Internal tides of near-semidiurnal frequencies were observed in October 2000 at the ADCP1 station located in oceanic waters (18° 17.478’ N, 67° 48.155’ W), about 10 nautical miles to the NNE of Mona Island with a depth of 481 m. The... more
Internal tides of near-semidiurnal frequencies were observed in October 2000 at the ADCP1 station located in oceanic waters (18° 17.478’ N, 67° 48.155’ W), about 10 nautical miles to the NNE of Mona Island with a depth of 481 m. The maximum observed height (crest to trough) in the euphotic zone was 26 m. The presence of a shallow pycnocline (40 m) and high Brunt-Väisälä frequencies were due to maximum influence of the Orinoco River on the Mona Passage waters at this time of the year. These conditions facilitated the propagation of the internal tide throughout the middle of the euphotic zone. Maximum concentrations of chlorophyll-a (1.2 mg Chl-a m-3) occurred during high upward velocities (> 40 m h-1). Phytoplankton was displaced upward to a zone above the pycnocline receiving higher light intensities. Additionally, increases in vertical eddy diffusivity above 6 x 10-3 m-2 s-1 were observed one hour before the arrival of the internal tide trough. Soon after that, increments in primary productivity of the order of 0.05 mg C m-3 h-1 were measured. The advection of phytoplankton by strong upward currents into a layer of higher irradiance, and increments in vertical eddy diffusivity, were responsible for distorting the typical V-shaped spatial-temporal pattern of primary productivity. Higher values of primary productivity were observed near the wave trough, than those observed during periods of maximum solar irradiance at noon. Significant changes in the attenuation coefficient (from 0.03 m-1 to 0.05 m-1) for the following SeaWiFS bands: 412, 443, 490 and 512 nm corresponded to events of maximum upward velocities and higher diffusivity. These processes seem to be easy to detect in oceanic waters, out of the influence from high nutrient load waters due to rivers.
This paper describes a numerical study of the two dimensional flow of a linearly stably stratified fluid around a body (vertical or horizontal thin strip) towed horizontally at constant velocity 𝑈. The dimensionless parameters governing... more
This paper describes a numerical study of the two dimensional flow of a linearly stably stratified fluid around a body (vertical or horizontal thin strip) towed horizontally at constant velocity 𝑈. The dimensionless parameters governing this problem are the internal Froude number 𝐹𝑟=𝑈𝑁𝐿⁄, the Reynolds number ℜ𝑒=𝑈𝐿𝜈⁄, the ratio of intrinsic length scales 𝐶=𝛬𝐿⁄ and the Peclet number 𝑃𝑒=𝑈𝐿𝜅𝑆⁄ (L is the dimension of the strip; N is buoyancy frequency, 𝛬 is stratification length scale, 𝜈 is kinematic viscosity and 𝜅𝑆 is the solute diffusivity). The set of the dimensionless parameters define both the conditions of numerical and of small scale laboratory modeling of environmental flows. The Fields of velocity, density and their gradients were computed and visualized as well as the wave internal propagation. Measurements on velocity structure functions and the effect of wave-turbulence interaction on mixing and intermittency are used to describe the topology of the flow showing the relationship between the Richardson number and the blocked wake thickness.
Internal waves describe the (linear) response of an incompressible stably stratified luid to small perturbations. The inclination of their group velocity with respect to the vertical is completely determined by their frequency. Therefore... more
Internal waves describe the (linear) response of an incompressible stably stratified luid to small perturbations. The inclination of their group velocity with respect to the vertical is completely determined by their frequency. Therefore the reflection on a sloping boundary cannot follow Descartes' laws, and it is expected to be singular if the slope has the same inclination as the group velocity. In this paper, we prove that in this critical geometry the weakly viscous and weakly nonlinear wave equations have actually a solution which is well approximated by the sum of the in- cident wave packet, a reflected second harmonic and some boundary layer terms. This result confirms the prediction by Dauxois and Young, and provides precise estimates on the time of validity of this approximation.
An interaction of internal solitary waves with the shelf edge in the time periods related to the presence of a pronounced seasonal pycnocline in the Red Sea and in the Alboran Sea is analysed via satellite photos and SAR images.... more
An interaction of internal solitary waves with the shelf edge in the time periods related to the presence of a pronounced seasonal pycnocline in the Red Sea and in the Alboran Sea is analysed via satellite photos and SAR images. Laboratory data on transformation of a solitary wave of depression while passing along the transverse bottom step were obtained in a tank with a two-layer stratified fluid. The certain difference between two characteristic types of hydrophysical phenomena was revealed both in the field observations and in experiments. The hydrological conditions for these two processes were named the “deep” and the “shallow” shelf respectively. The first one provides the generation of the secondary periodic short internal waves – “runaway” edge waves – due to change in the polarity of a part of a soliton approaching the shelf normally. Another one causes a periodic shear flow in the upper quasi-homogeneous water layer with the period of incident solitary wave. The strength of the revealed mechanisms depends on the thickness of the water layer between the pycnocline and the shelf bottom as well as on the amplitude of the incident solitary wave.
En octubre del 2000 durante la misión oceanográfica Mona Challenge en el R/V Chapman fue cuando descubrimos cerca de la picnoclina-una zona donde el cambio en densidad del agua con profundidad es máximo-que ésta capa mostraba un valor... more
En octubre del 2000 durante la misión oceanográfica Mona Challenge en el R/V Chapman fue cuando descubrimos cerca de la picnoclina-una zona donde el cambio en densidad del agua con profundidad es máximo-que ésta capa mostraba un valor elevado del coeficiente de difusividad vertical turbulenta (> 4 x 10-3 m 2 s-1) a 64 m de profundidad. Además entre 90 m y 100 m de profundidad registramos valores entre 5 x 10-3 m 2 s-1 y 6 x 10-3 m 2 s-1. Estos valores superan unas 10 veces los valores esperados en el interior del océano. La estación oceanográfica ADCP-1 (18° 17.478' N, 67° 48.155' W), fue localizada a unas 10 millas náuticas al NNE de la Isla de Mona y a una profundidad de 481 m. Muy cerca al norte de la estación, se encuentra El Pichincho-una pequeña meseta a unos 250 m de profundidad-conocida como un área donde se genera una marea interna que puede alcanzar una altura de hasta 50 metros. Todo apuntaba que la marea interna era la responsable en generar la mezcla turbulenta que detectamos en ADCP-1. El interior del océano está establemente estratificado verticalmente debido a diferencias en densidad, donde las capas más densas están por debajo de las menos densas. Esta condición hace difícil la mezcla vertical. No obstante, olas no-lineales propagándose a los largo de la interface pueden erosionar esa estratificación vertical, provocando mezcla. La inestabilidad de cizalladura-el gradiente vertical de la velocidad de la corriente de la ola interna-es contrario al efecto estabilizador de la estratificación vertical por densidad y puede generar inestabilidad de Kelvin-Helmholtz (K-H), provocando así la mezcla turbulenta. La inestabilidad K-H es una inestabilidad primaria que se caracteriza por sus ondulaciones en forma de pronunciadas crestas rompiendo. El número Richardson de gradiente RiG=N 2 /S 2 cuantifica la tendencia a crecer de la inestabilidad K-H a pesar del efecto estabilizador de la estratificación vertical. Si el valor de RiG < 0.25 entonces podemos suponer que se genere la inestabilidad K-H. Valores reducidos del número de Richardson y altos valores del coeficiente de difusividad vertical turbulenta demostraron que una parte de la energía de la marea interna que se genera en el Pichincho se disipa mediante mezcla turbulenta en las inmediaciones del mismo y la otra parte se propaga hacia el Mar Caribe o el Océano Atlántico como una ola interna. La marea interna semidiurna es generada cuando las isopicnas son empujadas por la fuertes corrientes semidiurnas desviadas por la pendiente topográfica del Pichincho durante una parte del ciclo mareal. Cuando coinciden periodos de mareas vivas con condiciones de estratificación favorable para la generación de olas internas incrementa la altura de la marea interna. Por lo general, existen condiciones favorables a finales de septiembre, durante la marea equinoccial, también durante octubre y noviembre, cuando se reduce la profundidad de la picnoclina. Aumentos en la frecuencia boyante (frecuencia Brunt-Väisälä) a una profundidad de 300 m generalmente ocurren en mayo-junio y esto favorece la generación de olas internas en el Pasaje de la Mona. La cascada de pérdida de energía de la ola interna hasta desembocar en turbulencia comienza con la inestabilidad primaria K-H, pero no termina ahí, sino que se desarrollan inestabilidades secundarias en el interior de ésta.
250-m resolution MODIS images acquired by the Earth Observing System Terra and Aqua Satellites during sunglint conditions allowed us to survey high-frequency nonlinear internal solitary wave occurrences in the Indian Ocean. These images... more
250-m resolution MODIS images acquired by the Earth Observing System Terra and Aqua Satellites during sunglint conditions allowed us to survey high-frequency nonlinear internal solitary wave occurrences in the Indian Ocean. These images clearly show packets of internal solitary waves (ISWs) generated in the Sape and Sumba Straits during the extraordinary coincidence of the following astronomical factors: perigee, syzygy, zero lunar declination, spring equinox, longitude of the lunar node equal to 180°. The ISWs packets propagated west-southwest (2.24 m/s) and crossed 1503 km of Indian Ocean in about 8 days to excite 2.3-minutes coastal seiches in Flying Fish Cove – Christmas Island (AUS). The seiche amplitudes range between 0.1 m and 0.3 m. Satellite images and sea level data evidence the relationship between these two physical phenomena. Our finding sustains that ISWs can travel very long distances and impinge on distant archipelagos far from their generation region.
Two-dimensional, nonlinear and nonhydrostatic field-scale numerical simulations are used to examine the resuspension, dispersal and transport of mud-like sediment caused by the shoaling and breaking of long internal solitary waves on... more
Two-dimensional, nonlinear and nonhydrostatic field-scale numerical simulations are used to examine the resuspension, dispersal and transport of mud-like sediment caused by the shoaling and breaking of long internal solitary waves on uniform slopes. The patterns of erosion and transport are both examined, in a series of test cases with varying conditions. Shoreward sediment movement is mainly within boluses, while seaward movement is within intermediate nepheloid layers. Several relationships between properties of the suspended sediment and control parameters are determined such as the horizontal extent of the nehpeloid layers, the total mass of resuspended sediment and the point of maximum bed erosion. The numerical results provide a plausible explanation for acoustic backscatter patterns observed during and after the shoaling of internal solitary wavetrains in a natural coastal environment. The results may be useful in the interpretation of some sedimentary structures, and suggest an effective mechanism for offshore dispersal of muddy sediments.
Internal waves describe the (linear) response of an incompressible stably stratified fluid to small perturbations. The inclination of their group velocity with respect to the vertical is completely determined by their frequency. Therefore... more
Internal waves describe the (linear) response of an incompressible stably stratified fluid to small perturbations. The inclination of their group velocity with respect to the vertical is completely determined by their frequency. Therefore the reflection on a sloping boundary cannot follow Descartes' laws, and it is expected to be singular if the slope has the same inclination as the group velocity. In this paper, we prove that in this critical geometry the weakly viscous and weakly nonlinear wave equations have actually a solution which is well approximated by the sum of the incident wave packet, a reflected second harmonic and some boundary layer terms. This result confirms the prediction by Dauxois and Young, and provides precise estimates on the time of validity of this approximation.
Large amplitude (~100 m) and fast (3 m/s) mode-1 internal solitary wave packets generated by the interaction of strong tidal currents with a central sill (12.61°S, 60.84°E) in the Mascarene Plateau during perigean-spring tides... more
Large amplitude (~100 m) and fast (3 m/s) mode-1 internal solitary wave packets generated by the interaction of strong tidal currents with a central sill (12.61°S, 60.84°E) in the Mascarene Plateau during perigean-spring tides (28-SEP-2015; 14-NOV-2016), propagated northwestward and southeastward hundred of kilometers before impinged on Agalega (2 days later) and Rodrigues islands (3 days later), respectively. The arrival of the ISWs packets coincided with a simultaneous increase in coastal seiche activity at both islands.
