Estuarine Gravitational Circulation Research Articles

Coastal Upwelling and Estuarine Gravitational Circulation: A Feedback System in a Tropical Estuary in the South Atlantic

Coastal Upwelling and Estuarine Gravitational Circulation: A Feedback System in a Tropical Estuary in the South Atlantic

Relationship of salinity, temperature, pH, and transparency to dissolved oxygen in the Bouregreg estuary (Morocco): First results

Abstract Dissolved oxygen (DO) concentration is an essential indicator for assessment of estuary ecosystems. According to our knowledge, there is no research on the depth profiles of the DO and associated water quality variables of the entire Bouregreg estuary (Morocco). Water samples were collected from three stations at 0.5-m depth intervals during high and low tides (10/11/2021, 20/03/2022, and 02/07/2022). Our study shows that the DO concentrations ranged between 5.5 and 11.8 mg/L and demonstrated an inconsistent stratification pattern. When the tide was high and low in most of the study area, the bottom layer had a higher concentration of DO than the surface layer. There was a relatively high concentration of DO in the bottom layer of the estuary and a relatively low concentration in the surface layer of the estuary due to a high rate of freshwater flow on the surface (the release of Sidi Mohammed Ben Abdellah dam and the existence of estuarine gravitational circulation). Salinity, pH, water temperature, and water transparency ranged from 19.4 to 35.8 ppt, 7.0 to 8.7, 17 to 23 °C, and 16 to 21 cm, respectively, considering the depth profiles in the study area. The study provides scientific support for ecology operation and considerable advances in understanding the ecosystem dynamics of the Bouregreg estuary.

Vertical profile of dissolved oxygen and associated water variables in the Pasur-Rupsha estuary of Bangladesh

Vertical profile of dissolved oxygen (DO) and associated water variables were measured in the head of the Pasur-Rupsha estuary of Bangladesh. Water samples were collected from two stations at 0.60 m depth intervals during high and low tides from July, 2016 to January, 2017. DO concentration was ranged from 6.0 to 8.6 mg/l with maximum two units of variation in the profiles of the stations and demonstrated an inconsistent stratification pattern. The observed stratification pattern was remarkably triggered by a relatively high concentration of DO in the bottom layer than that of the surface layer during both the high and low tides in most of the study periods. High rate of freshwater flow at the surface; existence of estuarine gravitational circulation with 5–15 m depth profile; and lack of consumers in the bottom due to the high deposition of sediment particles were found responsible for the relatively high concentration of DO at bottom and low in surface layer of the estuary. Water temperature, salinity, pH, phytoplankton density and transparency were ranged from 22 to 32 °C, 0–6 ppt, 7.0 to 8.5, 1000–23000 ind/l and 13–17 cm respectively considering the depth profiles in the study periods. Temperature and plankton density were found significantly (P < 0.000) correlated with DO while the pH and salinity did not show any significant (P > 0.3) correlation. The ecosystem process associated with the vertical profile of DO explained in this study will provide considerable advances in understanding the ecosystem dynamics of the Pasur-Rupsha Estuary of Bangladesh.

Seasonal Changes of Estuarine Gravitational Circulation: Response to the Annual Temperature Change

Estuarine circulation is a vertical circulation that develops along the salinity gradient in estuaries and nearshore coastal waters. Fresher, and therefore less-dense, water flows out of the estuary in the surface layer, while a deeper inflow brings water from the open sea into the estuary. This study uses 7 years of in situ current measurements and hydrographic surveys to verify that this deeper inflow has two modes: a deep inflow that intrudes along the seabed, and a shallow-inflow that penetrates into the subsurface layer. These modes show seasonal variability, i.e., the deep-inflow mode occupies almost all of the winter season, whereas the shallow-inflow mode dominates during the summer. This mode change may play a key role in oxygen and carbon dioxide (CO2) dynamics in estuaries and nearshore coastal waters. When the transition from deep- to shallow-inflow begins in spring, a cold water mass forms on the seabed in the upper estuary. This cold water mass is isolated from heating sources and oxygenated water; consequently, the cold water mass becomes hypoxic and accumulates both inorganic nutrients and CO2 during spring and summer. When the transition from the shallow- to deep-inflow occurs, the CO2, which is trapped in the bottom-water, is emitted to the atmosphere. The mechanism that causes the seasonal mode change in estuarine circulation is driven by the spatial inhomogeneity in the heating or cooling of the lower layer, which generates a horizontal density gradient in the layer. This mechanism highlights the importance of temperature in estuarine dynamics, which has not been extensively studied previously.

A Modeling Study on the Influence of Sea-Level Rise and Channel Deepening on Estuarine Circulation and Dissolved Oxygen Levels in the Tidal James River, Virginia, USA

The impact of channel deepening and sea-level rise on the environmental integrity of an estuary is investigated using a three-dimensional hydrodynamic-eutrophication model. The model results show that dissolved oxygen (DO) only experienced minor changes, even when the deep channel was deepened by 3 m in the mesohaline and polyhaline regions of the James River. We found that vertical stratification decreased DO aeration while the estuarine gravitational circulation increased bottom DO exchange. The interactions between these two processes play an important role in modulating DO. The minor change in DO due to channel deepening indicates that the James River is unique as compared with other estuaries. To understand the impact of the hydrodynamic changes on DO, both vertical and horizontal transport timescales represented by water age were used to quantify the changes in hydrodynamic conditions and DO variation, in addition to traditional measures of stratification and circulation. The model results showed that channel deepening led to an increase in both gravitational circulation strength and vertical stratification. Saltwater age decreased and vertical exchange time increased with increases in channel depth. However, these two physical processes can compensate each other, resulting in minor changes in DO. A comparison of the impact of a sea-level rise of 1.0 m with channel deepening scenarios was conducted. As the sea level rises, the vertical transport time decreases slightly while the strength of gravitational circulation weakens due to an increase in mean water depth. Consequently, DO in the estuary experiences a moderate decrease.

Retrospect and perspective of the estuarine turbidity maximum zone researches

Turbidity maximum zone (TMZ) is the dynamic turbid water areas within limits in an estuary, where the concentrations of total suspended solids (TSS) are steadily and significantly higher than in the upstream river and the downstream sea. TMZ is a special phenomenon of the process of suspended sediment movement and transport in estuaries, which reflects the interaction of particulate matter and turbulent energy between rivers and oceans. TMZ has long been a hot topic for scientific inquiries and engineering innovations among a broad spectrum of scholars, government agencies, engineering corporations, and communities. The studies of TMZ has a long history of over 80 years since it was proposed in the last century. Based on a comprehensive review, the paper summarizes the main findings and progress of the TMZ studies and provides prospects of TMZ research in the future. The previous studies have focused on the basic characteristics of TMZ systematically and comprehensively, such as the TSS concentrations, and the sources and compositions TSS in estuaries. Although the formation and development of TMZ are affected by various factors and rather complex, estuarine gravitational circulation and tidal effects are considered as the primary mechanisms of TMZ formation and the main control factors of TMZ in different types of estuaries, including weakly, partially, or well-mixed estuaries. Besides, sediment re-suspension also plays an important role in TMZ formation. Additionally, dynamic evolution characteristics of TMZ from short to medium terms have been analyzed and discussed in great detail. It has been commonly agreed that these past and current studies of TMZ have made great progress in many aspects, and the findings and outcomes have made significant contributions to our theoretical understanding and practical management of TMZ. However, several areas warrant further exploration to advance the TMZ research, to improve our understanding of the spatial and temporal variations of TMZ, and to contribute to the sustainable management of TMZ for improving its surrounding natural eco-environment. The first one is to accurately detect, extract, and measure the size, depth, and volume of TMZ in estuaries. Where a TMZ is located in an estuary is a fundamental question and an important task for studying TMZ. However, current studies of detecting TMZ were mainly based on relatively rough description or some types of thresholds of TSS concentrations, or certain levels of turbidity criteria, which had low accuracies, were not consistent in different estuaries and times and showed weak applicability. Therefore, future works should focus more on the exploration of new methods and techniques for precisely detecting and extracting the spatiotemporal distributions of TMZ. Secondly, to comprehensively understand the whole features of TMZ, detection, and measurement of 3D distribution characteristics of TMZ in different conditions should be rigorously carried out in the future. Last but not least, future studies should pay more attention to the studies of the long term impacts of human activities and global environmental changes on spatial and temporal variations of TMZ to obtain a more profound understanding of the change dynamics of TMZ for better management policies.

Modeling of suspended sediment by coupled wave-current model in the Zhujiang (Pearl) River Estuary

A three-dimensional wave-current-sediment coupled numerical model is developed to understand the sediment transport dynamics in the Zhujiang (Pearl) River Estuary (ZRE), China. The model results are in good agreement with observed data, and statistics show good model skill scores. Numerical studies are conducted to assess the scenarios of suspended sediment in the ZRE under the effects of different forcing (river discharges, waves, and winds). The model results indicate that the estuarine gravitational circulation plays an important role in the development of estuarine turbidity maximum in the ZRE, particularly during neap tides. The increased river discharge can result in a seaward sediment transport. The suspended sediment concentration (SSC) in the bottom increases with both wave bottom orbital velocity and wave height. Because of the shallow water depth, the effect of waves on sediment in the west shoal is greater than that in the east channel. The southwesterly wind-induced wave affects the SSC more than those resulting from the northeasterly wind, while the northeasterly wind-driven circulation has a slightly greater influence on the SSC than that of the southwesterly wind. However, a steady southwesterly wind condition favors the increase of the SSC in the Lingding Bay more so than a steady northeasterly wind condition. If the other forcings are same, the averaged SSC under a steady southwesterly wind condition is about 1.1 times that resulting from a steady northeasterly wind.

Residence Time of a Highly Urbanized Estuary: Jamaica Bay, New York

Using a validated coupled hydrodynamic-tracer transport model, this study quantified the mean residence time in Jamaica Bay, a highly eutrophic lagoonal estuary in New York City. The Bay is a well-mixed to partially-stratified estuary with heavily-dredged bathymetry and substantial wastewater treatment plant effluent inputs that lead to seasonal hypoxia in some poorly-flushed deep-water basins. Residence time was computed for Jamaica Bay and its largest isolated deep basin, Grassy Bay. The response of residence time to freshwater discharge and wind forcing during summer 2015 was also investigated. The model results showed that the mean residence time, which represents the time required to flush out 63% of tracers released into the region of interest, was 17.9 days in Jamaica Bay and 10.7 days in Grassy Bay. The results also showed that some regions in Jamaica Bay retained their tracers much longer than the calculated residence time and, thus, are potentially prone to water quality problems. Model experiments demonstrated that summertime wind forcing caused a small increase in residence time, whereas freshwater discharge substantially reduced residence time. Freshwater inputs were shown to strongly enhance the two-layer estuarine gravitational circulation and vertical shear, which likely reduced residence time by enhancing shear dispersion. Due to the Bay’s small, highly-urbanized watershed, freshwater inputs are largely derived from the municipal water supply, which is fairly uniform year-round. This water helps to promote bay flushing, yet also carries a high nitrogen load from wastewater treatment. Lastly, the tidal prism method was used to create a simple calibrated model of residence time using the geometry of the study area and the tidal range and period.

Quantifying the impact of the major driving mechanisms of inter-annual variability of salinity in the North Sea

New 67-year long (1948–2014) gridded time series of salinity in the North Sea at all depths allowed to quantify, spatially resolved, the amount of inter-annual salinity variability explained by each of its driving mechanisms: sea level pressure (SLP), precipitation, river run-off, zonal and meridional winds and currents over the eastern North Atlantic. For the current data, not only annual averages but also their deviations, as measure of turbulence, were considered. Our results summarize and expand the knowledge gathered in the last 50years about the mechanisms driving inter-annual variability of salinity in the North Sea. Three mechanisms, uncorrelated with each other and acting over separate regions of the North Sea, arise as most important: (1) River run-off from continental Europe explains 50–80% of inter-annual salinity variations at lag 0 in the Southern and German Bights and the Norwegian Trench up to the connection with the North Atlantic, down to the seabed near the coasts and to the deep Norwegian Trench (100m); (2) Remote variations of salinity in the Rockall Trough explain 70% of salinity variations of the tongue of high salinity in the northwestern North Sea with a lag of one year and down the water column; (3) The Neva discharge explains 60% of salinity changes in Skagerrak and southern Norwegian trench at lag 0. An explanation for this correlation might be the Baltic freshwater outflow being modulated by the Neva discharge through intensification of the estuarine gravitational circulation. We confirmed known relations between river run-off, precipitation over continental Europe, SLP over northern Europe and zonal wind over western Europe. Linked to these changes, we found also changes of meridional wind north of Scotland favoring eastward Ekman transport of salty North Atlantic waters into the North Sea off the Norwegian coast. Excluding this only case, we found no significant correlation between wind-driven currents and North Sea salinity changes. This result supports the notion that the Atlantic inflow into the North Sea is mainly density-driven. Salinity in the region east of Scotland and northern England was alienated from all driving mechanisms tested. An explanation was found in concomitant canceling changes of the intensity of the North Sea circulation and the discharge of the river Tay.

Analytical study of lateral-circulation-induced exchange flow in tidally dominated well-mixed estuaries

In straight estuary channels, differential advection and the Coriolis force are the major driving mechanisms for lateral circulation. An analytical model was developed to explore the roles of the two mechanisms in the dynamics of tidally dominated well-mixed estuaries. The model provided a nondimensional parameter, Keh, a type of Kelvin number (considered as horizontal Kelvin number) to elucidate the relative importance of the two mechanisms. Differential advection is effective under small Keh, while the Coriolis force is effective under larger Keh. The critical value of Keh has an order of magnitude of 0.1 in well-mixed estuaries. Lateral circulations generate residual currents through the lateral advection term in the along-estuary momentum equation. When differential advection is effective, the lateral-advection-induced flow has a laterally sheared structure with the landward flow in the channel and seaward flows over shoals. When the Coriolis force is effective, it has a laterally sheared structure with the landward flow in the left part of the cross-section and the seaward flow in the right (facing ocean). When the two mechanisms are equally important, it has an asymmetric laterally sheared structure with a stronger seaward flow over the right shoal. Those lateral structures indicate that the lateral-circulation-induced flow generally reinforces the estuarine gravitational circulation.

The role of density gradients on tidal asymmetries in the German Bight

The dynamics of the German Bight associated with river plumes and fresh water intrusions from tidal flats have been studied with numerical simulations. The horizontal and vertical patterns of the M2, M4 and M6 tides revealed complex distortions along the bathymetric channels connecting the coast and the open sea. A major focus was on the surface-to-bottom change in tidal asymmetries, which provides a major control on draining the tidal flats around the Elbe and Weser River mouths. Comparisons between baroclinic and barotropic experiments demonstrated that the estuarine gravitational circulation is responsible for pronounced differences in surface and bottom asymmetries. These differences could be considered as a basic control mechanism for sediment dynamics. The most prominent area of tidal distortions, manifested by a delay of the tidal wave, was located between the estuarine turbidity maximum and the estuarine mouth north of Cuxhaven. This area was characterized by the strongest periodic convergence and divergence of the flow and by the largest salinity gradients. The enhancement of the gravitational circulation occurred during the transition between spring and neap tides. The large-scale dynamics and small-scale topographic features could impact the sediment distribution as there was a marked interplay in the channels between stratification and turbulence. Also an explanation has been given for the mechanisms supporting the existence of a mud area (Schlickgebiet) south of Helgoland Island, associated with trapping suspended particular matter.

Using Depth-Normalized Coordinates to Examine Mass Transport Residual Circulation in Estuaries with Large Tidal Amplitude Relative to the Mean Depth

Abstract Residual (subtidal) circulation profiles in estuaries with a large tidal amplitude-to-depth ratio often are quite complex and do not resemble the traditional estuarine gravitational circulation profile. This paper describes how a depth-normalized σ-coordinate system allows for a more physical interpretation of residual circulation profiles than does a fixed vertical coordinate system in an estuary with a tidal amplitude comparable to the mean depth. Depth-normalized coordinates permit the approximation of Lagrangian residuals, performance of empirical orthogonal function (EOF) analysis, estimation of terms in the along-stream momentum equations throughout depth, and computation of a tidally averaged momentum balance. The residual mass transport velocity has an enhanced two-layer exchange flow relative to an Eulerian mean because of the Stokes wave transport velocity directed upstream at all depths. While the observed σ-coordinate profiles resemble gravitational circulation, and pressure and friction are the dominant terms in the tidally varying and tidally averaged momentum equations, the two-layer shear velocity from an EOF analysis does not correlate with the along-stream density gradient. To directly compare to theoretical profiles, an extension of a pressure–friction balance in σ coordinates is solved. While the barotropic riverine residual matches theory, the mean longitudinal density gradient and mean vertical mixing cannot explain the magnitude of the observed two-layer shear residual. In addition, residual shear circulation in this system is strongly driven by asymmetries during the tidal cycle, particularly straining and advection of the salinity field, creating intratidal variation in stratification, vertical mixing, and shear.

Suspended sediment transport in the Deepwater Navigation Channel, Yangtze River Estuary, China, in the dry season 2009: 1. Observations over spring and neap tidal cycles

[1] The in situ data in the Deepwater Navigation Channel (DNC), Yangtze River Estuary (YRE), China, in the dry season 2009, shows spring tides associated with greater maximum velocities, more mixing, less stratification, and diffused fluid mud; whereas neap tides are associated with smaller maximum velocities, greater stratification, inhibited mixing, and stratified fluid muds. The balance of salt flux indicates the seaward salt transport is dominated by fluvial flows, and the landward salt transport is generated by compensation flows during spring tides, but shear effects during neap tidal cycles. The balance of suspended sediment flux illustrates the offshore sediment transport is dominated by fluvial flows as well, but the onshore transport is induced by tidal-pumping effects on spring tides, and shear effects on neaps. The suspended sediment transport is strongly affected by the salinity distribution and salinity-gradient-induced stratification in the DNC. The spring-neap asymmetry is generated by the estuarine gravitational circulation during low-flow conditions; while the flood-ebb asymmetric stratification within a tidal cycle is due to the semidiurnal tidally movement of the salt front.

Suspended sediment transport in the Deepwater Navigation Channel, Yangtze River Estuary, China, in the dry season 2009: 2. Numerical simulations

[1] A three-dimensional wave-current-sediment coupled numerical model with wetting and drying process is developed to understand hydrodynamics and sediment transport dynamics in the Deepwater Navigation Channel (DNC), the North Passage of the Yangtze River Estuary (YRE), China. The model results are in good agreement with observed data, and statistics show good model skill scores and correlation coefficients. The model well reproduces the spring-neap variation between a well-mixed estuary and a highly stratified estuary. Model results indicate that the estuarine gravitational circulation plays the most important role in the estuarine turbidity maximum (ETM) formation in the DNC. The upstream nonlocal sediment intrusion through the spillover mechanism is a major source of sediment trapping in the North Passage after the morphological changes. Numerical studies are conducted to show scenarios in the YRE under the effects of different forcings (river discharges, waves, and winds). Between these study cases, surface-wave-breaking relieves the sediment trapping and bottom-wave-current-interaction aggravates the bed erosion and elevates the SSC in the ETM; the former and the latter have the least and largest influence on the suspended sediment transport in the DNC. The wind effects have a greater influence on sediment trapping than the river discharges, and the steady northwesterly wind condition favors the siltation in the DNC most. The significance of density-driven turbidity current is also assessed, which can enhance the saline-water intrusion and suppress the turbulent mixing in the bottom boundary layer.

Modeling river plume dynamics with the HYbrid Coordinate Ocean Model

The dynamics of large-scale river plumes are investigated in idealized numerical experiments using the HYbrid Coordinate Ocean Model (HYCOM). The focus of this study is to address how the development and structure of a buoyant plume are affected by the outflow properties, as impacted by processes within the estuary and at the point of discharge to the coastal basin. Changes in the outflow properties involved vertical and horizontal redistribution of the river inflow and enhanced vertical mixing inside an idealized estuary. The development of the buoyant plume was evaluated in a rectangular, f-plane basin with flat and sloping bottom conditions and in the absence of other external forcing. The general behavior of a mid-latitude river plume was reproduced, with the development of a surface anticyclonic bulge off the estuary mouth and a surface along-shore coastal current which flows in the direction of Kelvin wave propagation (“downstream”); the momentum balance was predominantly geostrophic. Conditions within the estuary and the outflow properties at the river mouth (where observed profiles may be available) greatly impacted the fate of riverine waters. In flat bottom conditions, larger mixing at the freshwater source enhanced the estuarine gravitational circulation, promoting larger upward entrainment and stronger outflow velocities. Although the overall geostrophic balance was maintained, estuarine mixing led to an asymmetry of the currents reaching the river mouth and to a sharp anticyclonic veering within the estuary, resulting in reduced upstream flow and enhanced downstream coastal current. These patterns were altered when the plumes evolved in the presence of a bottom slope. The anticyclonic veering of the buoyant outflow was suppressed, the offshore intrusion decreased and the recirculating bulge was displaced upstream. The sloping bottom impacts were accompanied by enhanced transport and increased downstream extent of the coastal current in most cases. No major changes in the general properties and especially the vertical structure of the plumes were observed when the vertical coordinates were changed from cartesian–isopycnal, to sigma or to sigma–isopycnal. The findings offer a benchmark for coastal studies with HYCOM, where plume dynamics should be examined in tandem with additional circulation forcing mechanisms, resulting in transitions of the vertical coordinate system that are dictated by the prevailing dynamics.

The importance of estuarine gravitational circulation in the early life of the Bohai Penaeid Prawn

Estuaries along the southern shore of the Bohai Sea are the major habitat of the Bohai Penaeid Prawn ( Penaeus chinensis). Since the 80's, however, many of the rivers have been dammed. Field observations, as well as laterally integrated 2-D numerical experiments, were conducted to understand both the role of estuarine gravitational circulation and the impact of the damming of the rivers on the early life of the Bohai Penaeid Prawn. For a river with runoff, especially for small discharge, the gravitational circulation tends to transport the planktonic larvae in their metamorphosis phase near the river mouth where condition is favorable for survival. The gravitational circulation inside the estuary tends to transport the mysis phase larvae towards the upstream end of the estuary when the most part of the larvae suspended in the bottom layer, and it transports the post larvae to the low salinity near upstream side of the estuary when the larvae become benthic, if the larvae enter the estuary. Damming causes long periods of zero runoff in the river, resulting in the alteration of the estuarine circulation and in the change of the estuarine environment. In addition, excess evaporation may prematurely transport the planktonic larvae into the estuary. On the other hand, sudden release of a large volume of freshwater from behind the dam may exert undesirable stress on the larvae.

Dissolved oxygen stratification in two micro-tidal partially-mixed estuaries

The controlling physical factors for vertical oxygen stratification in micro-tidal, partially-mixed estuaries are discussed in this paper. A theoretical deduction shows that vertical stratification of dissolved oxygen (DO) concentration can be explained by the extended Hansen and Rattray's Central Region theory, which suggests that in addition to biological factors such as photosynthesis, biochemical oxygen demand (BOD), sediment oxygen demand (SOD), vertical DO profiles are mainly controlled by physical factors such as surface re-aeration, river flow, and estuarine gravitational circulation. Vertical mixing of DO from surface re-aeration and photosynthesis sets a DO profile of higher concentration near the surface and lower near the bottom. With a positive seaward longitudinal DO gradient, strong river flow and estuarine gravitational circulation can cause lower DO concentrations near the surface and higher near the bottom. The actual vertical oxygen profile is then determined by the relative magnitude of the above-mentioned mechanisms. It is sensitive to two parameters: (1) the strength of the gravitational circulation ( u E); and (2) the relative importance between biochemical oxygen demand and vertical diffusivity ( α). Vertical DO stratification usually becomes weaker as u E increases. The impact of gravitational circulation on vertical oxygen distribution becomes more important for a larger α. The impact of α on oxygen stratification is profound. As u E (and river flow) increases, DO stratification appears to be less sensitive to the value of α. Surface-to-bottom differences in DO concentrations (ΔDO) is negligible when α is small ( α < 0.5). As α increases, ΔDO increases under a weak to moderate gravitational circulation mode ( u E ≤ 5 cm s −1). Under a strong gravitational circulation mode, ΔDO becomes negative with a small α ( α < 2), and as α continues to increase, ΔDO becomes positive. The newly-deduced governing equation for vertical oxygen stratification is applied to two micro-tidal, partially-mixed estuarine systems: the Cape Fear River Estuary (CFRE) and the Pamlico River Estuary (PRE) of North Carolina. In the CFRE, although strong vertical salinity stratification exists, DO concentrations are usually well mixed. De-coupling between salinity stratification and oxygen stratification is mainly due to a relatively stronger estuarine gravitational circulation and higher freshwater inflow in the system. It appears that river flow and gravitational circulation are the dominant factors in controlling oxygen stratification in the CFRE. In contrast, vertical stratification of DO concentrations is closely correlated with that of salinity in the PRE. In the PRE, the estuarine gravitational mode and river flow are often both very weak, and DO stratification is very sensitive to the value of α. With negligible influence from tidal mixing, the system is more sensitive to vertical mixing regulated by salinity stratification and wind. As a result, vertical DO stratification is closely correlated with salinity stratification in the PRE.

The Role of Circulation in the Development of Hypoxia in Ise Bay, Japan

Seasonally occurring hypoxia in Ise Bay, Japan was studied by comparing flow structures and ages of water parcels. Age is defined as the elapsed time after isolation of the water parcel from the oxygen source. In this bay, bay water is strongly stratified and advective transport by the estuarine gravitational circulation dominates from April to October. In contrast, the strong tidal current keeps the water column well mixed and oxygen-rich from top to bottom in the bay-mouth strait, which connects the bay with the Pacific Ocean. The estuarine circulation in Ise Bay is influenced by the effect of the Earth's rotation; the well-mixed water intrudes into the eastern half of the bay under the pycnocline with the land on its right (looking down current). Surrounded by this strait water, relatively cooler water (cold pool) remains on the bottom in the western half of the bay, isolated from the seasonal heating. Comparing the temperature of water under the pycnocline in the bay with the temperature of the strait water, we can estimate an age (τ) of water parcels. This age coincides with the mean residence time, which has traditionally been used. This method gives an age to the water parcel at a point under the pycnocline in the bay.%Oxygen concentration under the pycnocline indicates a high correlation coefficient (R2>0·8) with age. The decrease of oxygen concentration during the period τ gives the oxygen consumption rate (OCR). The OCRs under the pycnocline were calculated from May to August. The OCR is approximately homogeneous independent of location in the bay. Moreover, monthly values of OCRs are also approximately constant from May to August, despite the mean temperature increase from 14·4 to 20·4°C during this period. Known physical processes explain 80% of the spatial and temporal variation in oxygen concentration. In contrast, biochemical processes provide homogeneous OCR and play only a minor role in the variation in oxygen concentration.

Turbulence closure modeling of estuarine stratification

Transient stratification in estuaries reflects competition between the stratifying influences of the vertical gravitational circulation and elastic straining of the longitudinal density gradient by vertical shear, set against the mixing influence of, principally, tidally generated turbulence. A one‐dimensional (vertical) numerical model of an estuary, using seven different parameterizations of vertical mass and momentum mixing coefficients from the literature, is used to make general predictions about the nature of the time‐dependent signal of stratification in an estuary. These predictions show good agreement with published predictions from simpler models and they demonstrate that, at higher frequencies (e.g., first and second species tides) stratification is produced primarily from elastic straining but that, at lower frequencies (due to the fortnightly spring‐neap modulation, for example) the estuarine gravitational circulation tends to dominate. The same model, with rotation included, is then used to hindcast a time series of stratification from Spencer Gulf, South Australia, in order to test parameterizations of mixing coefficients against real data. The character of the semidiurnal and diurnal stratification signal is better predicted by some schemes than others. Against qualitative and quantitative criteria, the Mellor and Yamada (1982) and Pacanowski and Philander (1981) schemes perform best. All schemes, however, underpredict the amplitude of the stratification signal at all timescales. At longer timescales, associated with the springs‐neaps cycle, this is believed to be due to a difference in the degree of geostrophic adjustment of the model and the observations, produced by the three‐dimensional topography of the observational site (Spencer Gulf, South Australia).

Residual currents in tidally dominated, well-mixed estuaries

An analytical model is developed to investigate the residual circulation originating from the interaction of the density field, river flow, and the nonlinear rectification of the periodic tides, in tidally-dominated, well-mixed estuaries. These residual flows effect the density field, which in turn drives the estuarine gravitational circulation. Previous analytical studies have excluded the tidal dynamics and have either imposed the density structure, thereby decoupling the problem, or assumed similarity solutions valid for specific regions within the estuary. Here, a theoretical model is developed that calculates, via a perturbation analysis, the lowest-order density field established from tidal mixing, tidally-induced residual flow, river runoff, and diffusion. Two special cases are considered, a long tidal river and a finite length, variable-breadth estuary. Once the density field is determined for each case, the density driven flow is calculated and added to the rectified tidal and river induced flows for both the Eulerian and Lagrangian reference frames. DOI: 10.1034/j.1600-0870.1993.00007.x