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Estuarine and coastal geomorphology, biogeochemistry, water quality, and coastal food webs in river-dominated shelves of the Gulf of Mexico (GoM) are modulated by transport processes associated with river inputs, winds, waves, tides, and deep-ocean/continental shelf interactions. For instance, transport processes control the fate of river-borne sediments, which in turn affect coastal land loss. Similarly, transport of freshwater, nutrients, and carbon control the dynamics of eutrophication, hypoxia, harmful algal blooms, and coastal acidification. Further, freshwater inflow transports pesticides, herbicides, heavy metals, and oil into receiving estuaries and coastal systems. Lastly, transport processes along the continuum from the rivers and estuaries to coastal and shelf areas and adjacent open ocean (abbreviated herein as “river-estuary-shelf-ocean”) regulate the movements of organisms, including the spatial distributions of individuals and the exchange of genetic information between distinct subpopulations. The Gulf of Mexico Research Initiative (GoMRI) provided unprecedented opportunities to study transport processes along the river-estuary-shelf-ocean continuum in the GoM. The understanding of transport at multiple spatial and temporal scales in this topographically and dynamically complex marginal sea was improved, allowing for more accurate forecasting of the fate of oil and other constituents. For this review, we focus on five specific transport themes: (i) wetland, estuary, and shelf exchanges; (ii) river-estuary coupling; (iii) nearshore and inlet processes; (iv) open ocean transport processes; and (v) river-induced fronts and cross-basin transport. We then discuss the relevancy of GoMRI findings on the transport processes for ecological connectivity and oil transport and fate. We also examine the implications of new findings for informing the response to future oil spills, and the management of coastal resources and ecosystems. Lastly, we summarize the research gaps identified in the many studies and offer recommendations for continuing the momentum of the research provided by the GoMRI effort. A number of uncertainties were identified that occurred in multiple settings. These include the quantification of sediment, carbon, dissolved gasses and nutrient fluxes during storms, consistent specification of the various external forcings used in analyses, methods for smooth integration of multiscale advection mechanisms across different flow regimes, dynamic coupling of the atmosphere with sub-mesoscale and mesoscale phenomena, and methods for simulating finer-scale dynamics over long time periods. Addressing these uncertainties would allow the scientific community to be better prepared to predict the fate of hydrocarbons and their impacts to the coastal ocean, rivers, and marshes in the event of another spill in the GoM.

Time series of current velocity profiles were obtained at the mouth and head of a Brazilian subtropical estuary, the Ararangua, to study its dry-period response to fortnightly tidal forcing, river pulses and wind forcing. The Ararangua exemplifies a series of river-dominated estuaries along southern Brazil with widths 30 m3/s but 200 m3/s appeared in two other occasions late in the observation period and caused seaward flushing of the entire system. Overall, the estuarine circulation resembled that of a fjord when river discharge was moderate and weaker: an active upper layer driven by river discharge moving over a semi-stagnant lower layer. Strongest subtidal exchange developed ∼2–3 days after neap tides, in contrast to other studies where fortnightly modulation is determined by vertical mixing from tides. In the lower estuary, subtidal flows seemed to result from a combination of vertical mixing, from tides and wind, and from tidal advection. Moreover, the lower estuary was much more sensitive to wind forcing than the upper estuary. The upper estuary responded preferentially to river discharge and tidal forcing.

Flow interactions between two inlets of Mobile Bay, Alabama, and the Gulf of Mexico are described with ship-mounted ADCP surveys obtained on consecutive days. These two inlets, Main Pass (MP) and Pass aux Herons (PaH), remain interdependent throughout the diurnal tidal cycle. A phase lag of 31.6° (~2.1 h) affects the diurnal tidal currents between the two openings in the lower Mobile Bay. Tidal discharge through each inlet exhibits a time lag, that results in a phase-dependent discharge ratio between the two inlets. Consequently, the magnitude and direction of exchange flows between the two inlets are time-dependent. Early in the rising tide, Mobile Bay receives seawater only through MP while simultaneously discharging estuarine water through PaH. During the beginning of falling tide, Mobile Bay receives seawater through PaH while simultaneously discharging estuarine water through MP. Net transports through MP and PaH during the study period are positive and negative, respectively. Thus, Mobile Bay receives seawater from the Gulf of Mexico through MP and discharges a portion of this seawater, along with estuarine water, to eastern Mississippi Sound through PaH. The characterization of magnitudes and spatial structures of exchange flows in the lower bay determine how buoyant suspended matter connects between the Gulf, Bay, and Sound.

Information on the transport and distribution of microplastics in coastal lagoons is scarce. This study provides the first evaluation of microplastic distribution in a hypersaline coastal lagoon and explores natural and anthropogenic factors that drive their location and transport. The study combines different field strategies: spatial distribution of microplastics in sediments, for September 2017 (wet season and peak use of the lagoon) and February 2018 (winter season, characterized by intense Northerly winds and least use of the lagoon); spatial distribution of microplastics in the water column in the winter season; ocean-lagoon exchanges of water and microplastics at the lagoon entrance during tidal cycles. Also, one-year records of water level variations along the lagoon provide connections between local pressure gradients and water fluxes. Statistical analyses indicate relationships between temporal variations of microplastic concentrations and human activities. Results show marked seasonality in sources and transport agents. During the summer, microplastics concentration was related to human activities. After this season, the accumulated precipitation in the continental karst region leads to an increase in the water level at the head of the lagoon. The resulting pressure gradient promotes seaward flushing of hypersaline water and of microplastics. At tidal (diurnal) time scales, measurements at the mouth of the lagoon revealed that more particles were collected in ebb than in flood. This variability underscores the need to resolve tidal variability for microplastic sampling in coastal lagoons and estuaries.

Hurricane Harvey reached Category 4 when it made landfall on the coast of Texas in late August 2017. Harvey not only affected the coastal region with wind speeds that peaked near 50 m/s, it also dumped ~7.6 × 1010 m3 of rain over 3 days. This rainfall was equivalent to the discharge of the Amazon River over the same period and made Harvey the wettest tropical cyclone to affect the United States. Winds and rainfall interacted to produce atypical storm surges along the coast and estuaries of Texas and compound flooding in the Houston region. Data from the NOAA's Center for Operational Oceanographic Products and Services provided information on water levels in this region. The highest water levels, 3 m above predicted, occurred from August 27th to 29th at Buffalo Bayou in the uppermost reaches of the Galveston-Trinity-Tabbs-Burnet Bay system. The peak surge occurred on Aug 29th because of the triple punch of a) the ocean wind stress and corresponding surge, plus the rainfall-related land-derived discharge from b) Buffalo Bayou and then from c) the San Jacinto River. Winds from the ocean persisted during that 3-day period and drove onshore water transport. This transport, together with anomalously high mean sea levels and the coastline modifications in the upper bay system, delayed the seaward motion of the land-derived discharge. Numerical model simulations that turned forcings on and off, highlighted the importance of the two river pulses in causing the widespread flooding. Simulations also underscored the influence of the interaction between land-derived discharge and ocean-derived surge along different parts of the Houston-Galveston Bay system.

Quarter-diurnal tidal constituents are linked to tidal asymmetries and play a role in the morphodynamic evolution of inlets, estuaries and lagoons. A two-year non-continuous dataset of current velocities and waves was used to improve understanding on the generation of quarter-diurnal harmonics. Data analysis provides evidence for the generation of quarter-diurnal harmonics by wind-induced waves. Data were collected at the entrance to the Arcachon Lagoon, a mixed-energy tidal inlet in Southwest France. Temporal variability of the quarter-diurnal tidal current was identified through wavelet analysis of the main velocity component (alongshore velocity). Quarter-diurnal velocity amplitudes, bottom and wave stresses, and significant wave height suggested a linkage between quarter-diurnal amplification and wave action. Both significant wave height and wave stress were coherent with quarter-diurnal current amplitudes at synoptic frequencies, most markedly in winter months of both years observed. These coherences corroborated the hypothesis that wind-induced waves enhance bottom stresses that can generate and augment quarter-diurnal currents in this region.

River discharge, precipitation, wind, water level and salinity measurements were used to investigate saltwater intrusion in a subtropical estuary. The upstream limit of saltwater intrusion was defined by the position of the 2 g/kg isohaline ( X 2 ) as it changed each tidal cycle. Observations were obtained from November 2014 to November 2015 along the Suwannee River, at Florida's Big Bend. Observations provided information to construct a first-order autoregressive Markov-chain model and predict X 2 . The model predicts X 2 from its position in the previous tidal cycle, from river discharge, from water level, from precipitation and from wind velocity. In order to capture the seasonality of the Suwannee River discharge, the analysis was done separately for high and low river discharge regimes. The high discharge regime for the particular year of observations coincided with the cold season and had a mean value of 360 m3/s. The low discharge regime overlapped with the warm season and had a mean of 198 m3/s. As expected, saltwater intrusion was inversely related to river discharge. Values of X 2 were 0.4 km for a discharge of 377 m3/s, and 6 km for a discharge of 195 m3/s. The autoregressive model was able to reproduce the observed seasonality of saltwater intrusion in the Suwannee River. The model was then used to project values of X 2 for different scenarios of climate change as represented by possible combinations of river discharge and water levels.

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.

Data from three 25 h hydrographical surveys conducted in a microtidal, tropical estuary were used to study seasonal changes of salt intrusion and its response to runoff and wind stress pulses. The tropical estuary was the Jamapa River estuary, located in the southwestern Gulf of Mexico. This tropical region features wet and dry seasons, and is sporadically affected by >10 ms−1 northerly wind events (3 to 5 days long) during the dry autumn and winter. During both, wet and dry seasons, the daily average river discharge commonly exhibits pulses of few days long, associated either with the passage of cold fronts or of tropical storms and hurricanes. Data collection campaigns took place on April 14 of 2010 (dry season), October 15 of 2010 (wet season) and January 11 of 2011 (dry and northerly wind season). In each campaign, an axial transect covered the entire estuary with 13 hydrographic stations, periodically sampled during a diurnal tidal cycle. The main findings can be summarized as: (1) The salt-wedge position was modulated primarily by river discharge. A maximum intrusion length of 5 km along Jamapa River estuary was observed under minimum river discharge. A critical value of river run off ( Q c ) for the presence or absence of a salt-wedge was found to be 70 m3s−1, which was close to the mean annual river discharge. The position of the salt wedge responded to runoff pulses over time scales of the order of hours. (2) Both, down-estuary winds (local action) and NW-winds (along-shore, remote action) pulses enhanced salt wedge intrusion and overwhelmed tidal action on the salt wedge position. (3) Tidal influence was evident on the position of the salt wedge and on the salinity of the bottom layer (only at the estuary entrance), under wind calm conditions.

We extend the 3D unstructured-grid model previously developed for the Upper Chesapeake Bay to cover the entire Bay and its adjacent shelf, and assess its skill in simulating saltwater intrusion and the coastal plume. Recently developed techniques, including a flexible vertical grid system and a 2nd-order, monotone and implicit transport solver are critical in successfully capturing the baroclinic responses. Most importantly, good accuracy is achieved through an accurate representation of the underlying bathymetry, without any smoothing. The model in general exhibits a good skill for all hydrodynamic variables: the averaged root-mean-square errors (RMSE's) in the Bay are 9 cm for sub-tidal frequency elevation, 17 cm/s for 3D velocity time series, 1.5 PSU and 1.9 PSU for surface and bottom salinity respectively, 1.1 °C and 1.6 °C for surface and bottom temperature respectively. On the shelf, the average RMSE for the surface temperature is 1.4 °C. We highlight, through results from sensitivity tests, the central role played by bathymetry in this estuarine system and the detrimental effects, from a common class of bathymetry smoothers, on volumetric and tracer fluxes as well as key processes such as the channel-shoal contrast in the estuary and plume propagation in the coast.

Transverse variations of tidal and subtidal flow were investigated in a macrotidal and convergent estuary. This was accomplished by combining data analysis of current velocities and water density with numerical modeling at the mid-reaches of the Gironde Estuary (France). Nonlinear mechanisms responsible for overtide generation and hence subtidal flows were found to vary across the estuary and from neap to spring tides. Subtidal flows were driven by a combination of internal asymmetry, tidal advective accelerations, nonlinear effects of water level variations, quadratic friction, and river discharge. The quarter-diurnal overtide band (D4) in the flow was generated by internal asymmetry and tidal advective accelerations during neap tide. The ratio of quarter-diurnal to squared semidiurnal bands (D4/D22) was largest (>0.3) in sections of the channel showing subtidal outflow. River discharge increased from neap to spring tides causing a subsequent increase of seaward subtidal currents. During spring tide, D4 was generated by tidal advective accelerations and quadratic friction combined with river discharge, rather than by internal asymmetry. The sixth-diurnal overtide (D6) in the flow was comparable to D4 for both neap and spring tides. Largest D6/D23 ratios were found in the shallowest cross-channel locations during both neap and spring tides.

Eddy viscosity parameterizations that depend on a gradient Richardson number Ri have been most pertinent to the open ocean. Parameterizations applicable to stratified coastal regions typically require implementation of a numerical model. Two novel parameterizations of the vertical eddy viscosity, based on Ri, are proposed here for coastal waters. One turbulence closure considers temporal changes in stratification and bottom stress and is coined the ‘regular fit'. The alternative approach, named the ‘lateral fit', incorporates variability of lateral flows that are prevalent in estuaries. The two turbulence parameterization schemes are tested using data from a Self Contained Autonomous Microstructure Profiler (SCAMP) and an Acoustic Doppler Current Profiler (ADCP) collected in the James River Estuary. The ‘regular fit' compares favorably to SCAMP-derived vertical eddy viscosity values but only at relatively small values of gradient Ri . On the other hand, the ‘lateral fit' succeeds at describing the lateral variability of eddy viscosity over a wide range of Ri. The modifications proposed to Ri-dependent eddy viscosity parameterizations allow applicability to stratified coastal regions, particularly in wide estuaries, without requiring implementation of a numerical model.

Data from field experiments were used to assess, for the first time, hydrodynamic and transport processes in three shallow low-inflow estuaries of Northeast Brazil (Ceara State). The estuaries of the Coco, Pacoti and Pirangi rivers exhibit anthropogenic stresses related to freshwater withdrawals and sewage discharges and morphological changes. Sampling was carried out during complete spring tidal cycles at each estuary during consecutive days on November 6, 7 and 8, 2010. November is the end of the dry period, and the results represented expected extreme low inflow conditions. All estuaries showed well mixed water columns because of frictional influences. Coco and Pacoti displayed ebb dominance while the Pirangi exhibited flood dominance, a result from morphological changes in the intertidal storage areas. Each estuary showed distinct longitudinal salinity distributions related to different water budgets, which were actually altered by human activities. The Coco estuary was hyposaline because of waste water discharge from the city of Fortaleza. This estuary represented a source of materials to the coastal sea as it showed net export of Suspended Particulate Matter (SPM) and chlorophyll. The Pacoti estuary was slightly hypersaline but nearly neutrally buoyant as temperature compensated for salinity variations. The Pacoti also exhibited net seaward transport of SPM and chlorophyll. The Pirangi estuary showed strong hypersalinity and inverse estuarine circulation as a result of multiple river dams. The negative water budget combined with mangrove depletion led to magnifying trapping processes, thus resulting in net import of salt, SPM and chlorophyll. The three cases exemplified how estuaries on the order 10 km long can be sensitive to human-induced changes. These human changes affect their circulation structure and their function as source or trap for materials at the land–ocean interface.

The variability of residual flows and saltwater intrusion was evaluated for the duration of a hurricane-related freshwater pulse in a subtropical estuary, the Suwannee River. Increased rainfall affected the river’s watershed when Hurricane Irma skirted the west coast of Florida in September 2017. The river remained short of reaching flood stages, but the increased river discharge was enough to modify the residual circulation and restrict saltwater intrusion. Current velocity profiles and hydrographic variables were collected in five tidal-cycle experiments to study the lower estuary’s response to the peak river discharge and its subsequent relaxation. Data were recorded in three cross-estuary transects. In addition, continuous salinity measurements were recorded for 48 days over the shallow portion of the seaward transect. Results showed that the subtidal water-level slope inside the estuary, between the mouth and a river station at Wilcox, modified the residual circulation and restricted the intrusion of salt water. During high tide, salt water entered the estuary due to tidal pumping even though residual circulation was seaward in the cross-section. Residual seaward flows persisted for the duration of the hurricane-related freshwater pulse. After relaxation of the freshwater pulse, saltwater intrusion was observed in the time series over the shallow portion. This saltwater intrusion responded to changes in wind forcing, with largest intrusions when the wind blew into the estuary, i.e., toward the N and NW.

Current velocity and hydrographic measurements were obtained in Mossoro estuary, a tropical semiarid estuary in Northeastern Brazil, to explore the hydrodynamic regime associated with marked seasonal variability in evaporation and precipitation. Measurement campaigns were performed during dry conditions in October 2012 and wet conditions in May 2013. Dry season observations showed along-estuary density gradients of order 0.001 kg/m 4 with salinity increasing upstream in the first 20 km up to a value of 70 g/kg. Salinity then decreased to a value of 48 g/kg at the upstream limit of the section. During this dry season, the residual flow structure changed drastically from spring to neap tides. Neap tide residual flows showed inflow of oceanic water at the surface and outflow of hypersaline water underneath. Spring tide residual flows showed net inflow throughout the water column in the middle of the section and outflow over the flanks, a laterally sheared structure consistent with tidally induced residual flows. In the wet season, along-estuary density gradients were of similar magnitude to the dry season but in the opposite direction. Residual flows in neap tides were as expected for density-driven flows: outflow at surface and inflow underneath. During spring tides, residual flows were dominated by outflows over the flanks and inflow in the middle of the section, once again consistent with tidally driven residuals. Advective accelerations from tides were compared to baroclinic pressure gradients, in the non-dimensional tidal Froude number ( Fr 0 ), to explore the changes from tidally driven to density-driven residual flows. Results indicated that residual flows in this estuary were driven by tidal rectification ( Fr 0 >> 1) in spring tides and by density-gradients ( Fr 0 Fr 0 .

Estuarine salt plugs can cause water quality problems and are usually attributed to hypersaline conditions in low-inflow estuaries, where evaporation rates exceed or match freshwater input rates. However, salt plugs may also develop in typical estuaries, in brackish waters, by an additional freshwater influence from the estuary's mouth. In this study, month-long and years-long data showed the formation of a salt plug by a freshwater pulse from the ocean into Barataria Bay, in the Mississippi River Delta. Month-long currents showed an inverse estuarine circulation at Barataria Pass with inflow in the surface and outflow underneath. A dimensionless analysis showed that the dynamics governing the inverse circulation was related to a density gradient reversal, which was related to a salt-plug. Empirical Orthogonal Functions applied to nine years of salinity values along the estuary showed that Mode 1 (EOF1) represented the salt-plug structure and explained 86% of the variability. The salt-plug structure was most pronounced during winter, under minimum river discharge, and least evident in summer. Wavelet coherence analysis showed an annual relationship between EOF1 weights and both, river discharge and along-channel wind-stress. This study shows evidence of a salt-plug development by buoyant water pulses from outside an estuary.

Year-long time series of water level are analyzed at five locations along the St. Johns River Estuary, Florida, to investigate propagation of subtidal pulses. Hilbert-transformed Empirical Orthogonal Functions (HEOFs) are obtained after a dominant seasonal signal is extracted from the data. These functions provide information on spatial structure and propagation phase of subtidal water level pulses. The first HEOF mode explains 96% of the subtidal variability and features an unusual spatial structure: amplitude attenuation (averaging 1 mm/km) to 55 km upstream, slight amplification (0.16 mm/km) over the middle 70 km, and attenuation (2.3 mm/km) over the final 18 km of the estuary. The phase suggests a shift from progressive to quasi-standing wave behavior at 55 km from the estuary mouth. Additionally, local minima in the phase suggest two sources of subtidal forcing: the coastal ocean and the upstream end. An analytical model describing the evolution of long waves through a channel with frictional damping is fit to the amplitude of HEOF mode 1. Solutions are obtained as a function of two parameters: the nondimensional length of the basin, κ, and the nondimensional frictional depth, δ. Values of κ between 0.55 and 0.67 and δ between 1.45 and 1.7 provide the best fit with the HEOF results (1% error or less). These values indicate a highly frictional environment in which the average subtidal wavelength is 10 times the basin length. Subtidal pulses in this estuary, therefore, behave as damped waves that can be represented with idealized models.

Residual currents induced by asymmetries in tidal mixing (ATM) were determined using a series of underway-current velocity profiles and salinity measurements, estimates of vertical mixing, and an analytical expression. Six different 12-hour transect surveys at the mouth of the James River were carried out under typical spring and neap tides and a range of river flows. The ATM-induced flows were found to be of the same order of magnitude as the density-driven flows. These flow profiles showed either a two or a three-layer structure in the channel and a two-layer structure over shoals. More stratification during ebb than flood periods (tidal straining) favored gravitational circulation, whereas reversed straining (more stratification at flood) opposed gravitational flow. The contribution of ATM-induced residuals increased as water column stratification increased. Lateral variations in ATM favored lateral shear in along-channel flow, enhancing the laterally sheared exchange. Observations were in good agreement with previous numerical and analytical model results. In particular, observational validation is presented on reverse tidal straining inducing residuals that act against gravitational circulation. Inconsistencies arise from the assumptions made in the models, namely no Coriolis forcing, no topographic features such as headlands, and simplified nature of stratification.

Tidal variability of lateral advection of momentum ( vu y , where u and v are along-estuary and lateral flows, respectively, and the subindex indicates differentiation with respect to the cross-estuary direction) was investigated in a coastal plain estuary with observations at Hampton Roads, which is the transition between the James River and Chesapeake Bay. Towed current velocity profiles and hydrographic profiles were captured during 9 expeditions in 2004 and 2005, to determine the intratidal and spatial changes in lateral advection of momentum and its contribution to along-channel flow. Curvature effects and lateral density gradients were important in driving lateral circulation and in modifying intratidal lateral advection of momentum. Lateral advection had the same order of magnitude as the baroclinic pressure gradient. Its contribution to the along-channel momentum balance was greatest during or just after peak flood and weakest at the end of ebb. During peak flood and peak ebb, the spatial distribution of vu y was seaward at the southern (left) side near surface and at the northern side (right) near bed (looking up-estuary), and landward in the rest of the channel. During slack periods the vu y structures were mostly landward. Observations were in good agreement with analytical model results during peak ebb and flood, but inconsistent during slack periods. The discrepancies between model results and field measurements can be attributed to bathymetry–density gradient interactions, which enhanced ebb-to-flood asymmetries in the along-channel and lateral flow.

[1] The role of asymmetric tidal mixing (ATM) in subtidal estuarine dynamics is investigated using a series of generic numerical experiments that simulate narrow estuaries under different stratification and external forcing conditions. The focus is on quantifying the characteristics of ATM-induced flow and its contributions to stratification and salt transport. The flow induced by ATM has a two-layer vertical structure in periodically stratified estuaries, similar to that of the density-driven flow. It has a three-layer vertical structure in the central regime of weakly stratified estuaries, and a reverse two-layer structure in highly stratified estuaries. The changes in vertical distribution of ATM-induced flows result from the influence of stratification on the covariance of eddy viscosity and vertical shear. Such covariance represents the driving force of ATM-induced flow in the tidally averaged momentum equation. Compared to density-driven flow, ATM-induced flow dominates in periodically stratified estuaries with strong tides, has the same order of magnitude in weakly stratified estuaries with moderate tides, and is less important in highly stratified estuaries with weak tides. In contrast to density-driven flow that always increases estuarine stratification and transports salt landward, the ATM-induced flow exhibits different behaviors because of its varying vertical structure. In estuaries with strong tides, ATM-induced flow tends to enhance stratification and to transport salt landward, similar to density-driven flow. In estuaries with weak tides, ATM-induced flow tends to reduce stratification and to transport salt seaward.

[1] The influence of bathymetry on an estuary plume at an estuary-shelf transition is studied with a three-dimensional ocean circulation model. To understand the response of the plume to bathymetry, several types of estuarine shapes and shelf geometries were adopted in this numerical study. The channel's shape and its width-to-depth aspect ratio affected the fate of the plume by determining flow characteristics inside the estuary. Moreover, the bathymetry of the shelf such as the shelf slope and the direction of a submarine channel defined the plume characteristics on the shelf. An estuarine channel with a triangular cross section generated relatively stronger exchange flows at midestuary than a rectangular cross section, which resulted in a larger surface plume over the shelf. The extension of the submarine channel onto the shelf favored increased plume water transport out to the shelf, a result of reduced frictional effects on the shelf. The orientation of the submarine channel changed the direction of the plume over the shelf, with no additional external forces. Two fronts developed at the edges of the submarine channel because of enhanced lateral shears in the flow. When the estuary was relatively wide compared to the internal Rossby radius (Kelvin number Ke ≥ 5), or when the relative strength of the freshwater discharge compared to the estuary width was weak (Rossby number Ro ≤ 0.05), the coastal plume did not expand up-shelf. In fact, results indicated that freshwater up-shelf transport in a coastal current, moving against Coriolis’ accelerations, was proportional to Ro.

Bathymetric effects on tidal currents are investigated at Main Pass, which is the primary inlet of Mobile Bay, Alabama. A 12-h ship-mounted ADCP survey, which covered nearly one-half of the diurnal tide during flood conditions, included 24 repetitions. The resulting velocity data demonstrate significant tidal variability in the horizontal and vertical current structure between the ship channel and the shoals. The diurnal tidal flows, the dominant tidal forcing, are 72° (4.8 h) ahead of the water level throughout shallower areas of Main Pass, indicating near-standing wave conditions. Moving across the mouth, a phase lag (5.37° or 20 min) develops with the deep channel tidal currents lagging the shoal region. The vertical tidal structure is also modified across the mouth where near-bottom flows change their direction first in the ship channel, while near-surface flows change their direction first over the western shoal. This may be related to the seaward pressure gradient associated with the relatively large (~1715 m 3 /s) freshwater discharge or the discharge interaction with a nearby opening, Pass-aux-Herons. Current magnitudes over the shoals and in the ship channel vary by as much as 1 m/s. Flows at the east side of Main Pass, close to Mobile Point, behave oppositely to those in the rest of the transect during the survey. This inconsistent flow pattern is caused by an anticyclonic eddy that is triggered by flow separation at Mobile Point.

[1] Measurements of velocity profiles and near-bottom temperature and pressure were used to determine turbulence properties at a point-source submarine groundwater discharge (brackish) in a tropical estuary. The turbulence properties estimated were Reynolds stress, turbulent kinetic energy production, and vertical eddy viscosity. Results showed a dominance of the zonal Reynolds stress component with maximum of 0.0025 m2 s−2 (2.5 Pa) at low tide. Turbulent kinetic energy production and vertical eddy viscosity values also reached maxima (0.98 W m−3 and ~10−1 m2 s−1, respectively) at low tides. Discharge of brackish water increased at low tides, relative to high tides, as indicated by vertical mean velocity and by mean velocity shear. These maxima were caused by decreasing hydrostatic pressure and likely increasing hydraulic head at the site of discharge. Increased turbulence at low tides was one order of magnitude larger than the turbulence caused elsewhere by tidal flows up to ~2.5 m s−1.

Con un microperfilador SCAMP (Self Contained Autonomous MicroProfile) se detectó un proceso de Doble Difusión Convectiva (DDC), en los canales Martínez y Baker de la Patagonia central, Chile. Este perfilador midió temperatura y salinidad en la columna de agua con una resolución del orden de milímetros. El muestreo se efectuó en diciembre 2011, en 19 estaciones oceanográficas de ~55 m de profundidad. Los registros del microperfilador permitieron detectar la formación de escaleras termohalinas que caracterizan la formación de la DDC. Por medio del ángulo de Turner se confirmó que debajo de una capa superficial fuertemente estratificada (7-10 m) se registraron eventos de DDC a profundidades entre 15 y 45 m. La DDC fue débil en la mayoría de los registros pero se evidenciaron eventos fuertes entre 15 y 40 m. En la capa subsuperficial, su origen se debió al intercalamiento de las masas aguas estuarinas (aguas frías y menos saladas) encima de las oceánicas (aguas más cálidas y saladas), mientras que en la superficie la interacción océano-atmósfera favoreció su ocurrencia. La DDC en conjunto con el viento, las mareas y las ondas internas, puede constituir un mecanismo importante para la mezcla vertical, influenciando los flujos de calor, el hábitat de las especies marinas en estas profundidades y la exportación de carbono hacia los sedimentos.

For at least 20 years, nadir altimetry satellite missions have been successfully used to first monitor the surface elevation of oceans and, shortly after, of large rivers and lakes. For the last 5–10 years, few studies have demonstrated the possibility to also observe smaller water bodies than previously thought feasible (river smaller than 500 m wide and lake below 10 km2). The present study aims at quantifying the nadir altimetry performance over a medium river (200 m or lower wide) with a pluvio-nival regime in a temperate climate (the Garonne River, France). Three altimetry missions have been considered: ENVISAT (from 2002 to 2010), Jason-2 (from 2008 to 2014) and SARAL (from 2013 to 2014). Compared to nearby in situ gages, ENVISAT and Jason-2 observations over the lower Garonne River mainstream (110 km upstream of the estuary) have the smallest errors, with water elevation anomalies root mean square errors (RMSE) around 50 cm and 20 cm, respectively. The few ENVISAT upstream measurements have RMSE ranging from 80 cm to 160 cm. Over the estuary, ENVISAT and SARAL water elevation anomalies RMSE are around 30 cm and 10 cm, respectively. The most recent altimetry mission, SARAL, does not provide river elevation measurements for most satellite overflights of the river mainstream. The altimeter remains “locked” on the top of surrounding hilly areas and does not observe the steep-sided river valley, which could be 50–100 m lower. This phenomenon is also observed, for fewer dates, on Jason-2 and ENVISAT measurements. In these cases, the measurement is not “erroneous”, it just does not correspond to water elevation of the river that is covered by the satellite. ENVISAT is less prone to get ‘locked’ on the top of the topography due to some differences in the instrument measurement parameters, trading lower accuracy for more useful measurements. Such problems are specific to continental surfaces (or near the coasts), but are not observed over the open oceans, which are flatter. To overcome this issue, an experimental instrument operating mode, called the DIODE/DEM tracking mode, has been developed by CNES (Centre National d’Etudes Spatiales) and has been tested during few Jason-2 cycles and during the first SARAL/AltiKA cycle. This tracking mode “forces” the instrument to observe a target of interest, i.e. water bodies. The example of the Garonne River shows, for one SARAL ground track, the benefit of the DIODE/DEM tracking mode for a steep-sided river reach, which is not detected using the nominal instrument operating mode. Yet, this mode relies on ancillary datasets (a priori global DEM and global land/water mask), which are critical to obtain river valley observation. The ultimately computed elevations along the satellite tracks, loaded on board, should have an absolute vertical accuracy around 10 m (or better). This case also shows, when the instrument is correctly observing the river valley, that the altimeter can detect water bodies narrower than 100 m (like an artificial canal). In agreement with recent studies, this work shows that altimeter missions can provide useful water elevation measurements over a 200 m wide river with RMSE as low as 50 cm and 20 cm, for ENVISAT and Jason-2 respectively. The seasonal cycle can be observed with the temporal sampling of these missions (35 days and 10 days, respectively), but short term events, like flood events, are most of the time not observed. It also illustrates that altimeter capability to observe a river is highly dependent of the surrounding topography, the observation configuration, previous measurements and the instrument design. Therefore, it is not possible to generalize at global scale the minimum river width that could be seen by altimeters. This study analyzes, for the first time, the potential of the experimental DIODE/DEM tracking mode to observe steep-sided narrow river valleys, which are frequently missed with nominal tracking mode. For such case, using the DIODE/DEM mode could provide water elevation measurements, as long as the on board DEM is accurate enough. This mode should provide many more valid measurements over steep-sided rivers than currently observed.

Bathymetry effects on the flow field at the transition between idealized estuaries and the adjacent ocean are studied with the Regional Ocean Modeling System (ROMS). Estuary width, depth, and channel direction at the shelf are used to determine flow characteristics in and out of an idealized estuary. The idealized estuary connects to an upstream boundary, where freshwater discharges, and an offshore tidal boundary. Tidally averaged salinity and flow structures are examined at the estuary mouth. A recirculation feature directly affects flow at the estuary mouth, especially in shallow and wide estuarine systems. The recirculation retards the exchange flow near the edges of the estuary mouth and consequently allows strengthening of the flow in the middle. The geometric shape of the estuarine channel affects the strength of the residual flow at the estuary mouth. The presence of an extended submarine channel on the shelf enhances the baroclinic circulation and stratification, and maximizes the salinity intrusion length without additional external forces. The direction of this submarine channel affects the exchange flow structures at the estuary/ocean transition zone in such a way that the salinity intrusion length increases with a left-turning channel (in the Kelvin wave sense of the fresh water flow direction). This is attributed to the competition between centrifugal and Coriolis forces. Flow characteristics described by the Kelvin and Ekman number, which outline channel geometric effects, are similar to previous studies.

The influence of nonlinear advection on estuarine exchange flow was investigated with observations at the transition between the James River and Chesapeake Bay, Hampton Roads, Virginia. Data were collected under different tidal forcing, wind forcing, and river discharge in 2004 and 2005. The relative contribution of nonlinear advective terms to the along-channel momentum balance had the same order of magnitude as pressure gradient and friction, verifying recent analytical and numerical model results. Both the magnitude and the spatial distribution of nonlinear advection showed fortnightly variability. Nonlinear advection was more influential on along-channel flow at spring tides than at neap tides because of increased tidal velocities, in a cross-sectionally averaged sense. The flow structures induced by each nonlinear advective process were investigated for the first time with observations. The lateral advection term υuy was found to enhance laterally sheared exchange acting along with Coriolis forcing at spring tides and opposing it at neap tides. Vertical advection wuz showed similar spatial distribution as υuy at spring tides but was vertically sheared (landward at middepth and seaward in the rest of the water column) at neaps. Longitudinal advection uux augmented landward flow in the channel.

Measurements of current velocity profiles during and after cresting of the Suwannee River in Northern Florida, USA, were used to investigate the effects of increased river discharge on subtidal flows near the estuarine transition with the Gulf of Mexico. Three moored velocity profilers were deployed across a lower estuary cross-section. The cross-section bathymetry consisted of a channel (∼5.5 m deep) near the western bank of the estuary that shoaled monotonically eastward. Two-layer gravitational exchange developed only in the deepest part of the cross-section during the river cresting and persisted for ∼20 days. After this ∼20-day period, the net flow decreased and was seaward throughout the water column. Net flows outside the channel were seaward throughout the observation period and were modulated by the river pulse. By comparing the estuarine response in the 5.5-m channel to theoretical responses driven by a dynamic balance between pressure gradient and stress divergence, a condition required for two-layered flow was proposed. Gravitational exchange flow should be expected when the ratio of density-driven flow to river-induced flow is greater than 0.23 to 0.28. Smaller values of this ratio should produce unidirectional, seaward flows after a river pulse. Two-layered flows restricted to the channel can be explained also with this ratio because of the sensitivity of density-driven flows to local depth and eddy viscosity. These findings need to be tested against observations in other systems affected by extreme freshwater pulses.

The spatial structure of tidal amplitude and phase in a simplified system of connected estuaries, an idealized version of Florida’s Intracoastal Waterway, is analyzed with a linear analytical model. This model includes friction, the earth’s rotation, and variable bathymetry. It is driven at the connection with the ocean by a co-oscillating tide. Model results compare well with observations of pressure and currents in a section of the Intracoastal Waterway on the east coast of Florida. The comparison suggests that the waterway is highly frictional, causing the amplitude of the water elevation and tidal velocity to decrease away from the inlets to a minimum in the middle of the waterway. The local phase relationship between velocity and water elevation changed nonlinearly from 90° with no friction to 45° with maximum friction. In moderately to highly frictional basins, the phase lag was consistently less than 45°.

Observations of residual exchange flows at the entrance to four subtropical estuaries, two of them semiarid, indicate that these flows are mainly tidally driven, as they compare favorably with theoretical patterns of tidal residual flows. In every estuary examined, the tidal behavior was that of a standing or near-standing wave, i.e., tidal elevation and tidal currents were nearly in quadrature. The pattern of exchange flow that persisted at every estuary exhibited inflow in the channel and outflow over the shoals. Curiously, but also fortuitously, this pattern coincides with the exchange pattern driven by density gradients in other estuaries. The tidal stresses and the residual elevation slopes should be the dominant mechanisms that drive such tidal residual pattern because the Stokes transport mechanism is negligible for standing or near-standing waves. Time series measurements from the semiarid estuaries showed fortnightly modulation of the residual flow by tidal forcing in such a way that the strongest net exchange flows developed with the largest tidal distortions, i.e., during spring tides. This modulation is opposite to the modulation that typically results in temperate estuaries, where the strongest net exchange flows tend to develop during neap tides. The fortnightly modulation on tidal residual currents could be inferred from previous theoretical results because residual currents arise from tidal distortions but is made explicit in this study. The findings advanced herein should allow the drawing of generalities about exchange flow patterns in subtropical estuaries where residual flows are mainly driven by tides.

Time series at eight locations in Chesapeake Bay and the adjacent inner shelf were used to determine the relative influence of the wind and barometric pressure effects on subtidal sea-level variability and slopes in the estuary. Special emphasis was placed on the lower Chesapeake Bay, where inverse barometric effects accounted for up to 32% of the subtidal sea-level variations, and wind forcing accounted for more than 67% of the variance. The wind frequency from any given direction varied from one station to another due to the nonsynoptic characteristics of atmospheric pressure in Chesapeake Bay. In the northern bay, northwesterly winds were most frequent in winter, and southerly winds were most frequent in summer. In the southern bay, northeasterly winds were most frequent in fall and winter, and southwesterly winds dominated in the summer. These winds produced sea-level responses as follows: northeasterly winds caused water to pile up in the southwestern corner of the bay, whereas southwesterly...

Microstructure and current velocity measurements were collected at a cross-channel transect in the James River under spring and neap tidal conditions in May 2010 to study cross-estuary variations in vertical mixing. Results showed that near-surface mixing was related to lateral circulation during the ebb phase of a tidal cycle, and that the linkage was somewhat similar from neap to spring tides. During neap tides, near-surface mixing was generated by the straining of lateral density gradients influenced by the advection of fresh, riverine water on the right side (looking seaward) of the transect. Spring tide results revealed similar findings on the right side of the cross section. However, on the left side, the straining by velocity shears acted in concert with density straining. Weak along-estuary velocities over the left shoal were connected to faster velocities in the channel via a clockwise lateral circulation (looking seaward). These results provided evidence that in the absence of direct wind forcing, near-surface vertical mixing can occur from mechanisms uncoupled from bottom friction.

Selective tidal stream transport is hypothesized as a dominant mechanism by which larvae of marine animals move through estuarine openings. For larvae moving from the shelf to estu- arine habitats, selective tidal stream transport proposes that larvae are higher in the water column during flood tide and lower in the water column during ebb tide. Although a number of studies conclude that selective tidal stream transport is the mechanism responsible for larval ingress, few studies consider alternative mechanisms or consider passive explanations for tidal patterns in larval distributions. We examined the biophysical mechanisms responsible for larval ingress into Chesa- peake Bay using an Eulerian approach. We made flux calculations for 3 species and partitioned flux estimates among 3 different ingress mechanisms (wind forcing, residual bottom inflow and tidal). For the Atlantic croaker Micropogonias undulatus (Sciaenidae), all 3 mechanisms of ingress contributed to the net up-estuary flux of larvae, but tidal mechanisms become more important for larger sizes. Net up-estuary flux of the Atlantic menhaden Brevoortia tyrannus (Clupeidae) was dominated by resid- ual bottom inflow and wind forcing. Ingress of the summer flounder Paralichthys dentatus (Para- lichthyidae) was dominated by tidal mechanisms, and the importance of tides increased with devel- opmental stage. We found little evidence for the hypothesis that tidal patterns in larval distributions resulted from passive processes (water mass-specific distributions, buoyancy, vertical mixing), thereby supporting the hypothesis that tidal patterns resulted from active behaviors. However, our estimates of vertical mixing were not direct and additional work is needed to examine the role of vertical mixing in influencing vertical distributions in areas with strong tides. We conclude that a combination of wind forcing, residual bottom inflow, and selective tidal stream transport are respon- sible for the ingress of larval fishes into Chesapeake Bay, and that the relative importance of the 3 mechanisms differs among species and changes with larval development.

We present the first measurements obtained with a combination of a towed acoustic Doppler current profiler (ADCP) and a conductivity–temperature–depth (CTD) recorder in the Rio de la Plata Estuary. Subtidal and tidal flows are described for austral winter and summer conditions. Data were collected at three transects 15 km long for 25 h during August 11–17, 1999 (austral winter) and February 2–6, 2000 (austral summer) at the transition zone between fresh and brackish waters. Two transects covered cross-estuary tracks off the northern and southern coastlines of the estuary. A central transect was oriented along the estuarine axis near the up-stream limit of saltwater intrusion. The observations indicated that the dominant dynamical processes were different at each of the transects sampled. The most relevant feature was the dominance of tidal currents in the southern side of the estuary and the lesser role they played in the northern side (Uruguayan coast). The pressure gradient produced by the freshwater outflow was modified differently at the southern than at the northern sides of the estuary. At the southern side tidal mixing, from strong tidal currents, allowed the development of gravitational circulation. At the northern side weak tidal currents allowed modifications by ambient forcing and surface stresses, consistent with theoretical results of surface-advected freshwater outflows.

Linkages among density, flow, and bathymetry gradients were explored at the entrance to the Chesapeake Bay with underway measurements of density and flow profiles. Four tidal cycles were sampled along a transect that crossed the bay entrance during cruises in April–May of 1997 and in July of 1997. The April–May cruise coincided with neap tides, while the July cruise occurred during spring tides. The bathymetry of the bay entrance transect featured a broad Chesapeake Channel, 8 km wide and 17 m deep, and a narrow North Channel, 2 km wide and 14 m deep. The two channels were separated by an area with typical depths of 7 m. Linkages among flows, bathymetry, and water density were best established over the North Channel during both cruises. Over this channel, greatest convergence rates alternated from the left (looking into the estuary) slope of the channel during ebb to the right slope during flood as a result of the coupling between bathymetry and tidal flow through bottom friction. These convergences were linked to the strongest transverse shears in the along-estuary tidal flow and to the appearance of salinity fronts, most markedly during ebb periods. In the wide channel, the Chesapeake Channel, frontogenesis mechanisms over the northern slope of the channel were similar to those in the North Channel only in July, when buoyancy was relatively weak and tidal forcing was relatively strong. In April–May, when buoyancy was relatively large and tidal forcing was relatively weak, the recurrence of fronts over the same northern slope of the Chesapeake Channel was independent of the tidal phase. The distinct frontogenesis in the Chesapeake Channel during the increased buoyancy period was attributed to a strong pycnocline that insulated the surface tidal flow from the effects of bottom friction, which tends to decrease the strength of the tidal flow over relatively shallow areas.

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.

Water density and velocity data from a ∼70-day period deployment across the entrance to the Chesapeake Bay were used in conjunction with wind velocity and sea level records to describe the response of the lower part of the estuary to forcing from Hurricane Floyd in mid-September 1999. Forcing associated with Floyd consisted of northeasterly winds during the 4 days prior to the passage of the storm eye over the lower Chesapeake Bay with hourly values that peaked at ∼21 m/s. After the passage of the eye of the storm on 16 September, winds shifted rapidly to northwesterly and peaked at 27 m/s. Buoyancy forcing from Floyd consisted of at least 2000 m3/s of direct freshwater input to the lower Chesapeake Bay. This was added to the runoff from the surrounding land that was already saturated with precipitation from tropical storm Dennis that affected the area of the lower bay in early September 1999. Northeasterly winds prior to the eye passage over the lower bay caused net inflow over the shallow northern half of the bay entrance and outflow in the deep channel to the south of the entrance. This outflow veered clockwise with depth. The change to northwesterly winds coincided with a pulse of freshwater that caused a salinity drop throughout the water column of up to 8 units in 1 day. Wind and river discharge set up a seaward barotropic pressure gradient force that drove net outflow everywhere across the entrance to the bay thus hindering any inflow and effectively flushing water out. The barotropic pressure gradient must have overcome the tendency of the baroclinic pressure gradient, related to the buoyancy input, to drive salty water into the estuary. It is estimated that approximately one-third of the net outflow was caused by wind forcing and two-thirds by freshwater discharge.

Water velocity data from acoustic Doppler current proe lers and electromagnetic current meters deployed at six separate locations across the entrance of the Chesapeake Bay from mid-April to early July of 1999 and from early September to mid-November of 1999 were used in conjunction with wind velocity and sea level records to describe the characteristics of the wind-induced subtidal volume exchange between the bay and the adjacent continental shelf. The current measurements were used to estimate volume e uxes associated with the local and remote wind-induced bay-shelf exchange over time scales of 2‐ 3 days. The results show that at these relatively short subtidal time scales (1) the net e ux integrated over the entrance to the estuary adequately describes the unidirectional (either ine ow or oute ow over the entire cross-section) barotropic volume e ux associated with the coastally forced remote wind effect, (2) during the e rst deployment there is always a bi-directional exchange pattern (ine ow and oute ow existing simultaneously over different parts of the cross-section) superimposed on the sectionally integrated unidirectional exchange, (3) the magnitude of the bi-directional transport associated with the local wind effect may be a signie cant fraction of the unidirectional transport associated with the remote wind effect, and (4) the relative importance of the local wind effect in producing estuary-shelf exchange changes appreciably with season, depending on the characteristic frequency of the wind events and the degree of stratie cation in the estuary.

We used an extensive temperature and salinity data set to develop a statistically meaningful way of estimating mean temperature and salinity from discrete measurements in the mouth of Chesapeake Bay. From April 1992 to December 1998, the Center for Coastal Physical Oceanography completed 73 monthly hydrographic sections at high spring tide across the mouth of Chesapeake Bay. Time series of area weighted mean bay mouth temperature (MBMT) and salinity (MBMS) were calculated. We found that at any time the temperature at any location in the section correlated with the MBMT with a r2 of 0.95 or better. A similar analysis for salinity showed that the best correlation was about 0.9 with many locations below 0.8. A correlation between MBMT and temperature at a nearby tide station indicated it was possible to estimate MBMT from the temperature at the tide station to ±0.74°C (90% confidence interval). Salinity was not measured at the tide station, but the correlation at a location in the section similar to the tide station indicates that MBMS can be estimated with an error of ±1.5 (90% confidence interval).

Water velocity and density profiles were obtained in late-spring and late-winter to document reversing mean circulation patterns at the entrance to a semiarid coastal lagoon, the Bay of Guaymas, in the Gulf of California, Mexico. The lagoon is shallow but the bathymetry at its entrance is similar to that of temperate estuaries with an asymmetrically positioned channel flanked by shoals. In late-spring the mean circulation at the entrance to the lagoon was driven by horizontal density gradients that arose from excess evaporation over precipitation in the area as evidenced by water density profiles. The lagoon exported relatively warm (25·8 °C) and salty (36·2) water to the Gulf of California through the channel. This export was consistent with inverse estuarine circulation influenced by bathymetry. In late-winter, the circulation at the entrance of the lagoon was mostly driven by wind stress that blew from the northwest, roughly along the main axis of the lagoon. Relatively cool (16·0) °C) and less salty (35·1) water from the Gulf of California was driven into the lagoon within the channel. Density gradients inside the lagoon seem to have played a secondary role in driving the circulation. The late-winter circulation was then estuarine-like, with outflow in the direction of the wind over the shallow areas and a compensatory inflow appearing in the channel as expected from theory of wind-driven flow over bathymetry. This estuarine-like circulation developed despite the lack of measurable freshwater input to the lagoon and was the opposite to that observed in late-spring. These observations then document a reversal in water exchange patterns from season to season in a semiarid coastal lagoon. The observations also constitute one of the few reported examples of flow over shoals driven in the same direction as the wind stress with a compensatory flow in the channel.

Water density and velocity data from two ;75-day deployments across the entrance to the Chesapeake Bay were used in conjunction with wind velocity and sea level records to describe the transverse structure of wind-induced subtidal exchange. Acoustic Doppler current proe lers, electromagnetic current meters, and conductivity-temperature-depth recorders were deployed at the entrance to the bay from mid-April to early July of 1999 and from early September to mid-November of 1999. Three main scenarios of wind-induced exchange were identie ed: (1) Northeasterly (NE) winds consistently drove water from the coast toward the lower Chesapeake Bay as well as water from the upper bay to the lower bay, which was indicated by the surface elevation slopes across the lower bay and along the bay. This resulted in water piling up against the southwestern corner of the bay. The subtidal e ow over the southern portion of the bay entrance was directed to the left of the wind direction, likely the result of the ine uence of Coriolis and centripetal accelerations on the adjustment of the sea level gradients. Over the northern shallow half of the entrance, the subtidal e ows were nearly depth-independent and in the same direction as the wind. (2) Southwesterly (SW) winds caused opposite sea level gradients (relative to NE winds), which translated into near-surface oute ows throughout the entrance and near-bottom ine ows restricted to the channels. This windinduced circulation enhanced the two-way exchange between the estuary and the adjacent ocean. (3) Northwesterly winds produced the same exchange pattern as NE winds. Water piled up against the southwestern corner of the bay causing net oute ow in the deep, southern area and downwind e ow over the shallow areas. Northwesterly winds greater than 12 m/s caused the most efe cient e ushing of the bay, driving water out over the entire mouth of the estuary.

Vessel-towed acoustic Doppler current profilers (ADCPs) have been widely used to measure velocity profiles. Since the instrument is usually mounted on a catamaran floating on the surface, previous studies have used the water surface as the reference level from which the vertical coordinate for the velocity profile is defined. However, because of the tidal oscillation, the vertical coordinate thus defined is time-dependent in an Earth-coordinate system, which introduces an error to the estimated harmonic constants for the velocity. As a result, the total transport will also be in error. This is particularly a problem in shallow waters where the tidal elevation is relatively large. Therefore tidal elevation needs to be resolved to make a correct harmonic analysis for the velocity. The present study is aimed at resolving the tidal elevation change in shallow water using a vessel-towed ADCP. Semidiurnal and diurnal tidal elevations across the lower Chesapeake Bay have been determined using a vessel-towed ADCP. Data from four cruises ranging from 25 to 92 hours in 1996 and 1997 are used. Water depth averaged every 30 s by the ADCP is studied by harmonic and statistical analysis. By selecting only the data within a narrow band (∼320 m) over the planned transect, we are able to improve the reliability of the data. We then grid the depth data along the 16 km transect into 200 equal segments and use harmonic analysis to resolve the semidiurnal and diurnal tidal variations within each segment. We find that (1) the depth data from the ADCP contain both semidiurnal and diurnal signals that can be resolved, from which the surface elevation can be inferred, (2) the major error appears to come from spatial variation of the depth, (3) the semidiurnal and diurnal tidal variations of elevation inferred over flat bottom topography account for almost 100% of the total variability, while those measurements over large bottom slopes account for a much lower percentage of the total variability, (4) at least 70% of the variability of depth can be explained by semidiurnal and diurnal tides if the bottom slope is smaller than 0.006, and (5) the spatial variation of both amplitude and phase of the elevation along the transect appears to be small with a slightly lower tidal amplitude at the south of the Chesapeake Bay entrance, consistent with the Coriolis effect. The inferred elevations from the ADCP readings are consistent with sea level measurements at a tide station 10 km inside the estuary.

Velocity profiles and time-series data were combined with conductivity‐temperature‐depth (CTD) casts to describe the general circulation at tidal and subtidal scales in a bay of semi-arid climate, Coquimbo Bay (308S), Chile. This was the first study that used a towed acoustic Doppler current profiler (ADCP) in coastal Chilean waters and is one of the very few in semiarid bays. The ADCP was towed for two semi-diurnal tidal cycles in early austral autumn, between March 23 and 24, 1997 along a triangular trajectory that covered most of the bay. Additional data consisted of moored current meters and CTD casts. The observations indicated the presence of a surface layer, above the pycnocline, that showed predominantly diurnal variability forced by the breeze regime and by tides. The tidal circulation in the surface layer featured amplitudes of 10cm/s within an anticyclonic gyre that occupied most of the bay. The subtidal circulation in the surface was characterized by a pair of counter-rotating gyres. The northernmost three-fourths of the bay showed an anticyclonic gyre, and the observations over the southern fourth implied a cyclonic gyre. The subtidal anticyclonic gyre had a counterpart rotating in opposite direction within a lower layer, underneath the pycnocline. The lower layer showed semidiurnal variability in addition to diurnal variability and was insulated by the pycnocline from heat and momentum fluxes through the air‐water interface. Circulations that resemble estuarine and anti-estuarine patterns were found associated with the subtidal gyres. A horizontal divergence related to a 10cm/s near-surface outflow around Point Tortuga, to the south of the bay entrance, allowed the development of upward motion o the Point, as evidenced by the tilt of the isopycnals at the entrance to the bay. # 2000 Elsevier Science Ltd. All rights reserved.

A set of velocity profiles obtained in the James River estuary with an acoustic Doppler current profiler was used in combination with the results of an analytic tidal model to depict the appearance of surface lateral flow convergences (∂v/∂y) during both flood and ebb stages of the tidal cycle. The bathymetry of the estuary was characterized by a main channel and a secondary channel separated by relatively narrow shoals. Lateral surface flow convergences appeared over the edges of the channels and were produced by the phase lag of the flow in the channel relative to the shoals. Flood convergences developed in the late tidal stages and ebb convergences appeared soon after maximum currents. Most of these convergences caused fronts in the density field and flotsam lines that also appeared over the edges of the channel and that lasted

Current velocity and water density profiles were obtained along two cross-estuary transects with the purpose of determining the fortnightly variability of the transverse dynamics in a partially stratified coastal plain estuary. The profiles were measured with a towed acoustic Doppler current profiler and a conductivity-temperature-depth recorder in the James River estuary, Virginia. The cross-estuary transects were sampled during the spring tides of October 26–27, 1996, and the ensuing neap tides of November 2–3, 1996. The transects were ∼4 km long, featured a bathymetry that consisted of a channel flanked by shoals, and were sampled repeatedly during two semidiurnal tidal cycles (25 hours) in order to separate semidiurnal, diurnal, and subtidal signals from the observations. This work concentrates on the subtidal transverse dynamics. The transverse baroclinic pressure gradients were larger during neap tides than during spring tides. During spring tides the advective accelerations were predominantly greater than the Coriolis accelerations, most markedly over the edges of the channel. Both effects combined with frictional influences to balance the pressure gradient in the transverse direction. During neap tides, advective accelerations were not as dominant over Coriolis accelerations as during spring tides. Also, during neap tides, Coriolis played a more relevant role, compared to spring tides, in combining with friction to balance the pressure gradient. This behavior was indicative of the momentum balance approaching gravitational circulation modified by the Earth's rotation, weak friction, and nonlinear advection during neap tides. The balance became more influenced by nonlinear advection and friction and less influenced by the Earth's rotation during spring tides. These results showed that transverse dynamics of a partially stratified estuary are far from being in geostrophic balance.

An innovative method is introduced to solve a two-dimensional, depth-averaged analytic model for narrow estuaries or tidal channels with arbitrary lateral depth variations. The solution is valid if the lateral variation of the amplitude of tidal elevation (|Δa|) is small, i.e., |Δa| ≪ a, where a is the amplitude of the tidal elevation. This assumption is supported by a 60-day observation of elevation in the James River Estuary using pressure sensors at both sides of a cross section of the estuary. The error introduced by the solution is of the order of |Δa|/a, which has a maximum of ∼5% in the James River Estuary. The propagation of the tidal wave (elevation) is therefore essentially one-dimensional (along the estuary), regardless of the depth distribution, whereas tidal velocity has a strong transverse shear and is three-dimensional in general. Dozens of depth functions in six groups of various forms are used to calculate the solution. The tidal velocity is highly correlated with the bathymetry. The largest amplitude of the along-channel velocity is in the deepest water. The phase of the along-channel velocity in the shallow water leads that in deep water, causing a delay in time of flood or ebb in the deep water. The transverse velocity is generally small in the middle of a channel but reaches its maximum over the edges of bottom slopes. The depth function has a significant effect on the ellipticity and the sense of rotation of the tidal ellipses. By fitting the observed phase of semidiurnal tide in the James River Estuary to the phase of the momentum equation, we have obtained optimal values of the drag coefficient: 1.5 × 10−3 and 1.8 × 10−3 for the spring and neap tides, respectively. Then we apply these values of the drag coefficient and the model to the James River Estuary using the real bathymetry. Results show remarkable agreement between the observations and the model along the transects for both spring and neap tides. The cross-channel phase difference of the along-channel velocity between the channel and the shoal is found to be ∼1 hour, a value consistent with that from the model. The model-estimated lateral variation of elevation is 2.5% of the tidal amplitude, which is slightly smaller than the observed value.

Density data from a lower Chesapeake Bay transect obtained after two northeasterly wind events were used to describe the effects of these events on the density field in the lower estuary. The first northeaster occurred in early August 1995 and the second northeaster was related to the passage of hurricane Felix off the lower bay in mid-August 1995. The latter northeaster prolonged a period of persistent winds from the N and NE that began in early August and caused storm surges of similar magnitude to the former northeaster. The salinity fields observed after the early August event suggested encroachment of coastal waters into the lower bay as reflected by high salinities throughout the transect. Two days after weakening of the winds related to Felix, the density distribution across the lower bay showed strongly stratified conditions. This behavior suggested that the inflow of coastal water into the lower bay and the wind mixing related to Felix combined to produce a vertically uniform density gradient perpendicular to the bay entrance that relaxed after the winds weakened. This weakening of the winds coincided with neap tidal currents, which were not energetic enough to maintain vertical homogeneity and must have allowed the self-adjustment of the density gradient and the seaward advection of relatively buoyant waters near the surface. These mechanisms were illustrated with simplified numerical experiments. The findings of this study are used to propose the hypothesis that, in general, enhanced stratification and flushing in the lower bay will ensue the relaxation of a northeasterly wind event, provided that this relaxation coincides with a weak friction regime, i.e., neap tides.

The spatial and temporal variability of water entering and leaving the Chesapeake Bay estuary was determined with a spatial resolution of 75 m. The four cruises during which the observations were made took place under different conditions of freshwater discharge, tidal phase, and wind forcing. The tidal variability of the flows was dominated by the semidiurnal constituents that displayed greatest amplitudes and phase lags near the surface and in the channels that lie at the north and south sides of the entrance. The subtidal variability of the flows was classified into two general scenarios. The first scenario occurred during variable or persistently non-southwesterly winds. Under these conditions there was surface outflow and bottom inflow in the two channels, inflow over the shoal between the two channels, and possible anticyclonic gyre formation over the shoal. The flow pattern in the channels was produced by gravitational circulation and wind forcing. Over the shoal it was caused by tidal rectification and wind forcing. The second scenario occurred during persistently southwesterly winds. The anticyclonic gyre over the shoal vanished suggesting that wind forcing dominated the tidal rectification mechanism over the shoal, while gravitational circulation and wind forcing continued to cause the flows in the channels. In both scenarios, most of the volume exchange took place in the channels.