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Dongxiao Wang - One of the best experts on this subject based on the ideXlab platform.
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interaction of a river plume with coastal upwelling in the northeastern south china sea
Continental Shelf Research, 2009Co-Authors: Li Li, Dongxiao WangAbstract:Observational and modeling studies were conducted to investigate the Pearl River plume and its interaction with the southwesterly driven upwelling circulation in the northern South China Sea during the summer. After exiting the Pearl River Estuary, the discharged freshwater generates a nearly Stationary bulge of freshwater near the entrance of the estuary. Forced by the Wind-Driven coastal upwelling Current, the freshwater in the outer part of the bulge flows downstream at the speed of the Current and forms a widening and deepening buoyant plume over the shelf. The plume axis gradually shifts offshore of the Current maximum as a result of Currents induced by the contrasting density at the nose Of plume and by the intensified Ekman drift in the Plume. In this plume-Current system, the fraction of the discharged freshwater volume accumulated in the bulge reaches a steady state and the Volume of newly discharged freshwater is transported downstream by the upwelling Current. Enhancement of stratification by the plume thins the Surface frictional layer and enhances the cross-shelf circulation in the upper water column such that the surface Ekman Current and compensating flow beneath the plume are amplified while the shoaling of the deeper dense water in the upwelling region changes minimally. The pressure gradient generated between the buoyant plume and ambient seawater accelerates the Wind-Driven Current along the inshore edge of the plume but retards it along the offshore edge. Along the plume, downward momentum advection is strong near the highly nonlinear Source region and a weaker upward momentum advection occurs in the far field over the shelf. Typically, the plume is shaped by the Current over the shelf while the Current itself is adjusting to a new dynamic balance invoked by the plume-induced changes of vertical viscosity and the horizontal pressure gradient. The spatial variation of this new balance leads to a coherent change in the cross-isobath transport in the upper water column during upwelling. (C) 2008 Elsevier Ltd. All Fights reserved.
Teixeira, Miguel A. C. - One of the best experts on this subject based on the ideXlab platform.
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A model for the Wind-Driven Current in the wavy oceanic surface layer: apparent friction velocity reduction and roughness length enhancement
'American Meteorological Society', 2018Co-Authors: Teixeira, Miguel A. C.Abstract:A simple analytical model is developed for the Current induced by the wind and modified by surface wind-waves in the oceanic surface layer, based on a first-order turbulence closure and including the effect of a vortex force representing the Stokes drift of the waves. The shear stress is partitioned between a component due to shear in the Current, which is reduced at low turbulent Langmuir number (La_t), and a wave-induced component, which decays over a depth proportional to the dominant wavelength (l_w). The model reproduces the apparent reduction of the friction velocity and enhancement of the roughness length estimated from Current profiles, detected in a number of studies. These effects are predicted to intensify as La_t decreases, and are entirely attributed to non-breaking surface waves. The Current profile becomes flatter for low La_t owing to a smaller fraction of the total shear stress being supported by the Current shear. Comparisons with the comprehensive dataset provided by the laboratory experiments of Cheung and Street show encouraging agreement, with the Current speed normalized by the friction velocity decreasing as La_t decreases and l_w increases if the model is adjusted to reflect the effects of a full wave spectrum on the intensity and depth of penetration of the wave-induced stress. A version of the model where the shear stress decreases to zero over a depth consistent with the measurements accurately predicts the surface Current speed. These results contribute towards developing physically-based momentum flux parameterizations for the wave-affected boundary layer in ocean circulation models
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A model for the Wind-Driven Current in the wavy oceanic surface layer: apparent friction velocity reduction and roughness length enhancement
'American Meteorological Society', 2018Co-Authors: Teixeira, Miguel A. C.Abstract:A simple analytical model is developed for the Current induced by the wind and modified by surface wind-waves in the oceanic surface layer, based on a first-order turbulence closure and including the effect of a vortex force representing the Stokes drift of the waves. The shear stress is partitioned between a component due to shear in the Current, which is reduced at low turbulent Langmuir number ($La_t$), and a wave-induced component, which decays over a depth proportional to the dominant wavelength. The model reproduces the apparent reduction of the friction velocity and enhancement of the roughness length estimated from Current profiles, detected in a number of studies. These effects are predicted to intensify as $La_t$ decreases, and are entirely attributed to non-breaking surface waves. The Current profile becomes flatter for low $La_t$ owing to a smaller fraction of the total shear stress being supported by the Current shear. Comparisons of the model with the comprehensive dataset provided by the laboratory experiments of Cheung and Street show encouraging agreement, with the Current speed decreasing as the wind speed increases (corresponding to decreasing $La_t$), if the model is adjusted to reflect the effects of a full wave spectrum on the intensity and depth of penetration of the wave-induced stress. A version of the model where the shear stress decreases to zero over a depth consistent with the measurements accurately predicts the surface Current speed. These results contribute towards developing physically-based momentum flux parameterizations for the wave-affected boundary layer in ocean circulation models.Comment: 13 pages, 8 figures, submitted to Journal of Physical Oceanograph
Jordi Colomer - One of the best experts on this subject based on the ideXlab platform.
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the role of surface vertical mixing in phytoplankton distribution in a stratified reservoir
Limnology and Oceanography, 2007Co-Authors: Teresa Serra, Javier Vidal, Xavier Casamitjana, Marianna Soler, Jordi ColomerAbstract:We investigated convection caused by surface cooling and mixing attributable to wind shear stress and their roles as agents for the transport of phytoplankton cells in the water column by carrying out two daily surveys during the stratified period of the Sau reservoir. Green algae, diatoms, and cryptophyceae were the dominant phytoplankton communities during the surveys carried out in the middle (July) and end (September) of the stratified period. We show that a system with a linear stratification and that is subject to weak surface forcing, with weak winds , 4ms 21 and low energy dissipation rate values of the order of 10 28 m 2 s 23 or lower, enables the formation of thin phytoplankton layers. These layers quickly disappear when water parcels mix because there is a medium external forcing (convection) induced by the night surface cooling, which is characterized by energy dissipation rates on the order of ,5 3 1028 m2 s23. During both surveys the wind generated internal waves during the entire diurnal cycle. During the day, and because of the weak winds, phytoplankton layers rise in the water column up to a depth determined by both solar heating and internal waves. In contrast, during the night phytoplankton mixes down to a depth determined by both convection and internal waves. These internal waves, together with the Wind-Driven Current generated at the surface, seem to be the agents responsible for the horizontal transport of phytoplankton across the reservoir.
Li Li - One of the best experts on this subject based on the ideXlab platform.
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interaction of a river plume with coastal upwelling in the northeastern south china sea
Continental Shelf Research, 2009Co-Authors: Li Li, Dongxiao WangAbstract:Observational and modeling studies were conducted to investigate the Pearl River plume and its interaction with the southwesterly driven upwelling circulation in the northern South China Sea during the summer. After exiting the Pearl River Estuary, the discharged freshwater generates a nearly Stationary bulge of freshwater near the entrance of the estuary. Forced by the Wind-Driven coastal upwelling Current, the freshwater in the outer part of the bulge flows downstream at the speed of the Current and forms a widening and deepening buoyant plume over the shelf. The plume axis gradually shifts offshore of the Current maximum as a result of Currents induced by the contrasting density at the nose Of plume and by the intensified Ekman drift in the Plume. In this plume-Current system, the fraction of the discharged freshwater volume accumulated in the bulge reaches a steady state and the Volume of newly discharged freshwater is transported downstream by the upwelling Current. Enhancement of stratification by the plume thins the Surface frictional layer and enhances the cross-shelf circulation in the upper water column such that the surface Ekman Current and compensating flow beneath the plume are amplified while the shoaling of the deeper dense water in the upwelling region changes minimally. The pressure gradient generated between the buoyant plume and ambient seawater accelerates the Wind-Driven Current along the inshore edge of the plume but retards it along the offshore edge. Along the plume, downward momentum advection is strong near the highly nonlinear Source region and a weaker upward momentum advection occurs in the far field over the shelf. Typically, the plume is shaped by the Current over the shelf while the Current itself is adjusting to a new dynamic balance invoked by the plume-induced changes of vertical viscosity and the horizontal pressure gradient. The spatial variation of this new balance leads to a coherent change in the cross-isobath transport in the upper water column during upwelling. (C) 2008 Elsevier Ltd. All Fights reserved.
Steven R Ramp - One of the best experts on this subject based on the ideXlab platform.
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subtidal Currents over the central california slope evidence for offshore veering of the underCurrent and for direct wind driven slope Currents
Deep-sea Research Part Ii-topical Studies in Oceanography, 2000Co-Authors: Marlene A Noble, Steven R RampAbstract:Abstract In February 1991, an array of six Current-meter moorings was deployed for one year across the central California outer shelf and slope. The main line of the array extended 30 km offshore of the shelf break, out to water depths of 1400 m. A more sparsely-instrumented line, displaced 30 km to the northwest, extended 14 km offshore. Though shorter, the northern line spanned similar water depths because the gradient of the topography steepened in the northern region. A poleward flow pattern, typical of the California underCurrent, was seen across both lines in the array over most of the year. The poleward flow was surface intensified. In general, the portion of the underCurrent that crossed the southern line had larger amplitudes and penetrated more deeply into the water column than the portion that crossed the northern line. Transport over the year ranged from 0 to 2.5 Sverdrups (Sv) poleward across the southern line; 0 to 1 Sv poleward across the northern line. We suggest the difference in transport was caused by topographic constraints, which tended to force the poleward flow offshore of the northern measurement sites. The slope of the topography steepened too abruptly to allow the poleward flow to follow isobaths when Currents were strong. When Current velocities lessened, a more coherent flow pattern was seen across both lines in the array. In general, the poleward flow patterns in the underCurrent were not affected by local winds or by the local alongshore pressure gradient. Nor was a strong seasonal pattern evident. Rather unexpectedly, a small but statistically significant fraction of the Current variance over the mid- and outer slope was driven by the surface wind stress. An alongshelf wind stress caused Currents to flow along the slope, parallel to the wind field, down to depths of 400 m below the surface and out to distances of 2 Rossby radii past the shelf break. The transfer functions were weak, 3–4 cm/s per dyn cm−2, but comparable to Wind-Driven Current amplitudes of 4–6 cm/s per unit wind stress over the middle shelf. Equatorward, alongshelf winds also caused water from 200–300 m over the slope to upwell onto the shelf as the surface water moved offshore.
