dynamique en profondeur est caractérisée par un écoulement basse fréquence fortement cisaillé, ainsi que des épisodes intermittents de marée interne semi-diurne. Les mesures récoltées dans les 500 premiers mètres quant à elles mettent en évidence les instabilités d'un champ d'ondes internes. La première étape afin de déterminer la contribution de ces processus dynamiques au mélange turbulent a été de valider les paramétrisations fine-échelle du mélange turbulent à partir de mesures microstructure. Ces paramétrisations ont ensuite permis d'estimer la variabilité du mélange turbulent à partir des données de mouillage. Les estimations de la dissipation turbulente sont de l'ordre de 10^-8 W/kg, soit deux ordres de grandeur plus élevées que les valeurs rencontrées dans l'océan intérieur.PARIS-BIUSJ-Sci.Terre recherche (751052114) / SudocSudocFranceF

Stratified Inertial Subrange Inferred from In Situ Measurements in the Bottom Boundary Layer of the Rockall Channel

International audienceDeep-ocean high-resolution moored temperature data are analyzed with a focus on superbuoyant frequencies. A local Taylor hypothesis based on the horizontal velocity averaged over 2 h is used to infer horizontal wavenumber spectra of temperature variance. The inertial subrange extends over fairly low horizontal wavenumbers, typically within 2 × 10−3 and 2 × 10−1 cycles per minute (cpm). It is therefore interpreted as a stratified inertial subrange for most of this wavenumber interval, whereas in some cases the convective inertial subrange is resolved as well. Kinetic energy dissipation rate ϵ is inferred using theoretical expressions for the stratified inertial subrange. A wide range of values within 10−9 and 4 × 10−7 m2 s−3 is obtained for time periods either dominated by semidiurnal tides or by significant subinertial variability. A scaling for ϵ that depends on the potential energy within the inertio-gravity waves (IGW) frequency band PEIGW and the buoyancy frequency N is proposed for these two cases. When semidiurnal tides dominate, ϵ ≃ (PEIGWN)3/2, whereas ϵ ≃ PEIGWN in the presence of significant subinertial variability. This result is obtained for energy levels ranging from 1 to 30 times the Garrett–Munk energy level and is in contrast with classical finescale parameterization in which ϵ ∼ (PEIGW)2 that applies far from energy sources. The specificities of the stratified bottom boundary layer, namely a weak stratification, may account for this difference

Near-inertial energy propagation inside a Mediterranean anticyclonic Eddy

International audienceMotivated by observations of a strong near-inertial wave signal at the base of the semipermanent anticyclonicCyprus Eddy during the 2010 Biogeochemistry from the Oligotrophic to the Ultraoligotrophic Mediterranean(BOUM) experiment, a numerical study is performed to investigate the role of near-inertial/eddy interactions inenergy transfer out of the mixed layer. A hybrid temporal–spatial decomposition is used to split all variables intothree independent components: slow (eddy) and fast (inertial oscillations 1 waves), which proves useful inunderstanding the flow dynamics. Through a detailed energy budget analysis, we find that the anticyclonic eddyacts as a catalyst in transferring wind-driven inertial energy to propagating waves. While the eddy sets the spatialscales of the waves, it does not participate in any energy exchange. Near-inertial propagation through the eddycore results in the formation of multiple critical levels with the largest accumulation of wave energy at the base ofthe eddy. A complementary ray-tracing analysis reveals critical-level formation when the surface-confined inertial rays originate within the negative vorticity region. In contrast, rays originating outside this region focus atthe base of the eddy and can propagate at depth