Добірка наукової літератури з теми "Stratification thermohaline"

Physical and numerical experiments were performed for a linearly stratified heat—salt system, uniformly heated at one endwall. The initial stratification was in the diffusive sense. Intrusions formed at the heated endwall and propagated out into the interior fluid. Three classes of flow were identified, based upon the gravitational stability ratio, Rp, and a lateral stability parameter, R1, For R1 > 1, a vertical lengthscale for the initial intrusion thickness was developed which agreed well with that observed in the physical experiments. In all cases, a region of salt fingering developed due to gradient reversal at the heated endwall. Two very distinct merging processes were observed depending on the specific flow class. The first process occurred under conditions of high gravitational and lateral stability, and appeared to be controlled by horizontal motions induced by the intrusions. The second process was observed under less stable conditions and was a result of vertical motions at the heated endwall within the intrusions themselves. In the least stable class of flow (low gravitational and lateral stability), the intrusions were found to be self-perpetuating in the sense that they continued to propagate following removal of the endwall heat flux.

A theoretical model is developed which illustrates the dynamics of layering instability, frequently realized in ocean regions with active fingering convection. Thermohaline layering is driven by the interplay between large-scale stratification and primary double-diffusive instabilities operating at the microscale – temporal and spatial scales set by molecular dissipation. This interaction is described by a combination of direct numerical simulations and an asymptotic multiscale model. The multiscale theory is used to formulate explicit and dynamically consistent flux laws, which can be readily implemented in large-scale analytical and numerical models. Most previous theoretical investigations of thermohaline layering were based on the flux-gradient model, which assumes that the vertical transport of density components is uniquely determined by their local background gradients. The key deficiency of this approach is that layering instabilities predicted by the flux-gradient model have unbounded growth rates at high wavenumbers. The resulting ultraviolet catastrophe precludes the analysis of such basic properties of layering instability as its preferred wavelength or the maximal growth rate. The multiscale model, on the other hand, incorporates hyperdiffusion terms that stabilize short layering modes. Overall, the presented theory carries the triple advantage of (i) offering an explicit description of the interaction between microstructure and layering modes, (ii) taking into account the influence of non-uniform stratification on microstructure-driven mixing, and (iii) avoiding unphysical behaviour of the flux-gradient laws at small scales. While the multiscale approach to the parametrization of time-dependent small-scale processes is illustrated here on the example of fingering convection, we expect the proposed technique to be readily adaptable to a wide range of applications.

A high degree of variation in hydrographic conditions is found in the so-called Iroise Sea, within less than 100 km of the west coast of Brittany. Tidal current maximal velocity, especially, ranges there from about 0·5 knot to more than 8 knots (locally, near the island of Ushant), i.e. practically as wide a range as found over the whole of north-west European shelf seas. Pelagic ecosystems accordingly exhibit a high degree of variety, related not only to classical inshore-offshore gradients, but also to the extent of vertical mixing or stratification. Areas where different physical and biological conditions prevail are generally separated by rather clearcut boundaries. The better-known of these is the Ushant thermal front, which runs in summer across the whole entrance to the English Channel, but also extends into the Iroise. In addition, freshwater runoff results in thermohaline stratification, or at least in the existence of thermohaline vertical gradients, in the two major bays of the west coast of Brittany. The relevant area is limited seawards by a thermohaline front, the Iroise inner front (Grail & Le Fèvre, 1967; Le Fèvre & Grall, 1970), beyond which are found the well-mixed waters inshore of the Ushant front. Fig. 1 sums up these hydrographic patterns in the area taken here into consideration.

In this study we examine two-component shear flows that are stable with respect to Kelvin–Helmholtz and to double-diffusive instabilities individually. Our focus is on diffusively stratified ocean regions, where relatively warm and salty water masses are located below cool fresh ones. It is shown that such systems may be destabilized by the interplay between shear and thermohaline effects, caused by unequal molecular diffusivities of density components. Linear stability analysis suggests that parallel two-component flows can be unstable for Richardson numbers exceeding the critical value for non-dissipative systems $(Ri=1/4)$ by up to four orders of magnitude. Direct numerical simulations indicate that these instabilities transform the initially linear density stratification into a series of well-defined horizontal layers. It is hypothesized that the combined thermohaline–shear instabilities could be ultimately responsible for the widespread occurrence of thermohaline staircases in diffusively stable regions of the World Ocean.

Abstract. We present and analyze high-resolution observational data of thermohaline structure and currents acquired in the Gulf of Finland (Baltic Sea), using an autonomous buoy profiler and bottom-mounted acoustic Doppler current profiler during July–August 2009. Vertical profiles of temperature and salinity were measured in the upper 50-m layer with a 3 h time resolution, and vertical profiles of current velocity and direction were recorded with a 10 min time resolution. Although large temporal variations of vertical temperature and salinity distributions were revealed, it was possible to define several periods with quasi-stationary vertical thermohaline structure. These quasi-stationary stratification patterns persisted for 4–15 days and were dominated by certain physical processes: upwelling, relaxation of upwelling, estuarine circulation and its wind-induced reversal, and downwelling. Vertical profiles of current velocities supported the concept of synoptic-scale, quasi-stationary periods of hydrophysical fields, characterized by distinct layered flow structures and current oscillations. To estimate the contribution of different processes to the changes in stratification, a simple conceptual model was developed. The model accounts for heat flux through the sea surface, wind mixing, wind-induced transport (parallel to the horizontal salinity gradient) in the upper layer, and estuarine circulation. It reproduced observed changes in vertical stratification reasonably well. The largest discrepancies between observations and model results were found when water motions across the Gulf and associated vertical displacements of isopycnals (upwelling or downwelling) were dominant processes.

Au moyen de simulations numériques, ce travail identifie les mécanismes qui pilotent le mélange au sein d'une stratification thermique et massique. Le mélange est provoqué par la convection naturelle induite au sein d'un liquide contenu dans un réservoir cylindrique vertical et chauffe latéralement. La stratification se présente initialement sous la forme de deux couches séparées par une interface diffusive d'épaisseur finie, la couche inferieure étant plus chaude et plus concentrée en soluté. La principale motivation de cette étude est la compréhension du phénomène de rollover, un mélange au sein des réservoirs de gaz naturel liquéfié (GNL) induisant une soudaine augmentation de l'évaporation à la surface libre. Le premier chapitre fait le point des connaissances sur le phénomène de rollover. Le deuxième chapitre propose une description du système en termes de groupements sans dimensions. L'épineux problème du respect des conditions de similitude est abordé. Le troisième chapitre expose les méthodes numériques employées dans les simulations. L'accent est mis sur les problèmes numériques qui constituent le premier obstacle à la réalisation de la similitude. Le délicat problème de viscosité artificielle, récurrent en simulation numérique, est caractérisé quantitativement dans la configuration qui nous intéresse. Ces considérations numériques sont suivies d'aspects physiques par l'étude d'une part d'un mélange dans l'eau salée, d'autre part d'un mélange dans le GNL. Le quatrième et dernier chapitre est axé sur l'évolution du GNL avec évaporation en surface libre. Des structures convectives originales sont mises en évidence. Ce chapitre propose une vision du rollover dont on espère qu'elle puisse inspirer les modélisateurs du rollover.

Dans le Golfe de Guinée (GG), les masses d’eau douce provenant des décharges des fleuves et les taux de précipitations élevés contribuent à la stratification en densité de la couche superficielle océanique, et jouent un rôle clé dans la modulation des interactions air-mer. Cependant, les variations thermohalines des couches superficielles au sein des panaches d’eau douce du GG sont encore mal connues, car très peu observées et documentées. L’objectif principal de cette thèse est donc d’étudier et de documenter la variabilité spatiale à mésoéchelle horizontale (10-100 km) et verticale (0-100m), intra-saisonnière à saisonnière de la structure 3D thermohaline dans les panaches d’eau douce du GG, et notamment les panaches des fleuves Congo et Niger. Tout d’abord, à l’aide des données d’observations satellite SSS SMOS, notre étude a montré que les panaches d’eau douce dans cette région s’étendent vers l’océan du large suivant deux régimes de propagation. Durant la période de septembre à janvier, ils se propagent vers le large en direction Nord-Ouest tandis que de janvier à avril, ils se redirigent vers le Sud-Ouest, où leur extension maximale est observée en avril. Le reste de l’année, de mai à août, est marqué par un épisode de salinisation de surface, où les panaches d’eau douce se dissipent avec une extension minimale observée en août. L’analyse du bilan de salinité dans la couche mélangée de surface a permis de mettre en évidence les principaux processus physiques contrôlant la variabilité saisonnière de la salinité au sein de ces panaches d’eau douce. Ce diagnostic a montré que les processus d’advection horizontale et les flux d’eau douce associés aux précipitations et aux décharges des fleuves expliquent principalement de la distribution offshore des masses d’eau de faible salinité dans cette région. Dans le panache du Congo en particulier, l’advection horizontale de salinité est principalement expliquée par la dérive d’Ekman du vent de surface. Ensuite, nous avons montré que la distribution offshore du panache du Congo aux échelles intra-saisonnières est associée à des couches de barrière de sel d’une part, et à des profils verticaux de densité en marches d’escalier d’autre part. Dans une étude de cas (au 31/03/216), nous avons montré que la stratification thermohaline en marches d’escalier observée, résulterait de la dynamique de cisaillement entre le flux d’Ekman de surface associée à la distribution offshore (Nord-Ouest) du panache du Congo, et le flux géostrophique (Sud-Est) associé aux masses d’eau de subsurface de l’océan ouvert à l’Ouest, plus denses et plus salées. Enfin, à partir d’une approche lagrangienne, nous avons mis en évidence l’origine et la structuration à grande échelle des masses d’eau impliquées dans la forte stratification haline observée au large du Congo. Cette étude a montré le fort cisaillement des courants à l’oeuvre au niveau des gradients halins au sein de la colonne d’eau associée à ces profils
In the Gulf of Guinea (GG), freshwater originated from river discharges and high precipitation rates contribute to the upper ocean density stratification, and play a key role in modulating air-sea interactions. However, the thermohaline variations of the ocean upper layers within the freshwater plumes in the GG are still poorly known, as they are poorly observed and documented. The main objective of this thesis is therefore to study and document the spatial variability at horizontal mesoscale (10-100 km) and vertical (0-100m), from intra-seasonal to seasonal time scales of the thermohaline 3D structure in the freshwater plume areas of the GG: mainly the Congo and Niger Rivers plumes. First, using SSS SMOS satellite data, our study showed that freshwater plumes in this region extend towards the open ocean following two propagation regimes. During September to January, they propagate northwestward while from January to April they redirect to the southwest, where their maximum extension is observed in April. The rest of the year, from May to August, is marked by a surface salinization episode, where the freshwater plumes dissipate with a minimum extension observed in August. A salinity budget analysis in the surface mixed layer allowed highlighting the main physical processes controlling the seasonal variability of salinity within these freshwater plumes. We showed that horizontal advection processes and freshwater fluxes by precipitation and river discharges are the main contributors of low SSS distribution in this region. In the southeastern Gulf of Guinea, off Congo, the horizontal SSS advection is dominated by Ekman wind-driven currents. Second, we showed that the offshore distribution of the Congo plume on intra-seasonal time scales is associated with salt barrier layers and with thermohaline staircases profiles. In a case study (for 2016/03/31), we showed that the observed thermohaline staircases would result from the shear dynamics between the surface Ekman flow associated with the offshore (North-Westward) distribution of the Congo plume, and the geostrophic (South-Eastward) flow associated with the denser and saltier subsurface water masses of the open ocean to the west. Finally, using a Lagrangian approach, we have highlighted the origin and large-scale structuring of water masses involved in the strong haline stratification observed off Congo. This study showed the strong shear of the currents associated with the vertical salinity gradients within the water column associated with the staircases profiles

The mixing and entrainment processes existing in oceanic overflows, e.g., Denmark Strait Overflow (DSO), affect the global thermohaline circulation. Owing to limited spatial resolution in global climate prediction simulations, the small-scale dynamics of oceanic mixing must be properly modeled. A series of experiments are performed in an Oceanic Overflow Facility to study the mixing and entrainment of a gravity current along an inclined plate, flowing into a steady ambient medium. At small values of the Richardson number, the shear dominates the stabilizing effect of the stratification and the flow at the interface of the current becomes unstable, resulting in turbulent mixing. In addition, the level of turbulence is enhanced by an active grid device. Using PIV and PLIF to measure, respectively, the velocity and density fields, we characterize the statistical properties of the mixing. We also study the entrainment of the ambient fluid by the flow. An accurate parametrization of the mixing and entrainment can be a valuable input for ocean circulation models.