Abstract
It is pointed out that accounting for an ocean surface velocity dependence in the wind stress τ can lead to a significant reduction in the rate at which winds input mechanical energy to the ...geostrophic circulation. Specifically, the wind stress is taken to be a quadratic function of Ua − uo, where Ua and uo are the 10-m wind and ocean surface velocity, respectively. Because |Ua| is typically large relative to |uo|, accounting for a uo dependence leads only to relatively small changes in τ. The change to the basin-averaged wind power source, however, is considerably larger. Scaling arguments and quasigeostrophic simulations in a basin setting are presented. They suggest that the power source (or rate of energy input) is reduced by roughly 20%–35%.
The study concerns the water level (WL) evolution in lagoons under the influence of tides and river fluxes. We derive new approximate analytical solutions of the Stigebrandt (1980) equations and ...apply them to the Nokoué Lagoon (Benin), a large tropical coastal lagoon fed by substantial river input.
•Nokoué Lagoon's water circulation and residence times strongly vary with seasons.•They are sensitive to river inflow, tide, salinity gradient and wind forcing.•Mean water residence times are of few ...days (months) during flood (low-water) periods.•Tides, river inflows and salinity gradients favor evacuation, wind favors retention.•Above a critical water flow of 50-100 m3/s, the lagoon is rapidly flushed (<5 days).
Seasonal water circulation and residence times in the large (150 km2) and shallow (1.3 m average dry season depth) Nokoué Lagoon (Benin) are analyzed by means of numerical simulations using the three-dimensional SYMPHONIE model. The average circulation during the four primary hydrological periods throughout the year are studied in detail. Despite the lagoon's shallowness, significant disparities between surface and bottom conditions are observed. During the flood season (September-November), substantial river inflow (∼1200 m3/s) leads to nearly barotropic currents (∼7 cm/s), ‘directly’ linking rivers to the Atlantic Ocean. Rapid flushing results in short water residence times ranging from 3 to 16 days, with freshwater inflow and winds driving lagoon dynamics. During the salinization period (December-January) the circulation transforms into an estuarine pattern, characterized by surface water exiting and oceanic water entering the lagoon at the bottom. Average currents (∼2 cm/s) and recirculation cells are relatively weak, resulting in a prolonged residence time of approximately 4 months. Circulation during this time is dominated by tides, the ocean-lagoon salinity gradient, wind, and river discharge (∼100 m3/s). During low-water months (February to June), minimal river inflow and low lagoon water-levels prevail. Predominant southwest winds generate a small-scale circulation (∼3 cm/s) with a complex pattern of multiple recirculation and retention cells. Residence times vary from 1 to 4 months, declining from February to June. During the lagoon's desalination season (July-August), increasing river inflows again establish a direct river-ocean connection, and average residence times reduce to ∼20 days. Notably, a critical river discharge threshold (∼50-100 m3/s) is identified, beyond which the lagoon empties within days. This study highlights how wind-driven circulation between December and June can trap water along with potential pollutants, while river inflows, tides, and the ocean-lagoon salinity gradient facilitate water discharge. Additionally, it explores the differences between residence and flushing times, as well as some of the limitations identified in the simulations used.
This study, based on five years of monthly in situ data collected from 2018 to 2022, examines the seasonal and interannual fluctuations of suspended particulate matter (SPM) concentration in Nokoué ...Lagoon, Benin. Seasonally, SPM exhibits significant variations primarily influenced by changes in river discharge. During low-flow periods (December to May), SPM concentrations are relatively low (<15 mg L-1) throughout the lagoon. During this time, slight temporal variations are correlated with wind energy and likely associated with wind-induced resuspension of sediments. This is confirmed by slightly higher concentrations of SPM in the bottom layers compared to the surface. Resuspension appears to be lower in the west than in the east, likely due to the increased presence of acadjas (brush parks) in the west, reducing fetch and wind intensity, thus decreasing resuspension. At the onset of the river flood period (July–August), associated with the West African monsoon, increased river flow generates a significant turbid plume extending from the northeast of the lagoon to the Cotonou channel, connecting the lagoon to the Atlantic Ocean. SPM levels then increase considerably (>100 mg L−1), with a pronounced SPM gradient from the western to eastern regions of the lagoon. The less dense freshwater laden with sediment from the rivers flows over the denser saline water of the lagoon, leading to slightly higher SPM concentrations in the surface layers. Between September and November, SPM concentration gradually decreases as river flows reach their peak values. Thus, on a seasonal scale, the relationships between SPM and river discharge show a temporal lag, resulting in a clockwise hysteresis cycle. This is explained by the early mobilization of fine sediments during rising river flows, followed by reduced sediment availability and dilution effects as the flood peaks. On an interannual scale, SPM variations are relatively low with slight temporal shifts observed in the formation and expansion of the turbid plume and peak SPM levels. The total SPM mass in the lagoon ranges from 0.2 to 0.3 × 104 tonnes during low-flow periods to 20-30 × 104 tonnes at the onset of flooding. We also discuss uncertainties associated with SPM determination, estimated at approximately 5–15%. This study leverages a unique database in West Africa and provides valuable insights into the hydro-sedimentary dynamics of Nokoué Lagoon.
•Nokoué Lagoon is characterized by strong seasonal variability in SPM.•River flows primarily drive the seasonal variability of SPM.•Wind influences SPM concentration during low-water periods.•A turbid plume of high SPM, with inorganic matter, extends in the eastern lagoon.•SPM in the western lagoon is organic matter-dominated and relatively weaker.
This study investigated the main water-level (WL) variability modes of Nokoué Lagoon in Benin (West-Africa). The average WL ranges between 1.3 and 2.3 m between the low- and high-water seasons. ...Seasonal as well as weak interannual variations between 2018 and 2019 are driven by rainfall regime over the catchment and associated river inflow. At sub-monthly scales, the lagoon is tidally choked: ocean tides can reach 90 cm, whereas in the lagoon semi-diurnal and diurnal tides hardly reach few centimeters. Choking conditions vary with river inflow and ocean tide amplitude, correctly represented by a simple tidal choking model. Diurnal modulation and asymmetry of the tide are stronger (weaker) during high (low) water period. We also observed WL variations of ±5–10 cm at a fortnightly frequency, stronger during wet (high-water) season. Superimposed on the seasonal, fortnightly and tidal WL variations, we further observed short-term high-frequency seiche events. Mostly observed during dry (low-water) conditions, they are characterized by typical standing-wave oscillations of 5–10 cm amplitudes and 3 h periods. They are forced by the passage of fast-moving squall-lines that induce strong wind variations, heavy rainfalls and rapid drop-off of the air temperature. Results obtained in this study provide useful metrics for the validation of flood forecasting models to be implemented in Benin, and elsewhere on the West African coastline.
•Nokoué Lagoon is strongly tidally choked due to bottom friction in the Cotonou channel.•WL and tidal choking conditions depend on extreme seasonal hydrological cycle.•Fortnightly WL variations (±5–15 cm) are observed and are stronger during wet season.•High-frequency seiche events have 5–10 cm amplitudes and 3 h periods.•Seiche events are forced by the passage of fast-moving squall-lines.
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•Strong interannual variability of the sediment dynamics in the Gulf of Lion.•Massive sediment accumulation near the Rhone River mouth (∼56 % of the river inputs)•Sediment deficit on ...the inner shelf due to the long-term reduction of the Rhone inputs.•Storm-induced sediment accumulation in the southwestern Gulf of Lion.•Cold winters mainly impact the southwestern and outer shelf and the submarine canyons.
A simulation based on a hydro-sedimentary model was conducted for the period between summer 2010 and spring 2012 in the Gulf of Lion (northwestern Mediterranean Sea) to understand the spatial and temporal variability of sediment transport, erosion and deposition on the continental shelf and slope. Datasets of both simulated and observed current, temperature and suspended matter from the shelf and the Cap de Creus Canyon which is the main export route towards the continental slope, were first compared to assess the reliability of the simulation. The simulation shows the massive sediment accumulation near the Rhone River mouth (∼56 % of the inputs), the accretion along the mid-shelf mud belt, and the impact of dense shelf water cascading on sediment resuspension and erosion inside the Cap de Creus Canyon. The two studied autumn–winter periods were strongly contrasted in terms of meteorological conditions and subsequent impacts on the sediment dynamics. During the first period (2010–2011) dominated by marine storms, the shelf sediment underwent strong changes, the Rhone River sediment load accumulated in a relatively small area, stock of littoral sands moved to the inner shelf (20–40 m) while inner shelf fine particles fed the mid-shelf mud belt and the upper Cap de Creus Canyon. During the second period (2011–2012) with very little marine wind and a particularly cold winter, sediment on the shelf underwent little change except for a southwestward growth of the Rhone River prodeltaic deposit. Sediment from the southwestern end of the shelf as well as from the upper Cap de Creus Canyon was flushed toward deeper reaches by dense shelf water cascading. Cascading also had a more moderate impact in the various canyons incising the continental shelf. Our work supports the view of an unbalanced sedimentary system, with a deficit mainly over the inner shelf, whose main driver is probably the strong and fast reduction of particulate matter inputs from the Rhone River (by a factor of 4 in less than one century).
Numerical modeling was used to provide a new estimate of the amount of 137Cs released directly into the ocean from the Fukushima Daiichi nuclear power plant (NPP) after the accident in March 2011 and ...to gain insights into the physical processes that led to its dispersion in the marine environment during the months following the accident. An inverse method was used to determine the time‐dependent137Cs input responsible for the concentrations observed at the NPP's two liquid discharge outlets. The method was then validated through comparisons of the simulated concentrations with concentrations measured in seawater at different points in the neighborhood of the plant. An underestimation was noticed for stations located 30 km offshore. The resulting bias in the release inventory was estimated. Finally, the maximum 137Cs activity released directly to the ocean was estimated to lie between 5.1 and 5.5 PBq (Peta Becquerel = 1015 Bq) but uncertainties remain on the amount of radionuclides released during the first few days after the accident. This estimate was compared to previous ones and differences were analyzed further. The temporal and spatial variations of the 137Cs concentration present in the coastal waters were shown to be strongly related to the wind intensity and direction. During the first month after the accident, winds blowing toward the south confined the radionuclides directly released into the ocean to a narrow coastal band. Afterwards, frequent northward wind events increased the dispersion over the whole continental shelf, leading to strongly reduced concentrations.
Key Points
Assessment of cesium‐137 released from Fukushima power plant
Dispersion of radionuclides induced by winds
Consequences of tidal dynamics on hydro-sedimentary processes are a recurrent
issue in estuarine and coastal processes studies, and accurate tidal
solutions are a prerequisite for modeling sediment ...transport, especially in
macro-tidal regions. The motivation for the study presented in this
publication is to implement and optimize a model configuration that will
satisfy this prerequisite in the frame of a larger objective in order to study the
sediment dynamics and fate from the Red River Delta to the Gulf of Tonkin from
a numerical hydrodynamical–sediment coupled model. Therefore, we focus on
the main tidal constituents to conduct sensitivity experiments on the
bathymetry and bottom friction parameterization. The frequency-domain solver
available in the hydrodynamic unstructured grid model T-UGOm has been used
to reduce the computational cost and allow for wider parameter explorations.
Tidal solutions obtained from the optimal configuration were evaluated from
tide measurements derived from satellite altimetry and tide gauges; the use
of an improved bathymetry dataset and fine friction parameter adjustment
significantly improved our tidal solutions. However, our experiments seem to
indicate that the solution error budget is still dominated by bathymetry
errors, which is the most common limitation for accurate tidal modeling.
The South China Sea throughflow (SCSTF) connects the South China Sea (SCS) with neighboring seas and oceans, transferring surface water of the global thermohaline circulation between the Pacific and ...Indian oceans. A configuration of the SYMPHONIE ocean model at high resolution (4 km) and including an explicit representation of tides is implemented over this region, and a simulation is analyzed over 2010-2018. Comparisons with in situ and satellite data and other available simulations at coarser resolution show the good performance of the model and the relevance of the high resolution for reproducing the spatial and temporal variability of the characteristics of surface dynamics and water masses over the SCS. The added value of an online computation of each term of the water, heat, and salt SCS budgets (surface, lateral oceanic and river fluxes, and internal variations) is also quantitatively demonstrated: important discards are obtained with offline computation, with relative biases of â¼40 % for lateral oceanic inflows and outflows.
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