Generation of internal solitary waves by frontally forced intrusions in geophysical flows

International audienceInternal solitary waves are hump-shaped, large-amplitude waves that are physically analogous to surface waves except that they propagate within the fluid, along density steps that typically characterize the layered vertical structure of lakes, oceans and the atmosphere. As do surface waves, internal solitary waves may overturn and break, and the process is thought to provide a globally significant source of turbulent mixing and energy dissipation. Although commonly observed in geophysical fluids, the origins of internal solitary waves remain unclear. Here we report a rarely observed natural case of the birth of internal solitary waves from a frontally forced interfacial gravity current intruding into a two-layer and vertically sheared background environment. The results of the analysis carried out suggest that fronts may represent additional and unexpected sources of internal solitary waves in regions of lakes, oceans and atmospheres that are dynamically similar to the situation examined here in the Saguenay Fjord, Canada

High-frequency frontal displacements south of san jorge gulf during a tidal cycle near spring and neap phases biological implications between tidal states

San Jorge Gulf (SJG) is a region of high biological productivity that supports important shrimp (Pleoticus muelleri) and hake (Merluccius hubbsi) fisheries, as well as high marine biodiversity associated, in part, with a tidal front located in the southern part of the gulf. In situ high-resolution cross-frontal measurements were collected using a remotely operated towed vehicle to characterize the three-dimensional structure of the tidal front and to investigate how its position varies during the semidiurnal tidal cycle (high/low) and the spring to neap transition, together with its impact on the distribution of nutrients and chlorophyll-a. Estimates of tidal height and flow velocity derived from a numerical model support the conclusion that frontal displacements mostly result from advec-tion by cross-frontal tidal currents. The frontal position was also modified by baroclinic instabilities that significantly distort the front. Measurements reveal intrusions of low-salinity, nutrient-rich waters from the mixed side into the pycnocline on the stratified side cause a subsurface chlorophyll-a peak near the neap phase. Most prior studies of fronts in the SJG have been limited to their surface manifestations because they were conducted using satellite images. This article aims to contribute to the understanding of the complex southern tidal front dynamics, highlighting that maximum primary productivity occurs in a subsurface layer that is not visible by satellite sensors.Fil: Carbajal, Juan Cruz. Centro Para El Estudio de Sistemas Marinos (cesimar), Conicet-cenpat; ArgentinaFil: Rivas, Andres Lujan. Centro Para El Estudio de Sistemas Marinos (cesimar), Conicet-cenpat; ArgentinaFil: Chavanne, Cédric. Institut Des Sciences de la Mer de Rimouski; Canad

High-frequency frontal displacements south of san jorge gulf during a tidal cycle near spring and neap phases biological implications between tidal states

San Jorge Gulf (SJG) is a region of high biological productivity that supports important shrimp (Pleoticus muelleri) and hake (Merluccius hubbsi) fisheries, as well as high marine biodiversity associated, in part, with a tidal front located in the southern part of the gulf. In situ high-resolution cross-frontal measurements were collected using a remotely operated towed vehicle to characterize the three-dimensional structure of the tidal front and to investigate how its position varies during the semidiurnal tidal cycle (high/low) and the spring to neap transition, together with its impact on the distribution of nutrients and chlorophyll-a. Estimates of tidal height and flow velocity derived from a numerical model support the conclusion that frontal displacements mostly result from advec-tion by cross-frontal tidal currents. The frontal position was also modified by baroclinic instabilities that significantly distort the front. Measurements reveal intrusions of low-salinity, nutrient-rich waters from the mixed side into the pycnocline on the stratified side cause a subsurface chlorophyll-a peak near the neap phase. Most prior studies of fronts in the SJG have been limited to their surface manifestations because they were conducted using satellite images. This article aims to contribute to the understanding of the complex southern tidal front dynamics, highlighting that maximum primary productivity occurs in a subsurface layer that is not visible by satellite sensors.Fil: Carbajal, Juan Cruz. Centro Para El Estudio de Sistemas Marinos (cesimar), Conicet-cenpat; ArgentinaFil: Rivas, Andres Lujan. Centro Para El Estudio de Sistemas Marinos (cesimar), Conicet-cenpat; ArgentinaFil: Chavanne, Cédric. Institut Des Sciences de la Mer de Rimouski; Canad

Observations of vortices and vortex Rossby waves in the lee of an island

A 9-month deployment of high-frequency radio (HFR) current meters and moored ADCPs in the lee of Oahu, Hawaii, gives some dynamical insights in the generation and evolution of vortices in the lee of islands. For mountainous islands lying in relatively strong and steady winds, such as the Hawaiian archipelago in the trade winds, vortices can be generated by Ekman pumping associated with orographic wind stress curls. An anticyclone generated in the lee of Oahu in October 2002 reaches a negative absolute vorticity for 4 days, before quickly decaying and broadening, possibly as a result of inertial instability. A large cyclone, generated in December 2002 in the lee of Hawaii, drifted northwestward and stalled southwest of Oahu in March-April 2003. Vortex Rossby waves developing on its periphery were observed by the HFRs with a northeastward phase propagation, 110 km wavelength and 16 days period

Phytoplankton Ecology During a Spring-Neap Tidal Cycle in the Southern Tidal Front of San Jorge Gulf, Patagonia

Tidal fronts are interfaces that separate stratified from mixed waters. The stratified surface zone of a front has lower inorganic nutrient concentrations than the mixed side, and thus, phytoplankton assemblages are expected to differ from one side of the front to the other. Here, we characterize the physics, nutrient dynamics, and biology of the southern front in San Jorge Gulf (SJG), Argentina, during a spring-neap tidal cycle. Baroclinic instabilities influence the shape and position of the front and presumably play an important role in the horizontal transport across the front. The highest phytoplankton biomass concentrations were found in the waters of the stratified side of the front during neap tide, with picophytoplankton, cyanobacteria, and nanophyto-plankton being the main contributors to the total autotrophic biomass. Bacteria contribute the most to heterotrophic biomass. In contrast, during spring tide, the carbon contribution of microphytoplankton was higher than during neap tide. In the mixed side, cells photoacclimate to optimum light conditions, suggesting that cells near the surface, which are probably photoinhibited, and cells below the euphotic zone, which are light-limited, are quickly advected by turbulent vertical motions to depths with optimal irradiance conditions.Facultad de Ciencias Naturales y Muse

Phytoplankton ecology during a spring-neap tidal cycle in the southern tidal front of san jorge gulf, patagonia

Tidal fronts are interfaces that separate stratified from mixed waters. The stratified surface zone of a front has lower inorganic nutrient concentrations than the mixed side, and thus, phytoplankton assemblages are expected to differ from one side of the front to the other. Here, we characterize the physics, nutrient dynamics, and biology of the southern front in San Jorge Gulf (SJG), Argentina, during a spring-neap tidal cycle. Baroclinic instabilities influence the shape and position of the front and presumably play an important role in the horizontal transport across the front. The highest phytoplankton biomass concentrations were found in the waters of the stratified side of the front during neap tide, with picophytoplankton, cyanobacteria, and nanophyto-plankton being the main contributors to the total autotrophic biomass. Bacteria contribute the most to heterotrophic biomass. In contrast, during spring tide, the carbon contribution of microphytoplankton was higher than during neap tide. In the mixed side, cells photoacclimate to optimum light conditions, suggesting that cells near the surface, which are probably photoinhibited, and cells below the euphotic zone, which are light-limited, are quickly advected by turbulent vertical motions to depths with optimal irradiance conditions.Fil: Flores Melo, Elizabeth Ximena. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina. Institut des Sciences de la Mer de Rimouski; CanadáFil: Schloss, Irene Ruth. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Austral de Investigaciones Científicas; Argentina. Universidad Nacional de Tierra del Fuego; Argentina. Institut des Sciences de la Mer de Rimouski; CanadáFil: Chavanne, Cédric. Institut des Sciences de la Mer de Rimouski; CanadáFil: Almandoz, Gaston Osvaldo. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina. Facultad de Ciencias Naturales y Museo, Universidad Nacional de la Plata; ArgentinaFil: Latorre, Maite Pilmayquen. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Centro Nacional Patagónico. Centro para el Estudio de Sistemas Marinos; ArgentinaFil: Ferreyra, Gustavo Adolfo. Institut des Sciences de la Mer de Rimouski; Canadá. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Austral de Investigaciones Científicas; Argentin

Le point sur les marées d’équinoxes dans l’estuaire du Saint-Laurent

Selon une étude publiée dans Le Naturaliste canadien, les marées de l’estuaire du Saint-Laurent ne seraient pas plus grandes aux équinoxes qu’à d’autres moments de l’année. Cette idée de marées plus grandes aux équinoxes relèverait plutôt d’une croyance populaire sans fondements théoriques, répandue même parmi la communauté scientifique. Il est démontré ici que ces conclusions contredisent en fait la théorie des marées et découlent d’un problème de sous-échantillonnage des données de marées. Une analyse basée sur 30 ans d’observations horaires du niveau de la mer à la station Rimouski montre que, statistiquement, les marées sont bel et bien plus énergétiques autour des équinoxes, en accord avec la théorie des marées. Cependant, cette conclusion statistique n’indique pas nécessairement que les marées les plus fortes d’une année particulière se rencontrent précisément aux jours des équinoxes puisque l’augmentation du marnage vers les équinoxes est faible comparativement aux variations de marnages associées au cycle vive-eau/morte-eau (2 semaines) qui n’est pas synchronisé avec les équinoxes. La théorie des marées est expliquée simplement et un schéma est proposé pour illustrer l’influence du cycle équinoxe-solstice sur les marées.According to a study published in Le Naturaliste canadien, tides in the St. Lawrence Estuary would not be any greater at the equinoxes than they are at any other moment of the year. According to the study, the idea that tides may be larger at the equinoxes is a popular unfounded belief that is widespread, even among scientists. Here, it is demonstrated that these conclusions contradict the theory of tides and arise from a problem in the subsampling of tide data. A statistical analysis based on 30 years of hourly water level observations at Rimouski, shows that tides are indeed more energetic around the equinoxes, which is in agreement with the theory of tides. However, this conclusion does not necessarily indicate that the greatest tides always appear precisely at the equinoxes in a given year. This is because the increase in the tidal range as the equinoxes are approached is rather small in comparison with the large neap-spring tidal variability, which is not synchronized with the equinoxes. The theory of tides is explained in simple terms and a schematic diagram is proposed to explain the effect on tides during the equinox-solstice cycle

Storm‐induced turbidity currents on a sediment‐starved shelf: Insight from direct monitoring and repeat seabed mapping of upslope migrating bedforms

International audienc

Observations of vortex rossby waves associated with a mesoscale cyclone

Short-wavelength (L~100 km) Rossby waves with an eastward zonal phase velocity were observed by high-frequency radio Doppler current meters and moored ADCPs west of Oahu, Hawaii, during spring 2003. They had Rossby numbers Ro=|?|f| = O(1), periods of 12-15 days, and phase speeds of 8-9 cm s-1, and they were surface trapped with vertical e-folding scales of 30-170 m. They transferred horizontal kinetic energy to the background flow of a mesoscale cyclone lying 160-190 km west of Oahu, revealed by altimetry. The waves approximately satisfied the dispersion relation of vortex Rossby waves propagating through the radial gradient of potential vorticity associated with the cyclone. Vertical shear of the background currents may also affect wave propagation. Theoretical studies have shown that vortex Rossby waves provide a mechanism by which perturbed vortices axisymmetrize and strengthen and may be important to the dynamics of oceanic vortices