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Spring neap variations in stratification and chlorophyll fluorescence were studied during the summers of 2011–2014 in a tidal sea (Yatsushiro Sea, Japan) using monitoring data and hydrodynamic models. Vertical profiles of salinity, temperature and chlorophyll fluorescence were collected nearly weekly from nine stations in this sea during the same period. Composite analysis using vertical profiles of density clearly indicated enhancement of the stratification during the neap tide and a vertically mixed water column during the spring tide in the tidal area. Interregional differences were revealed in the variation of chlorophyll fluorescence with the spring neap tidal cycle. More notable increases in chlorophyll fluorescence were observed during the neap tide in the tidal area around the narrow strait than in the inner area. Temporal stratification led to an increase in the chlorophyll fluorescence in the tidal strait during the neap tide. [ABSTRACT FROM AUTHOR]

Maputo Bay is a tidally-energetic embayment, influenced by strong rainfall and associated river runoff during the wet season. Literature shows that salinity can regulate the nutrient cycle in mangrove estuarine ecosystems affecting the early life stages in these habitats which sustain the economically important shrimp stocks. The freshwater flow into Maputo Bay is for a part controlled by dam systems on its main rivers. In this work we investigate how varying flows from the Incomati and Maputo rivers interact with the tide to influence the evolution of the bay's salinity field. A 3- dimensional hydrodynamic model was applied to Maputo Bay, improving a previously published model through vertical and temporal refinement and recalibration, leading to a more accurate representation of the semidiurnal and fortnightly stratification-mixing cycles occurring during the wet season. However, the model still predicts salinities lower than those found in observations. An analysis showed that uncertainties in the salinity field increased towards the mouth of the Maputo River indicating the uncertainty of the modeled flow at the catchment as a possible cause of the underestimation of salinity in the bay. A set of experiments of varying Maputo and Incomati river flows show that for a flow as frequent as 5 times per year, the buoyancy input and the associated density driven flow contributed less than the tide in forcing the bay-shelf exchange. The inverse is observed for results where a flow with a 5-year return period leads to a more efficient bay-shelf exchange during neap tides than during spring tides. For this scenario, the results suggest that the estuarine plume was arrested inside the bay by tidal stirring during spring tide and released during neap tide when tidal stirring was subdued. The analysis of the separate and joint effects of the two rivers in setting the salinity field in Maputo Bay show a difference in extent of influence area between the rivers. These results suggest that the management of the freshwater inflow from the Maputo River can be crucial for maintaining lower salinities over parts of the bay to sustain the economically important marine resources. [ABSTRACT FROM AUTHOR]

A three-dimensional hydrodynamic model is used to investigate intratidal spring-neap variations of turbulent mixing and stratification in the Strait of Singapore and its adjacent Johor estuary area. The waters in the Johor estuary are ebb dominant because of interaction of the OÍ--K1-M2 tidal constituents. In most estuaries mixing rates during the flood are larger than during the ebb because of a flood-dominant tidal asymmetry. The Johor estuary is an example where tidal asymmetry supports stronger mixing during ebb tides, whereas similar to other estuaries, tidal straining promotes stable stratification during the ebb. Therefore an analysis of intratidal variation in stratification reveals the relative importance of both mechanisms. Vertical profiles of salinity, flow velocity, and eddy diffusivity show a marked asymmetry between flood and ebb tides. Small changes in the tidal currents substantially influence the eddy diffusivity, energy dissipation rate, and build up of stratification, which is explained using the nondimensional Richardson number. The estuary is found completely mixed during flood tide and slightly stratified during ebb tide, suggesting tidal straining (generating ebb stratification) is more important for the intratidal variation in mixing and stratification than tidal asymmetry (flood stratification). [ABSTRACT FROM AUTHOR]

Abstract: To investigate how salinity changes with abrupt increases and decreases in river discharge, three surveys were conducted along six sections around the Yellow River mouth before, during and after a water regulation event during which the river discharge was increased from ∼200 to >3000m3 s−1 for the first 3 days, was maintained at >3000m3 s−1 for the next 9 days and was decreased to <1000m3 s−1 for the final 4 days. The mean salinity in the Yellow River estuary area during the event varied ∼1.21, which is much larger than its seasonal variation (∼0.50) and interannual variation (∼0.05). Before the event, a small plume was observed near the river mouth. During the event, the plume extended over 24km offshore in the surface layer in the direction of river water outflow. After the event, the plume diminished in size but remained larger than before the event. The downstream propagation of the plume (as in a Kelvin wave sense) was apparent in the bottom layer during the second survey and in both the surface and bottom layers during the third survey. The plume sizes predicted by the formulas from theoretical studies are larger than those we observed, indicating that factors neglected by theoretical studies such as the temporal variation in river discharge and vertical mixing in the sea could be very important for plume evolution. In addition to the horizontal variation of the plume, we also observed the penetration of freshwater from the surface layer into the bottom layer. A comparison of two vertical processes, wind mixing and tidal mixing, suggests that the impact of wind mixing may be comparable with that of tidal mixing in the area close to the river mouth and may be dominant over offshore areas. The change in Kelvin number indicates an alteration of plume dynamics due to the abrupt change in river discharge during the water regulation event. [Copyright &y& Elsevier]

The article discusses the hyperpycnal flows off the Huanghe River in China, and the relationship of the hyperpycnal flows to tidal cycles. It mentions that high concentrations of sediment input from river and tide modulations had initiated the hyperpycnal flows at the mouth of the river. It states that the tidal behaviors of hyperpycnal flows are associated with the suspended sediment concentration (SSC), salinity and stratification of water columns.

Maputo Bay, on the coast of Mozambique, is a tidally energetic, subtropical embayment in a region subjected to strong seasonal rainfall. Data from moored current meters, tide gauges and monthly bay-wide surveys were used to characterise the evolution of the density structure on seasonal, fortnightly and semi-diurnal time-scales and its relation to tidal forcing. The bay is subjected to large seasonal variations in freshwater input (10-103 m3 s-1) and pronounced fortnightly variations in tidal amplitude with a spring:neap tide ratio varying between 3.6 and 9.0 with a corresponding variation in tidal stirring power input (10-3-1 W m-3). During the dry season, the water column was continuously fully mixed with weak horizontal density gradients. In contrast, during the wet season, freshwater buoyancy induced marked horizontal salinity gradients and stratification, which was pronounced around the time of neap tides. This stratification was largely eroded at spring tides but semi-diurnal, periodic stratification was still evident. A potential energy anomaly model was used to demonstrate that this periodic component of stratification was largely a result of tidal straining acting locally, with an additional contribution of stratified water advected from outside the bay during the last stages of the flood tide. Simulations using the Delft3D-Flow hydrodynamic model gave a realistic account of the tidal regime in the bay, but achieved only qualitative success in the hindcasting of the changes in water column structure and horizontal exchange. The model indicated a wet season salinity deficit of 2-3 times that of the observed values and wet season temperatures were ~2-3 °C higher than those observed. The model also underestimated the intensity of stratification in the wet season. These shortcomings all suggest an overestimate of vertical mixing by the model's turbulence closure scheme. [ABSTRACT FROM AUTHOR]

Abstract: A three-dimensional numerical model of the seasonal evolution of the hydrography and circulation of the Gulf of California predicts a quasi-permanent anticyclonic gyre in the central part of the gulf, over San Pedro Mártir Basin (SPMB), just South of the mid-gulf archipelago. A historical hydrographic data bank is used to search for evidence of the predicted gyre. Despite 363 useful profiles (in 15 cruises from 1939–1996), sampling in the area of interest only allowed climatological average distributions of temperature and salinity to be obtained, from which dynamic height and geostrophic currents were calculated. The average distributions of temperature, salinity and density all show a concave shape over SPMB, which translate into a dynamic height dome and therefore into and anticyclonic gyre. Vertical sections (across and along the gulf axis) of geostrophic velocity show that the gyre occupies most of the water column, which is 800m deep in the center of SPMB. Experiments with the numerical model indicate that stratification is necessary, and that the tidal currents are responsible for the formation of the gyre; this occurs through nonlinear interactions (friction and advection) and vertical mixing, mechanisms that are enhanced in SPMB due to the proximity of the sills and channels of the midriff archipelago, where internal tides are generated. [Copyright &y& Elsevier]