Baroclinic Pressure Gradient Research Articles

Subinertial flow patterns in a tropical coral reef system of the southwestern gulf of Mexico

One-year subinertial flow profiles and near-bottom temperatures were recorded in a tropical coral reef of the southwestern Gulf of Mexico to determine whether baroclinicity affects flow distributions. The subinertial flow at four sites was predominantly influenced by three empirical modes of variability. The first mode (explaining 78% of the variance) was mostly associated to wind stress (r = 0.62). Its temporal variability had greatest energy during the northerly wind season (October–May), related to atmospheric cold fronts, at periods of 2–10 days and at 20–30 days. Mode 2 (with 6% of the variance) was related to sea-level slopes, linked also to northerly wind events; this mode induced an anticyclonic circulation around the reefs, although a less intense cyclonic circulation was also observed. Mode 3 (with 5% of the variance) was associated to a baroclinic pressure gradient that drove bidirectional flow profiles: Near-surface flows moving in opposite direction to near-bottom flows. Mode 3 dominated in the vertically stratified summer (June–September), the rainy season, but it was also occasionally observed during northerly winds. Near-bottom summer intrusions of relative cold water were baroclinically driven, linked to Mode 3, and unrelated to Ekman dynamics. Modes 2 and 3 of variability described additional flow distributions relative to the behavior already proposed for the reefs on the southwestern continental shelf of the Gulf of Mexico.

The roles of wind and baroclinic processes in cross-isobath water exchange within the Bohai Sea

Considering the significance of water exchange between the nearshore and offshore waters to shallow-water biogeochemical processes, the influences of wind and baroclinic processes on water exchanges across the 10-m (E10) and 20-m isobaths (E20) in the Bohai Sea during ice-free cycles of 1998–2019 were investigated using a three-dimensional Finite Volume Community Ocean Model (FVCOM). The two-way analysis of variance illustrated that both E10 and E20 showed significant seasonal and spatial variations. The cross-isobath fluxes were the strongest during summer and the weakest during fall, with offshore (onshore) transport in the upper (bottom) layer. The net volume transport was at O (10−2) Mm3/s (millions of cubic meters per second) in Bohai and Liaodong Bays, which was an order of magnitude greater than that in Laizhou Bay. Further numerical experiments revealed that wind regulated the upper-layer E10 via the wind-induced advection, while it altered E20 mainly through the enhanced mixing and the subsequent weakening of the baroclinic pressure gradient force in the subsurface layer. In addition, the thermal gradient driven cross-isobath volume transports in Bohai and Liaodong Bays were most prominent during summer but negligible during fall. Those along the 10-m isobath in Laizhou Bay, however, were induced by the salinity gradient from the buoyant fluxes of the Yellow River all year round. With supplemental contributions of the baroclinic processes related to the cold pool of the Bohai Sea, and the wind-induced advection and mixing in the surface and subsurface layers, respectively, Laizhou Bay's E20 had a four-layer vertical structure during summer.

Applications of generalized vertical coordinates in ocean circulation models

Two types of generalized vertical coordinates are introduced to reduce model errors in calculating the baroclinic pressure gradient (BPG), advection term and horizontal mixing in tracer equations in a three-dimensional (3D) ocean circulation model. These two types of vertical coordinates are defined as the generalized terrain-following coordinate (GTC) and generalized piecewise terrain-following coordinate (GPTC). The main feature of the GTC is the use of a series of constant-depth levels in the water column above the sea bottom. The GPTC differs from the GTC in which the depth differences between two adjacent grids are reduced, especially in the bottom layer over steep topography. Performances of these two vertical coordinate systems are assessed first in two types of idealized numerical experiments with a tall seamount and continuously varying bottom slope respectively. The model results in the tall seamount experiment demonstrate that both the GTC and GPTC perform well in reducing the model errors in the BPG over the steep topography, and greatly alleviate the spurious diapycnal mixing in terrain-following coordinate models. In the continuously varying bottom slope experiment, both the GTC and GPTC perform better than the z-level coordinate in simulating the 3D currents. The performances of the GTC and GPTC are further assessed by examining prognostic simulations of 3D currents and hydrography in the Yellow and East China Seas with realistic external forcing and bottom topography. The model results in the third experiment demonstrate that the GPTC restrains the overestimated upward climbing of the bottom cold water in comparison with other coordinates. The GPTC also performs better than the GTC and the conventional sigma-layer coordinate in simulating the shoreward intrusion of relatively high-salinity subsurface waters, intermediate low-salinity waters and the Yellow Sea bottom cold waters. By comparison, the GTC simulates the Kuroshio, the Slope Counter-Current and their seasonal variability significantly better than other coordinates.

Modelling study on estuarine circulation and its effect on the turbidity maximum zone in the Yalu River Estuary, China

The along-channel, lateral estuarine circulation and the related baroclinic effects on the turbidity maximum zone (TMZ) of the Yalu River Estuary (YRE) were explored in our study based on the Finite Volume Coastal Ocean Model (FVCOM). The results show that along-channel estuarine flow in YRE had a two-layer-structure with seaward flow in the surface layer and landward flow in the bottom layer. The along-channel bottom landward flow can transport sediment to the upper estuary and influence the location of the TMZ, which was mainly induced by asymmetric tidal mixing. Then, for the variation in cross-channel circulation (lateral circulation), the TMZ moved westwards after reclamation, due to the decreased eastward residual flow resulting from weakened eastward barotropic and baroclinic pressure gradient forcing. This study qualitatively demonstrated the baroclinic effects and human-induced impacts on estuarine subtidal flow and sedimentation. Although site-specific, these findings have wider implications for sediment transport in other medium-scale estuaries.

High and Variable Drag in a Sinuous Estuary With Intermittent Stratification

AbstractIn field observations from a sinuous estuary, the drag coefficient based on the momentum balance was in the range of , much greater than expected from bottom friction alone. also varied at tidal and seasonal timescales. was greater during flood tides than ebbs, most notably during spring tides. The ebb tide was negatively correlated with river discharge, while the flood tide showed no dependence on discharge. The large values of are explained by form drag from flow separation at sharp channel bends. Greater water depths during flood tides corresponded with increased values of , consistent with the expected depth dependence for flow separation, as flow separation becomes stronger in deeper water. Additionally, the strength of the adverse pressure gradient downstream of the bend apex, which is indicative of flow separation, correlated with during flood tides. While generally increased with water depth, decreased for the highest water levels that corresponded with overbank flow. The decrease in may be due to the inhibition of flow separation with flow over the vegetated marsh. The dependence of during ebbs on discharge corresponds with the inhibition of flow separation by a favoring baroclinic pressure gradient that is locally generated at the bend apex due to curvature‐induced secondary circulation. This effect increases with stratification, which increases with discharge. Additional factors may contribute to the high drag, including secondary circulation, multiple scales of bedforms, and shallow shoals, but the observations suggest that flow separation is the primary source.

Mechanisms of Lateral Spreading in a Near-Field Buoyant River Plume Entering a Fjord

Observations collected from a fast-flowing buoyant river plume entering the head of Doubtful Sound, New Zealand, were analysed to examine the drivers of plume lateral spreading. The near-field plume is characterised by flow speeds of over 2 ms−1, and strong stratification (N2 > 0.1 s−2), resulting in enhanced shear which supports the elevated turbulence dissipation rates (ϵ > 10−3 W kg−1). Estimates of plume lateral spreading rates were derived from the trajectories of Lagrangian GPS surface drifters and from cross-plume hydrographic transects. Lateral spreading rates derived from the latter compared favourably with estimates derived from a control volume technique in a previous study. The lateral spreading of the plume was driven by a baroclinic pressure gradient toward the base of the plume. However, spreading rates were underestimated by the surface drifters. A convergence of near-surface flow from the barotropic pressure gradient concentrated the drifters within the plume core. The combination of enhanced internal turbulence stress and mixing at the base of the surface layer, and the presence of steep fjord sidewalls likely reduced the rate of lateral spreading relative to the theoretical spreading rate. The estimates of plume width from the observations provided evidence of scale-dependent dispersion which followed a 4/3 power law. Two theoretical models of dispersion, turbulence and shear flow dispersion, were examined to assess which was capable of representing the observed spreading. An analytical horizontal shear-flow dispersion model generated estimates of lateral dispersion that were consistent with the observed 4/3 law of dispersion. Therefore, horizontal shear dispersion appeared to be the dominant mechanism of dispersion, thus spreading, in the surface plume layer.

The interaction of buoyant coastal river plumes with mangrove vegetation and consequences for sediment deposition and erosion in a tidal environment

To identify the drivers of sediment deposition within a mangrove-lined river delta, we developed a three-dimensional numerical model in an idealized domain. Our model system is based on the Firth of Thames, a modern flat-fronted, bay-head deltaic system, in the Hauraki Gulf, in the North Island of New Zealand. The interactions of the buoyant river plume with mangroves are examined, including sediment deposition and erosion patterns. The morphological study presented here is restricted to cohesive sediments, whose patterns are observed after 4 months of morphological evolution. For the non-vegetated case, represented by a spatially uniform roughness, sediment deposited throughout the central model domain with a formation of a cuspate delta at the river mouth. However, in the vegetated case (with vegetated areas represented by an enhanced and spatially varying roughness coefficient), sediment deposition occurred in the regions of forests and flats, whereas the fringe areas experienced erosion. The sediment deposition patterns along cross-sections were qualitatively consistent with field observations from the Firth of Thames. Analysis of the across-transect and along-transect momentum balances demonstrates that the predominant balance between the bottom shear stress and baroclinic pressure gradient largely controls the sediment deposition in the riverine sections of the domain. However, close to the center of the plume, vertical advection promotes suspension of the sediments in the fringe region during the flood tide.

Initial time dependence of wind- and density-driven Lagrangian residual velocity in a tide-dominated bay

The nonlinear effect of the summer southeast wind and density on the 3D structures of the full Lagrangian residual velocity (LRV) was quantified for a generally nonlinear system, using Jiaozhou Bay (JZB), China as the test site. In the tidally energetic JZB, the basic patterns of the wind- and density-driven full LRVs were found to be consistent with semi-analytical solutions but highly dependent on initial times. The wind-driven full LRVs at different tidal phases flowed similarly downwind over shoals and upwind in the deep region; however, the main branches could migrate across nearly half of the bay. A density-driven, clockwise flow was dominant in the western inner bay at low tide, but it almost disappeared at high tide. The effect of density generally enhanced the outward flow in the surface layer and inward flow in the bottom layer. Along-trajectory integrated momentum balances indicated that viscosity was the main factor responsible for the time dependence of the wind-driven full LRVs, while viscosity, barotropic and baroclinic pressure gradients were the main drivers of the intra-tidal variations in the density-driven full LRVs. Generally, the summer wind and density had opposing effects, although their influence was weaker than that of the tide and could not change the patterns of the tide-driven full LRVs. When analysing the effects of wind and density on the coastal circulation in JZB, both the 3D structures and the possibility of a high initial tidal phase dependency should be considered.

Using Tracer Variance Decay to Quantify Variability of Salinity Mixing in the Hudson River Estuary

AbstractThe salinity structure in an estuary is controlled by time‐dependent mixing processes. However, the locations and temporal variability of where significant mixing occurs is not well‐understood. Here we utilize a tracer variance approach to demonstrate the spatial and temporal structure of salinity mixing in the Hudson River Estuary. We run a 4‐month hydrodynamic simulation of the tides, currents, and salinity that captures the spring‐neap tidal variability as well as wind‐driven and freshwater flow events. On a spring‐neap time scale, salinity variance dissipation (mixing) occurs predominantly during the transition from neap to spring tides. On a tidal time scale, 60% of the salinity variance dissipation occurs during ebb tides and 40% during flood tides. Spatially, mixing during ebbs occurs primarily where lateral bottom salinity fronts intersect the bed at the transition from the main channel to adjacent shoals. During ebbs, these lateral fronts form seaward of constrictions located at multiple locations along the estuary. During floods, mixing is generated by a shear layer elevated in the water column at the top of the mixed bottom boundary layer, where variations in the along channel density gradients locally enhance the baroclinic pressure gradient leading to stronger vertical shear and more mixing. For both ebb and flood, the mixing occurs at the location of overlap of strong vertical stratification and eddy diffusivity, not at the maximum of either of those quantities. This understanding lends a new insight to the spatial and time dependence of the estuarine salinity structure.

The role of advection and density gradients in driving the residual circulation along a macrotidal and convergent estuary with non-idealized geometry

Due to variations in channel depth, width, and lateral bottom profile, estuarine residual flows can exhibit significant variation in magnitude and transverse structure along macrotidal and convergent estuaries. This article explores the along-channel residual flow (magnitude and transverse structure), forcing mechanisms and their variations along the Gironde estuary in France. With emphasis on the role of density gradient and the advective accelerations in the along channel momentum balance, the study outlines the along-channel residual flows and forcing mechanisms over the neap-spring tidal cycle during high and low river discharge conditions. The results demonstrate that the density-driven flow contribution to total residual flows is, approximately, 75% (along-channel averaged) during neap tide and 18% during spring tide for both high and low river discharge scenarios. Owing to the complex lateral variation in the channel depth and the constriction near the mouth, advective accelerations play a major role in altering the residual flow lateral structure. However, the relative importance of advection reduces in the main body of the estuary where the channel is widened with poor lateral variation in the bottom depth. The results suggest advection and the baroclinic pressure gradient produce a laterally sheared along-channel residual flow with inflow in the channel and outflow over the shoals during neap tide. During spring tide, this lateral structure is produced due to advection. The results show that even in a homogenous system, advection can induce a flow with a structure that mimics the density-driven flow. The article shows that along macrotidal estuaries the presence of complex morphological features can affect the residual flow dynamics. As such, the residual flow in these systems should be schematized not only by considering the lateral variation of bathymetry, but also the along channel complexity.

Lateral Circulation and Associated Sediment Transport in a Convergent Estuary

AbstractThe Coupled Ocean–Atmosphere–Wave–Sediment Transport (COAWST) modeling system was used to explore lateral circulation and associated sediment transport in a convergent estuary with existence of channel and shoals. The lateral momentum balance was found to be largely geostrophic with a barotropic pressure gradient balanced by a baroclinic pressure gradient and Coriolis forcing during most time of a tidal cycle, except at the maximum ebb/flood. At the east side (right side looking landward), due to estuarine convergence, barotropic forcing strongly modulates lateral momentum balance and generates eastward residual sediment transport during spring tides. But during neap tides, the enhanced lateral density gradient induces strong lateral baroclinic forcing to cause westward residual sediment flux. At the west side, baroclinic forcing strongly influences the lateral momentum balance and generates westward sediment transport. Together, lateral circulation tends to distribute sediment from the channel to both the side shoals during spring tides but generates westward sediment flux during neap tides. The diagnostic analysis of the lateral gradient of suspended sediment concentration (SSC) demonstrates that lateral sediment trapping is strongly influenced by the lateral circulation and the lateral asymmetry in bed erosion during spring tides, whereas lateral advection becomes the controlling factor during neap tides. Mechanisms for the lateral circulation and sediment transport can be extended to other convergent estuaries.

Modelling lateral circulation during flood season at the Oujiang River Estuary, China

Lateral flow plays a critical role in estuarine dynamics. It can influence longitudinal diffusivity, creates strong vertical mixing, transports sediment laterally and modifies along-channel momentum budget. The Oujiang River Estuary is an important river estuary at eastern China coast. This study aims to investigate the structure of lateral circulation and its driving mechanism under saline environment by a FVCOM model. The patterns of lateral flow varied significantly over tidal stages and critical cross-sections, which are influenced by the topography, tidal current and salinity variations during flood season. The results showed that no evident lateral circulation existed at upper part from the mouth for most time, except that weak circulation was developed near the mouth during neap tide with small river discharge for this macrotidal estuary. By contrast, distinct lateral circulation was generally developed at lower part of the estuary over a tidal cycle. Flow regime was balanced between bottom friction and centrifugal force at upper part, while off the mouth, lateral baroclinic pressure gradient due to the tilting of isopycnals was expected to be an important mechanism of lateral circulation during highly stratified ebb tide. The results in this paper also showed the satisfactory performance of the model, which provided a reasonable estimate of the driving forces of the lateral circulation.

Variability of the Deep Overflow through the Kerama Gap Revealed by Observational Data and Global Ocean Reanalysis

Herein, the temporal variability of the deep overflow through the Kerama Gap between the East China Sea and the Philippine Sea is investigated based on observational data combined with reanalysis data obtained during 2004–2011. The observations and model results show a strong bottom-intensified flow intruding into the deep Okinawa Trough. The observed deep overflow shows intraseasonal variations that are enhanced from August to November. The variability in the deep overflow via the Kerama Gap is well-correlated with the density changes near its sill depth in the Philippine Sea. Additionally, some portion of the dense water originates from a region east of Miyakojima, which can be related to the northeastward-flowing Ryukyu Current at intermediate depths. In contrast, three extreme deep overflow events indicate that the arriving mesoscale eddies propagated from the east resulted in an increase in the density near the Kerama Gap sill than that on the Okinawa Trough side. The density difference associated with the baroclinic pressure gradient across the Kerama Gap forced the deep overflow into the Okinawa Trough. The volume transport of the deep overflow computed by integrating the cross-sectional velocity and through hydraulic theory are 0.14 and 0.11 Sv (1 Sv = 106 m3/s), respectively.

On the Formation Dynamics of the North Equatorial Undercurrent

AbstractBased on a physics-oriented modeling study, we investigate the underlying forcing processes of the North Equatorial Undercurrent (NEUC). Made up of large-scale (~90%) and mesoscale (~10%) components, the NEUC weakens eastward with a longitude-independent seasonality. The large-scale component reflects the effect of the meridional baroclinic pressure gradient force (PGF_BC). The vertical velocity shear forms the eastward NEUC, when the PGF_BC exceeds the meridional barotropic pressure gradient force (PGF_BT). The mesoscale variability with alternating jets is linked to the wind stress curl in different regions of the tropical North Pacific. Spatially, the NEUC has a northern (NEUC_N) and a southern branch (NEUC_S), which are mainly attributed to the transports from Luzon Undercurrent (LUC) and Mindanao Undercurrent (MUC), respectively. The LUC of ~3 Sv (1 Sv ≡ 106 m3 s−1) feeds the NEUC_N in summer, while the MUC of ~4 Sv fuels the NEUC_S in autumn and the two branches do not coexist. The total NEUC transport peaks in August/September, and there exist three distinct periods in a 1-yr cycle: the non-NEUC period in winter, the LUC-driven period in summer, and the MUC-driven period in autumn. Based on the layer-integrated vorticity equation, we diagnose quantitatively that the variation of the NEUC is dominated by the lateral planetary vorticity influx from the LUC and the MUC. These external influxes interact with the internal dynamics of pressure torques and stress curls in the NEUC layer, to jointly govern the NEUC and its variability. Meanwhile, the nonlinearity due to relative vorticity advection near the coast modulates the strength of the NEUC.

The hydrodynamics of the Bons Sinais Estuary: The value of simple hydrodynamic tidal models in understanding circulation in estuaries of central Mozambique

The Bons Sinais Estuary, located in the central Mozambique, with the highest tidal range in the Western Indian Ocean, is shallow (∼10m), long (∼28km) and receives freshwater only during the wet season. These characteristics give the Estuary distinctive hydrodynamics and ecological features. Current measurements and longitudinal CTD profiling were carried out during dry season, July 2011 and November 2012, and wet season, February 2012 and March 2013. The Estuary is partially to well-mixed most of the time, with Simpson Number (Si) ≤0.5. One-D, depth and width integrated, tidal hydrodynamic model, forced by the modelled tides at the mouth, was applied to understand the major mechanisms governing the estuarine dynamics. Barotropic forcing dominated the baroclinic pressure gradient forcing and the currents were found to be mostly tidal driven. Accordingly, the hydrodynamic model described the observed current velocities well in the dry season (R2≈0.95) and to a lesser degree in the wet season (R2≈0.80). The remaining 5%–20% may be attributed to other hydrodynamic forcing that were not captured in the tidal model, requiring further investigation at higher temporal and spatial scales to fully characterize the system dynamics. These may include the identified baroclinic forcing from freshwater inputs and the influence of the complex geometry at the study site. The initial hydrodynamic field study and simple modelling approach that was applied may inform studies of higher-order processes and management tools at the Bons Sinais Estuary, and similar systems (e.g. sediment dynamics and accommodation space, and diagnostic models), and the result can contribute to understanding the drivers of longer-term ecological and geomorphological dynamics of the estuary.

Modeling the westward transversal current in the southern Yellow Sea entrance: a case study in winter 2007

The westward transversal current (TC) in the southern Yellow Sea entrance was investigated during winter 2007 using a numerical ocean model. The three-dimensional structures and dynamics of the westward TC were highlighted. The model-simulated monthly mean current fields showed that the strong westward TC was limited to the upper water column (shallower than 50 m) and extended westward from 126° E to 124° E. Momentum balance analysis indicated that the westward TC was mainly regulated by a quasi-geostrophic balance. Both the sea surface elevation–related barotropic pressure gradient force and the density-related baroclinic pressure gradient force controlled the intensity and direction of the westward TC. Sensitivity model experiments focusing on tide and wind demonstrated that the westward TC was intensified when tide was ignored and was greatly weakened when wind was excluded. Lagrangian particle tracking experiments were also performed, to investigate the relationship between the Yellow Sea Warm Current and the westward TC in the frontal region. The characteristics of the water in the Yellow Sea Warm Current were affected by the properties of the water in the frontal zone, especially in the sub-surface and lower layers. The westward TC might act as an important bridge, connecting the frontal zone with the Yellow Sea Warm Current flowing along the western Yellow Sea trough.

Channel curvature effects on estuarine circulation in a highly stratified tropical estuary: The São Francisco river estuary (Brazil)

The effects of the channel curvature on estuarine circulation were assessed in the São Francisco river estuary. This estuary is a deltaic front system, with a highly stratified salinity distribution. The assessment was based on field experiments where currents and salinity and temperature were monitored during a semi-diurnal tidal cycle in two cross-sections with distinct curvatures (radii of 2 and 8 km). The least-squares method was used to determine the mean velocity (residual velocity), and the tidal velocity amplitude and phase for the M2 harmonic. The results for the M2 harmonic were compared to an analytical solution that considers a simplified momentum balance between barotropic pressure gradient and bottom friction. The model skill for these two results was 0.44 and 0.63 for sections #A and #B, respectively. The differences between these results indicate that there are other dynamically influential forces for the longitudinal momentum balance besides those included in the analytical solution. The baroclinic pressure gradient presented values two orders of magnitude greater than the bottom friction, demonstrating the dynamic importance of this force. Although centrifugal acceleration is a component of the lateral momentum balance, it also plays a role in the distribution of longitudinal currents in the cross-section, displacing the maximum currents towards the exit of the curve. In addition to the channel curvature effect, the influence of the bathymetry was also observed on the tidal amplitude in the cross-sections and on the depth-averaged residual longitudinal velocities along the sections, with the highest average speed in the shallowest areas of each section.

Hydrodynamics and suspended particulate matter retention in macrotidal estuaries located in Amazonia-semiarid interface (Northeastern-Brazil)

The aim of the current study was to determine the nature of the seasonal variability of the Suspended Particulate Matter (SPM) fluxes from the drainage basin to the estuary in a macrotidal region (Northeastern Brazil), and the estuarine response to a seawater intrusion regarding sediment deposition, which will support the understanding of the global transport of materials at the continent-ocean interface. Thermohaline structure data was acquired using a Conductivity, Temperature, and Depth (CTD) probe with a sampling frequency of 4 Hz. Suspended particulate material was measured by gravimetric measurements applied to exact filtered volume samples. The outflows were measured through the use of an Acoustic Doppler Current Profiler (ADCP) with frequency of 1.5 MHz. The horizontal thermal and saline gradients varied from warmer and less saline waters (2014) to cooler and saline waters (2015). The gradient behavior when linked to volume transport and SPM flows, suggests a minimization of the fluvial flows in 2015, easing the advance of coastal water (CW) towards the inner estuary, leading to an inversion of the baroclinic pressure gradient. The bottom saline front, generated by the entrance of coastal water masses, caused an increase in SPM concentrations due to increased fluid density, resuspension of previously deposited sediment, and erosion of banks. High concentrations of SPM indicate higher volume transport suggesting a hydraulic barrier due to the change/inversion of the baroclinic pressure gradient, resulting in water and material retention. Material deposition was observed during neap tide, while during spring tide the material is resuspended, increasing the concentration, generating cycles of deposition and erosion during the neap-spring tides. The sediment in suspension that reach the estuary, even with low fluvial volume, stay in this environment forming new islands because of deposition. High deposition rates or sediment cycling, if generated by the hydraulic barrier, may indicate that the flows of SPM from the continental drainage to the estuary and adjacent continental shelf are interrupted and the residence time is increased.

Intratidal and Subtidal Circulation in a Tropical Estuary during Wet Season: The Maroni, French Guiana

Observations of water level, current velocity, river discharge, wind and salinity were collected in the Maroni estuary, on the border of French Guiana and Suriname during the wet season of 2018 to explore subtidal circulation patterns. Measurements are complimented by the application of analytical models with an aim to diagnose forcing mechanisms responsible for producing subtidal flows during the day of data collection and to extrapolate these findings to other time periods with variable wind and river forcing. Subtidal along-channel flows were found to be dominated by river discharge, with seaward directed velocities found throughout the channel section reaching 40 cm s − 1 . This pattern was altered with strong southwesterly winds, which produced and inverse gravitational circulation pattern despite the elevated river discharge. Secondary, or cross-channel flows, displayed a three-layer vertical structure in the main channel due to a combination of channel curvature and tidal asymmetry in the lateral baroclinic pressure gradient. The pressure gradient was produced by a salinity intrusion front that only manifested in the channel during flood tide. This is the first comprehensive study of tidal and subtidal flow dynamics in the Maroni estuary.

Study of Lateral Flow in a Stratified Tidal Channel‐Shoal System: The Importance of Intratidal Salinity Variation

AbstractLateral flow significantly contributes to the near‐bottom mass transport of salinity in a channel‐shoal system. In this study, an integrated tripod system was deployed in the transition zone of a channel‐shoal system of the Changjiang Estuary (CE), China, to observe the near‐bottom physics with high temporal/spatial resolution, particularly focusing on the lateral‐flow‐induced mass transport. These in situ observations revealed a small‐scale salinity fluctuation around low water slack during moderate and spring tidal conditions. A simultaneous strong lateral current was also observed, which was responsible for this small‐scale fluctuation. A high‐resolution unstructured‐grid Finite‐Volume Community Ocean Model has been applied for the CE to better understand the mechanism of this lateral flow and its impact on salinity transport. The model results indicate that a significant southward near‐bed shoal‐to‐channel current is generated by the salinity‐driven baroclinic pressure gradient. This lateral current affects the salinity transport pattern and the residual current in the cross‐channel direction. Cross‐channel residual current shows a two‐layer structure in the vertical, especially in the intermediate tide when the lateral flow notably occurred. Both observation and model results indicate that near‐bottom residual transport of water moved consistently southward (shoal to channel). Mechanisms for this intratidal salinity variation and its implications can be extended to other estuaries with similar channel‐shoal features.