Recent research in estuaries challenges the long-standing paradigm of the gravitationally driven estuarine circulation. In estuaries with relatively strong tidal forcing and modest buoyancy forcing, the tidal variation in stratification leads to a tidal straining circulation driven by tidal variation in vertical mixing, with a magnitude that may significantly exceed the gravitational circulation. For weakly stratified estuaries, vertical and lateral advection are also important contributors to the tidally driven residual circulation. The apparent contradiction with the conventional paradigm is resolved when the estuarine parameter space is mapped with respect to a mixing parameter M that is based on the ratio of the tidal timescale to the vertical mixing timescale. Estuaries with high M values exhibit strong tidal nonlinearity, and those with small M values show conventional estuarine dynamics. Estuaries with intermediate mixing rates show marked transitions between these regimes at timescales of the spring-neap cycle.

[1] Numerical simulations of the Hudson River estuary using a terrain-following, threedimensional model (Regional Ocean Modeling System (ROMS)) are compared with an extensive set of time series and spatially resolved measurements over a 43 day period with large variations in tidal forcing and river discharge. The model is particularly effective at reproducing the observed temporal variations in both the salinity and current structure, including tidal, spring neap, and river discharge-induced variability. Large observed variations in stratification between neap and spring tides are captured qualitatively and quantitatively by the model. The observed structure and variations of the longitudinal salinity gradient are also well reproduced. The most notable discrepancy between the model and the data is in the vertical salinity structure. While the surface-to-bottom salinity difference is well reproduced, the stratification in the model tends to extend all the way to the water surface, whereas the observations indicate a distinct pycnocline and a surface mixed layer. Because the southern boundary condition is located near the mouth the estuary, the salinity within the domain is particularly sensitive to the specification of salinity at the boundary. A boundary condition for the horizontal salinity gradient, based on the local value of salinity, is developed to incorporate physical processes beyond the open boundary not resolved by the model. Model results are sensitive to the specification of the bottom roughness length and vertical stability functions, insofar as they influence the intensity of vertical mixing. The results only varied slightly between different turbulence closure methods of k-e, k-w, and k-kl.

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Abstract. The response of a surface-trapped river plume to an upwelling favorable wind is studied using a three-dimensional model in a simple, rectangular domain. Model simulations demonstrate that the plume thins and is advected offshore by the cross-shore Ekman transport. The thinned plume is susceptible to significant mixing because of the vertically sheared horizontal currents. The Ekman dynamics and shear-induced mixing cause the plume to evolve to a quasi-steady uniform thickness, which can be estimated by a critical Richardson number criterion. Although the mixing rate decreases slowly in time, mixing continues under a sustained upwelling wind until the plume is destroyed. Mixing persists at the seaward plume front because of an Ekman straining mechanism in which there is a balance between the advection of cross-shore salinity gradients and vertical mixing. The plume mixing rate observed is similar to the mixing law obtained by previous studies of one-dimensional mixing, although the river plume mixing is essentially twodimensional. IntroductionIt has long been recognized that local winds play an important role in the dynamics of river plumes. A theoretical study by Csanady [1978] Although the basic tendency for the plume to spread offshore during upwelling winds has been observed in the aforementioned studies, none of these studies quantifies the plume motions in response to upwelling winds nor determines whether or not the Ekman physics are the only important part of the dynamical balance. Fong et al. [1997] provide one of the first quantitative tests of the plume response to alongshore winds based on observations of the western Gulf of Maine plume. They find that the motions at the seaward front of the plume are approximately described by an Ekman-dominated alongshore momentum balance. It is likely that the Ekman physics is important for the entire plume behavior, and one might expect the Ekman response to place strong constraints on how the structure of the plume is modified during an up- welling favorable wind event. The previous studies suggest that one consequence of upwelling winds is to thin the plume. However, the details of this thinning process have not been quantified or described in any detail.The wind-induced mixing of a river plume has received little attention in previous studies. Masse and Murthy [1992] observe the spreading of the thermally driven Niagara River plume to behave qualitatively consistently with the Ekman response. They suggest that the secondary effect of winds is to mix the plume and ambient waters. They argue that strong upwelling winds will enhance plume mixing by blowing the plume offshore and weakening the vertical density gradients. The enhanced shears induced by the thinning plume may make the plume more susceptible to shear-induced turbulent mixing. Souza and Simpson [1997] also note that winds may be important in driving mixing in a plume but do not describe the mechanism by which it would be accomplished. There remains the questions of how the thinning and sp...

Eddies with length scales of 1–10 km are commonly observed in coastal waters and play an important role in the dispersion of water‐borne materials. The generation and evolution of these eddies by oscillatory tidal flow around coastal headlands is investigated with analytical and numerical models. Using shallow water depth‐averaged vorticity dynamics, eddies are shown to form when flow separation occurs near the tip of the headland, causing intense vorticity generated along the headland to be injected into the interior. An analytic boundary layer model demonstrates that flow separation occurs when the pressure gradient along the boundary switches from favoring (accelerating) to adverse (decelerating), and its occurrence depends principally on three parameters: the aspect ratio [b/a], where b and a are characteristic width and length scales of the headland; [H/CDa], where H is the water depth, CD is the depth‐averaged drag coefficient; and [Uo/σa], where Uo and σ are the magnitude and frequency of the far‐field tidal flow. Simulations with a depth‐averaged numerical model show a wide range of responses to changes in these parameters, including cases where no separation occurs, cases where only one eddy exists at a given time, and cases where bottom friction is weak enough that eddies produced during successive tidal cycles coexist, interacting strongly with each other. These simulations also demonstrate that in unsteady flow, a strong start‐up vortex forms after the flow separates, leading to a much more intense patch of vorticity and stronger recirculation than found in steady flow.

The dynamics of lateral circulation in an idealized, straight estuary under varying stratification conditions is investigated using a three-dimensional, hydrostatic, primitive equation model in order to determine the importance of lateral circulation to the momentum budget within the estuary. For all model runs, lateral circulation is about 4 times as strong during flood tides as during ebbs. This flood-ebb asymmetry is due to a feedback between the lateral circulation and the along-channel tidal currents, as well as to time-varying stratification over a tidal cycle. As the stratification is increased, the lateral circulation is significantly reduced because of the adverse pressure gradient set up by isopycnals being tilted by the lateral flow itself. When rotation is included, a timedependent, cross-channel Ekman circulation is driven, and the tidally averaged, bottom lateral circulation is enhanced toward the right bank (when looking toward the ocean in the Northern Hemisphere). This asymmetry in the tidally averaged bottom lateral circulation may lead to asymmetric sediment transport, leading to asymmetric channel profiles in straight estuaries. For the weakly stratified model run, advection due to lateral currents is a dominant term in both the along-channel and cross-channel momentum equations over a tidal cycle and for the tidally averaged momentum equations. In the tidally averaged, along-channel momentum equation, lateral advection acts as a driving term for the estuarine exchange flow and can be larger than the along-channel pressure gradient. Therefore, it should not be ignored when estimating momentum budgets in estuaries.

The subtidal salt balance and the mechanisms driving the downgradient salt flux in the Hudson River estuary are investigated using measurements from a cross-channel mooring array of current meters, temperature and conductivity sensors, and cross-channel and along-estuary shipboard surveys obtained during the spring of 2002. Steady (subtidal) vertical shear dispersion, resulting from the estuarine exchange flow, was the dominant mechanism driving the downgradient salt flux, and varied by over an order of magnitude over the spring-neap cycle, with maximum values during neap tides and minimum values during spring tides. Corresponding longitudinal dispersion rates were as big as 2500 m 2 s Ϫ1 during neap tides. The salinity intrusion was not in a steady balance during the study period. During spring tides, the oceanward advective salt flux resulting from the net outflow balanced the time rate of change of salt content landward of the study site, and salt was flushed out of the estuary. During neap tides, the landward steady shear dispersion salt flux exceeded the oceanward advective salt flux, and salt entered the estuary. Factor-of-4 variations in the salt content occurred at the spring-neap time scale and at the time scale of variations in the net outflow. On average, the salt flux resulting from tidal correlations between currents and salinity (tidal oscillatory salt flux) was an order of magnitude smaller than that resulting from steady shear dispersion. During neap tides, this flux was minimal (or slightly countergradient) and was due to correlations between tidal currents and vertical excursions of the halocline. During spring tides, the tidal oscillatory salt flux was driven primarily by oscillatory shear dispersion, with an associated longitudinal dispersion rate of about 130 m 2 s Ϫ1.