Up-estuary Winds Research Articles

Responses of hydrodynamic circulation to the climatic change of wind forcing in a subtropical estuary

Under the background of climate change, long-term trends in atmospheric-marine processes have been observed over the past decades. Being a subtropical estuary influenced by East Asian monsoon wind, the Pearl River Estuary (PRE) is chosen as a prototype to investigate the response of estuarine hydrodynamics to the local climatic regime shift in wind forcing. Analyses from ERA5 wind data and local meteorological observations have revealed that summer wind over PRE (prevailed by the up-estuary wind) was decreasing in magnitude and shifting anticlockwise from southwesterly to southeasterly over the past 42 years (1979–2020). Using a fully calibrated three-dimensional numerical model, the hydrodynamic circulation of PRE were simulated for 42 years with realistic daily wind forcing and repeated annual cycles for other forcing fields. Both the least-squares fitting and the EOF analysis demonstrate that the summertime longitudinal flow was strengthened while the lateral circulation was weakened over the past 42 years. Comparative analyses of the results obtained from the process-oriented numerical experiments revealed that the decreasing wind speed is the dominant factor for the strengthening of the longitudinal circulation, while the shifting wind direction is the dominant factor for the weakening of the lateral circulation. Using calculated long-term trends in wind forcing, wind conditions are predicted for 2100. The model results indicate that by 2100, there will be a further strengthening of the longitudinal circulation and a further weakening of the lateral circulation during the summer. The momentum budget reveals that the baroclinic pressure gradient and the diffusion term are the dominant terms modulating the changes of hydrodynamic circulation. The water mass in the PRE will become saltier, and the exchange between the deep channel and shallow shoals of the estuary will be slowed down in the future. The impact of these changes on brackish water habitats and local ecosystems needs further study.

Dynamic evolution of a secondary turbidity maximum under various forcing conditions in a microtidal estuary

Axial survey and in situ mooring in microtidal Masan Bay were conducted to reveal the mechanism for movement of secondary turbidity maximum (STM) and sediment resuspension. The Bay is characterized by sluggish water circulation with a vertically well-mixed water column. The strength of the residual estuarine current (Δu¯) was enhanced by the combined effects of decreased tidal currents, increased precipitation, and down-estuary (northerly) winds, whereas Δu¯ was weakened by increased tidal currents and up-estuary (southerly) winds. The variability of current asymmetry originated from Δu¯ contributed to the entrapment of suspended sediments, creating a mobile sediment pool in central regions of the Bay. The sediments resuspended from the mobile sediment pool were more influenced by residual currents than the tidal currents, and they remained in suspension near bed. When the down-estuary winds were applied to the mobile sediment pool, the near-bed sediments were readily resuspended to form an STM > 40 mg l−1. The STM moved up-estuary by persistent high Δu¯ and down-estuary by an intermittent low Δu¯. Over the entire measurement periods, the STM moved by at least 450 m toward the up-estuary. As the consolidated sediment layer was exposed by the movement of the STM, near-bed currents were ineffective in resuspending sediments from the consolidated layer even though the winds provided sufficient current strength. Due to the depletion of available sediments for resuspension, the STM was not fully generated. This study highlights that the STM in a microtidal estuary can be determined by the combined function of various forcings such as tide, wind, and precipitation.

A local eddy viscosity parameterization for wind-driven estuarine exchange flow. Part I: Stratification dependence

A new parameterization for the estuarine turbulent eddy viscosity coefficient is developed considering the influence of wind forcing and feedback between stratification and shear. The emerging tidally averaged eddy viscosity profile AvT is parameterized as parabolic under well-mixed conditions, and is composed of a skewed-Gaussian-like form for the upper layer, and a parabolic form for the bottom layer under stratified conditions. The precise shape of the profiles depends parametrically on the bottom boundary layer thickness, the bulk Richardson number, and the Wedderburn number. The parameterized AvT profiles show excellent agreement with profiles obtained from numerical models. To explore the importance of vertically varying AvT with regard to exchange processes, an analytical model is designed. This one-dimensional model is based on a balance between frictional forces and pressure gradient. The resulting exchange flow is analyzed over the relevant parameter space that is associated with horizontal and vertical stratification through the bulk Richardson number, and the bi-directional wind stress via the Wedderburn number. Down-estuary wind enhances the up-estuary flow near the bottom and down-estuary flow near the surface driving an exchange flow pattern typically associated with gravitational circulation. Up-estuary wind results in either a two-layer inverted circulation opposing the gravitational circulation, or a three-layer flow that is up-estuarine at the surface with classical two-layer circulation underneath. Three-layer flow emerges with a weak wind. With increasing runoff velocity, three-layer flow transitions to a single layer flow under weak stratification conditions.

The Relative Importance of Wind Straining and Gravitational Forcing in Driving Exchange Flows in Tidally Energetic Estuaries

AbstractIn straight tidal estuaries, residual overturning circulation results mainly from a competition between gravitational forcing, wind forcing, and friction. To systematically investigate this for tidally energetic estuaries, the dynamics of estuarine cross sections is analyzed in terms of the relation between gravitational forcing, wind stress, and the strength of estuarine circulation. A system-dependent basic Wedderburn number is defined as the ratio between wind forcing and opposing gravitational forcing at which the estuarine circulation changes sign. An analytical steady-state solution for gravitationally and wind-driven exchange flow is constructed, where tidal mixing is parameterized by parabolic eddy viscosity. For this simple but fundamental situation, is calculated, meaning that the up-estuary wind forcing needs to be 15% of the gravitational forcing to invert estuarine circulation. In three steps, relevant physical processes are added to this basic state: (i) tidal dynamics are resolved by a prescribed semidiurnal tide, leading to caused by tidal straining; (ii) lateral circulation is added by introducing cross-channel bathymetry, smoothly increasing from 0.47 (flat bed) to 1.3 (parabolic bed) due to an increasing effect of lateral circulation on estuarine circulation; and (iii) full dynamics of a real tidally energetic inlet with highly variable forcing, where results from a two-dimensional linear regression.

Ocean-estuary exchange variability in a large tropical estuary

Ocean-estuary exchange variability was studied from 2008 to 2014 at a low latitude coastal plain estuary, Todos os Santos Bay, Brazil, near 13°S. Wind data from Climate Forecast System Reanalysis (CFSR) were used to complement the results of a high-resolution regional ocean model, ROMS. Volume exchange occurred under two distinct circulation patterns, namely Circ+ and Circ−, that matched the down-estuary (spring/summer) and up-estuary (autumn/winter) wind regimes respectively. Circ+ promoted water-column stratification, while Circ− lasted longer, induced more volume exchange, and reversed the estuarine circulation. In addition, a clockwise lateral circulation (looking into the estuary) occurred during Circ+, whereas a counterclockwise lateral circulation was observed during Circ−. Connectivity was observed between Jaguaripe River and Todos os Santos Bay entrance. Increases in Jaguaripe River discharge, in conjunction with up-estuary winds, promoted a northward advection of the river plume toward the main entrance of the estuary, creating a lateral density gradient over Todos os Santos Bay' western shoal. Our results demonstrate that findings about estuarine circulation at higher latitudes are applicable to low latitude estuaries when dynamical depth and/or basin's width indicate that Earth's rotation is relevant.

Impact of tides and winds on estuarine circulation in the Pearl River Estuary

The Finite-Volume Coastal Ocean Model (FVCOM) is used to simulate the dynamics of the circulation in the Pearl River Estuary (PRE), validated against the in-situ measurements of a cruise survey. The model simulation agrees well with observed sea level at local tide gauge stations and reproduces observed characteristics of the currents. A simplified model run without wind forcing reveals that there is a significant spring-neap variation in the along-estuary circulation. Additional sensitivity runs that include up-estuary and/or down-estuary winds are also implemented. These results reveal that the cross-estuary adjustment of stratification and mixing induced by along-estuary winds is stronger than the along-estuary wind straining effect; these yields enhanced stratification and weakened mixing under the up-estuary wind. During spring tide, the response of the along-estuary circulation to the along-estuary wind forcing is weak, while during neap tide, the up-estuary and down-estuary winds may induce substantial variations in the along-estuary circulation. The analysis on the momentum budget and turbulent stress reveals the different dynamic response of the estuarine circulation in the PRE to different wind forcing conditions, identifying a significant modification to estuarine circulation by the wind-tide interaction.

Observations of Wind Forcing Effects on Estuary Length and Salinity Flux in a River-Dominated, Microtidal Estuary, Mobile Bay, Alabama

AbstractUsing long-term records (~11 years) of salinity and 390 days of ADCP data, aspects of the estuary length and salinity flux were evaluated in Mobile Bay under a range of river discharge, tidal, and wind conditions. The temporal variability in the salinity structure was represented by the estuary length and showed a relationship of with respect to river forcing, similar to values reported in San Francisco Bay and Delaware Bay. Local wind forcing was observed to play a role in modifying this relationship, in which estuary length responded asymmetrically to along-channel winds with up-estuary winds, reducing the estuary length. To further explore potential salinity transport changes associated with the wind, a 1D salinity flux was calculated using the ADCP and salinity profile data. River discharge was the main forcing condition driving seasonal changes in salinity flux. At shorter time scales, the wind became a dominant forcing condition and drove large changes in the salinity flux during low-discharge periods. At all discharge levels, down-estuary wind conditions enhanced the shear and subtidal exchange. During up-estuary wind conditions, the two-layer flow was inhibited and reduced the exchange. These results indicate that in a shallow microtidal system, wind can play a large role in modifying the estuary length on scales comparable to the spring–neap changes observed in other systems.

Wind effects on the spring phytoplankton dynamics in the middle reach of the Chesapeake Bay

The spring phytoplankton bloom, bridging nutrient loading and summer hypoxia in the Chesapeake Bay especially in its middle reach, is highly sensitive to physical forcing. In addition to riverine input, winds could be a noticeable driver of estuarine phytoplankton variability, affecting its water quality and ecosystem health. In this study, wind impacts on the spring bloom were investigated utilizing the 29-year (1985–2013) monitoring data and an existing FVCOM-ICM estuarine water quality model. The retrospective analysis revealed that winds exerted a significant control on the spring algal biomass in the middle bay, which could be as important as the effects of riverine inputs. According to the model simulation, the mid-bay spring algal bloom was mainly fueled by the riverine nutrients, and its biomass was higher on the western than that on the eastern flank. Numerical experiments indicated a negative relationship between stratification and the overall algal biomass. It was found that up-estuary (southerly) winds could induce upwelling, thus enhance primary production along the western shore, and increase phytoplankton transport to deeper waters, making up-estuary winds most effective in the mid-bay transverse phytoplankton enrichment. Given that the regional wind pattern is tightly coupled with the large-scale climatic variability, it is recommended to fully consider climatic effects on the lower food web in ecosystem management in the future.

Baroclinic Effects on Wind-Driven Lateral Circulation in Chesapeake Bay

AbstractThe 2-month-long mooring data were collected in a straight midsection of Chesapeake Bay to document the lateral circulation driven by along-channel winds. Under upestuary winds, the lateral circulation featured a clockwise (looking into estuary) circulation in the surface layer, with lateral Ekman forcing as the dominant generation mechanism. Under downestuary winds, however, the lateral circulation displayed a structure dependent on the Wedderburn number W: a counterclockwise circulation at small W and two counterrotating vortices at large W. The surface lateral velocity was phase locked to the along-channel wind speed. Analysis of the streamwise vorticity equation showed that the strength and structure of the lateral circulation in this stratified estuary were largely determined by the competition between the tilting of planetary vorticity by along-channel currents and lateral baroclinic forcing due to sloping isopycnals. Under strong, downestuary winds, the lateral baroclinic forcing offset or reversed the tilting of planetary vorticity on the western half of the estuarine channel, resulting in two counterrotating lateral circulation cells. A bottom lateral flow was observed in the deep channel and appeared to be generated by lateral Ekman forcing on the along-channel currents.

Observed flow variability along the thalweg, and on the coastal slopes of the Gulf of Finland, Baltic Sea

Bottom-mounted ADCP measurements from 10 installations, collected between 2009 and 2014 and each lasting several months, are analysed in order to distinguish between different flow regimes, and to detect variability (a) along the thalweg of the elongated basin, with different regimes in summer and in winter, and (b) on the coastal slopes. In the deep thalweg area the mean flow speed amounts to 6–13 cm s−1, whereas the maximum speeds appear in winter near the bottom of the basin, and in summer within the halocline (around 70 m depth). The mean zonal flow component reveals a nearly depth uniform inflow during winter, and a layered inflow-outflow during summer. In years where up-estuary (W to SW) winds are stronger during the summer, inflow dominates in upper layers, and anti-estuarine outflow dominates in deeper layers. This causes the export of a salt wedge, and the weakening of haline stratification. Infra-low frequency zonal currents (i.e. excluding topographic waves etc. with periods of less than 10 days) have a structure which is uniform with depth for 53% of the time in winter; in summer, a layered structure is present 65% of the time. However, during both periods the reversed estuarine flow (inflow in upper layers and outflow in the bottom layer) appears, on average, for 30% of the time. The deep flow zonal component is well correlated with westward winds during summer (r = 0.84), and south-westward winds during winter (r = 0.77). On the coastal slopes, the speed of the currents are lower than in the thalweg region, and they decay with depth. In the vertical the flow exhibits a layered structure in both the winter and summer seasons.

Wind-driven residual circulation and related oxygen and nutrient dynamics in the Gulf of Finland (Baltic Sea) in winter

Establishment of distinct circulation patterns in the Gulf of Finland was observed by a targeted measurement campaign in winter 2013–2014. Strong and long enough up-estuary wind events caused a collapse of vertical stratification and development of a barotropic flow system consisting of an outflow in the open part and inflow along the coasts. In the periods without such unidirectional wind forcing, but when the water column remained weakly stratified, the residual barotropic inflow in the open gulf and outflow along the coasts was observed. In the case of moderate wind forcing, the three-layer vertical stratification and flow structure developed in the gulf. It is shown that the along-gulf expansion of the fresher water tongue in the surface layer as well as the up-estuary penetration of the saltwater wedge in the near-bottom layer followed well the long-term (monthly) changes in the cumulative along-gulf wind stress. The dynamics of the near-bottom saltwater wedge determined the extent of hypoxic bottoms and, as suggested by the firm correlation between the near-bottom phosphate concentration and salinity, the nutrient conditions in the near-bottom layer. The lateral transport of phosphorus, strengthened vertical stratification in the Northern Baltic Proper and suggested increase in the frequency of stratification collapses in the Gulf of Finland in winter predict that the eutrophication effects would not diminish significantly in this estuary in the nearest future.

Dynamics of wind-driven estuarine-shelf exchange in the Narragansett Bay estuary

Physical exchange between estuarine and continental shelf waters impacts flushing dynamics of the estuary and determines rates of ocean inputs of nutrients and plankton. To investigate the occurrence and propagation of shelf water intrusions into the Narragansett Bay estuary, we collected velocity data near the estuarine–shelf interface during three summer periods. These data were compared to environmental forcing factors, including wind velocity, tidal mixing and river discharge. Results suggest a background cyclonic flow within the two passages of the estuary based on mean inflow in the channel on the eastern side of the estuary and mean outflow on the western shoals. Within the lower estuary this background circulation was perturbed by channel-parallel winds. On the shelf outside of the estuary, winds parallel to the coast were associated with cross-shelf flow of deep water. Strong pulses in estuarine–shelf exchange flow fell into two wind-driven modes: responses to direct forcing by down-estuary wind or rebounds following relaxation or reversal of up-estuary wind. Rebound events were common, providing the most dramatic perturbations to the mean background estuarine circulation. A reduction in exchange between Narragansett Bay and shelf waters during prevailing up-estuary winds in the summertime and short-lived pulses in exchange flow under wind reversal events are expected to affect nutrient fluxes and dynamics of hypoxia in the estuary.

A Budget Analysis of Bottom-Water Dissolved Oxygen in Chesapeake Bay

A coupled hydrodynamic-dissolved oxygen model is developed to simulate the seasonal cycle of dissolved oxygen (DO) in Chesapeake Bay and investigate processes regulating summer hypoxia in the estuary. A budget analysis of DO in the bottom water reveals a balance between physical transport and biological consumption. In addition to the vertical diffusive flux, the longitudinal and vertical advective fluxes are important suppliers of DO to the bottom water. The longitudinal advective flux is affected not only by gravitational circulation but also by wind-driven currents. The vertical advective flux is affected by wind-driven lateral circulation and shows a strong dependence on wind speed and direction. Up-estuary winds weaken the landward bottom flow and generate a clockwise lateral circulation that exchanges DO between the deep channel and adjacent shoals, thereby reducing the longitudinal advective flux and increasing the vertical advective flux. In contrast, down-estuary winds amplify the longitudinal flux and reduce the vertical flux. During the summer, water column respiration contributes to about 74 % of the total biological consumption and sediment oxygen demand accounts for about 26 %. Sensitivity analysis model runs are conducted to analyze how changing river flows and winds affect the hypoxia prediction and oxygen budget balance. Due to the compensating changes in longitudinal and vertical fluxes, the hypoxic volume is relatively insensitive to changes in the river flow. In contrast, the timing and size of hypoxic volume changes with wind speed. Sensitivity analysis also shows that plankton oxygen production, water column respiration, and sediment oxygen demand all affect the hypoxia prediction and bottom oxygen balance.

Daily variation in ingress of fall-spawned larval fishes into Delaware Bay in relation to alongshore and along-estuary wind components

Identifying factors that affect ingress of larval fishes from offshore spawning areas into estuarine nurseries is important to improve understanding of variability in recruitment of many coastal marine species. This study investigated the ingress of larval Atlantic croaker (Micropogonias undulatus), Atlantic menhaden (Brevoortia tyrannus), and summer flounder (Paralichthys dentatus) at Roosevelt Inlet, near the mouth of Delaware Bay, USA in relation to short-term wind events. Nightly abundances, from November 15 to December 15, 2010, were analyzed with alongshore and along-estuary wind components (direction and speed) using cross-correlation analysis to determine if winds affect larval ingress. Ingress of Atlantic croaker and summer flounder correlated with along-estuary winds, whereas Atlantic menhaden showed no significant correlations with either alongshore or along-estuary winds. Although along-estuary winds during this period were predominantly down-estuary, Atlantic croaker ingress was correlated with positive along-estuary winds (blowing up-estuary), with a three-day lag; and a particularly large ingress peak occurred following the largest up-estuary wind peak. Ingress of summer flounder was correlated with negative along-estuary winds (blowing down-estuary), with a two-day lag. These results suggest that species-specific vertical position in the water column influenced ingress into Delaware Bay. The lag results also suggest that ingressing Atlantic croaker and summer flounder may have a pooling stage outside the mouth of Delaware Bay.

Utilising monitoring and modelling of estuarine environments to investigate catchment conditions responsible for stratification events in a typically well-mixed urbanised estuary

Estuarine health is affected by contamination from stormwater, particularly in highly-urbanised environments. For systems where catchment monitoring is insufficient, novel techniques must be employed to determine the impact of urban runoff on receiving water bodies. In the present work, estuarine monitoring and modelling were successfully employed to determine stormwater runoff volumes and establish an appropriate rainfall/runoff relationship capable of replicating fresh-water discharge due to the full range of precipitation conditions in the Sydney Estuary, Australia. Using estuary response to determine relationships between catchment rainfall and runoff is a widely applicable method and may be of assistance in the study of waterways where monitoring fluvial discharges is not practical or is beyond the capacity of management authorities. For the Sydney Estuary, the SCS-CN method replicated rainfall/runoff and was applied in numerical modelling experiments investigating the hydrodynamic characteristics affecting stratification and estuary recovery following high precipitation. Numerical modelling showed stratification in the Sydney Estuary was dominated by fresh-water discharge. Spring tides and up-estuary winds contributed to mixing and neap tides and down-estuary winds enhanced stratification.

Effects of winds on stratification and circulation in a partially mixed estuary

[1] Numerical experiments are conducted to investigate how axial winds affect stratification and circulation in the partially mixed estuary of Chesapeake Bay. In the absence of rotational effects, stratification in the estuary decreases following both down-estuary and up-estuary winds, but stratification experiences larger reduction and takes longer to recover under up-estuary winds. In the presence of rotational effects, wind-driven lateral circulations cause the lateral straining of density field and weaken the shear in the along-channel flows. Under the down-estuary winds, a counterclockwise lateral circulation steepens isopycnals in the cross-channel sections, while the Coriolis force acting on it decelerates the downwind current in the surface layer and the upwind-directed current in the bottom layer. Under the up-estuary winds, a clockwise lateral circulation flattens isopycnals in the cross-channel sections and reduces the shear between the surface and bottom currents. Hence, in the presence of rotational effects, the lateral straining offsets the effects of longitudinal straining such that the asymmetry in stratification reduction is significantly reduced between the down-estuary and up-estuary winds. Regime diagrams based on Wedderburn (W) and Kelvin (Ke) numbers are constructed to summarize the net effects of winds on estuarine stratification during both wind perturbation and postwind adjustment periods.

Response of a Large Stratified Estuary to Wind Events: Observations, Simulations, and Theory for Long Island Sound

Abstract The response to wind events in the Long Island Sound (LIS), a large macrotidal estuary influenced by rotation and stratification, is studied using long-term ferry-based current observations near the mouth, unstratified and stratified numerical simulations forced with along-estuary winds, and analytic solutions based on linear barotropic theory. The observed wind-event velocity anomalies for down-estuary winds have surface-intensified downwind flows flanking a deeper central upwind flow. Response to up-estuary wind events has a weaker magnitude and a broader and thicker downwind flow. The downwind and upwind flows are more laterally aligned than vertically layered, as determined by a newly defined dimensionless lateral alignment index. Simulation results and analytic solutions share the gross spatial patterns of the observed response, though statistical measures indicate weak agreement. Along-estuary variations in the simulation results and analytic solutions follow similar trends and are strongly influenced by variations of the bathymetric cross section. Wind-event anomalies in the section-averaged dynamics are dominated by the along-estuary pressure gradient opposing wind stress. In the stratified simulation, wind-driven density advection, isopycnal straining, and stirring modify stratification, eddy viscosities, and baroclinic pressure gradients. The wind-event response of the baroclinic pressure gradient is 15% of the barotropic gradient but is dynamically linked to response differences to up-estuary and down-estuary winds. The wind-event response asymmetries near the mouth are in qualitative agreement with observations and are opposite to asymmetries closer to the head.

The Effects of Wind, Runoff and Tides on Salinity in a Strongly Tidal Sub-estuary

Measurements over an annual cycle of longitudinal and vertical salinity distributions in a small sub-estuary, the Tavy Estuary, UK, are used to illustrate the dependence of salt intrusion and stratification on environmental variables. The interpretations are aided by vertical profiling and near-bed data recorded in the main channel and on the mudflats. Generally, high water (HW) salt intrusion at the bed is close to the tidal limit and is dominated by runoff and winds, with decreasing salt intrusion associated with increasing runoff and increasing up-estuary winds (or vice versa). Tidal effects are not statistically significant because of two compensating processes: the long tidal excursion, which is comparable with the sub-estuary length for all but the smallest neap tides, and the enhanced, near-bed, buoyancy-driven salt transport that occurs at small neap tides close to the limit of saline intrusion. The effect of wind on HW surface salt intrusion in the main channel is not statistically significant, partly because it is obscured by the opposing local and estuary-wide effects of an up-estuary or down-estuary wind stress. These processes are investigated using a simple tidal model that incorporates lateral, channel–mudflat bathymetry and reproduces, approximately, observed channel and mudflat velocities. Surface salinity at HW increases with tidal range because of enhanced spring-tide vertical mixing—a process that also reduces salinity stratification. Stratification increases with runoff because of increased buoyancy inputs and decreases with up-estuary winds because of reduced near-bed salt intrusion. Stratification and plume formation are interpreted in terms of the bulk and estuarine Richardson Numbers, and processes at the confluence of the sub-estuary and main estuary are described.

Axial Wind Effects on Stratification and Longitudinal Salt Transport in an Idealized, Partially Mixed Estuary*

Abstract A 3D hydrodynamic model [Regional Ocean Model System (ROMS)] is used to investigate how axial wind influences stratification and to explore the associated longitudinal salt transport in partially mixed estuaries. The model is configured to represent a straight estuarine channel connecting to a shelf sea. The results confirm that wind straining of the along-channel salinity gradient exerts an important control on stratification. Two governing parameters are identified: the Wedderburn number (W) defined as the ratio of wind stress to axial baroclinic pressure gradient force, and the ratio of an entrainment depth to water depth (hs/H). Here W controls the effectiveness of wind straining, which promotes increases (decreases) in stratification during down-estuary (up-estuary) wind. The ratio hs/H determines the portion of the water column affected by direct wind mixing. While stratification is always reduced by up-estuary wind, stratification shows an increase-then-decrease transition when down-estuary wind stress increases. Such transition is a result of the competition between wind straining and direct wind mixing. A horizontal Richardson number modified to include wind straining/mixing is shown to reasonably represent the transition, and a regime diagram is proposed to classify the wind’s role on stratification. Mechanisms driving salt flux during axial wind events are also explored. At the onset and end of the wind events, barotropic adjustment drives strong transient salt fluxes. Net salt flux is controlled by the responses of subtidal shear dispersion to wind forcing. Moderate down-estuary winds enhance subtidal shear dispersion, whereas up-estuary winds always reduce it. Supporting observations from upper Chesapeake Bay are presented.

The influence of wind on the water age in the tidal Rappahannock River

Wind plays an important role in regulating mixing/stratification, estuarine circulation, and transport timescale in estuaries. A three-dimensional model was used to investigate the effect of wind on transport time by using the concept of water age (WA) in the tidal Rappahannock River, a western tributary of the Chesapeake Bay, USA. The model was calibrated for water level, current, and salinity. A series of experiments regarding the effects of wind on WA was conducted under various dynamic conditions. The effect of wind on transport timescale depends strongly on the competition between the wind and buoyancy forcings, and on the pre-status of the circulation. A down-estuary wind generally decreases WA along the estuary. An up-estuary wind increases WA substantially because it changes the vertical mixing and estuarine circulation more significantly. When the buoyancy forcing increases, the up-estuary wind effect decreases whereas the down-estuary wind effect increases. A 2-day period wind pulse with a maximum speed of 15 m s −1 can alter WA for 3 days; but the wind influence on WA lasts up to 40 days in the simulation. Both local and non-local wind forcings alter WA distribution. The local wind enhances vertical mixing and changes the gravitational circulation in the downstream portion of the estuary whereas it enhances transport in the freshwater portion of the estuary. Consequently, the local wind has a significant impact on WA distribution. In contrast, the non-local wind does not change the gravitational circulation significantly by imposing setup (setdown) of water level at the open boundary, resulting in a lesser impact on WA distribution.