A method for calculating subtidal estuarine exchange flow using an isohaline framework is described, and the results are compared with those of the more commonly used Eulerian method of salt flux ...decomposition. Concepts are explored using a realistic numerical simulation of the Columbia River estuary. The isohaline method is found to be advantageous because it intrinsically highlights the salinity classes in which subtidal volume flux occurs. The resulting expressions give rise to an exact formulation of the time-dependent Knudsen relation and may be used in calculation of the saltwater residence time. The volume flux of the landward transport, which can be calculated precisely using the isohaline framework, is of particular importance for problems in which the saltwater residence time is critical.
The shelf sources of estuarine inflow Brasseale, Elizabeth; MacCready, Parker
Journal of physical oceanography,
07/2021, Letnik:
51, Številka:
7
Journal Article
Recenzirano
Odprti dostop
Abstract
The inflow to an estuary originates on the shelf. It flushes the estuary and can bring in nutrients, heat, salt, and hypoxic water, having consequences for estuarine ecosystems and fjordic ...glacial melt. However, the source of estuarine inflow has only been explored in simple models that do not resolve interactions between inflow and outflow outside of the estuarine channel. This study addressed the estuary inflow problem using variations on a three-dimensional primitive equation model of an idealized estuarine channel next to a sloping, unstratified shelf with mixing provided by a single frequency, 12-hour tide. Inflow was identified using particle tracking, momentum budgets, and Total Exchange Flow. Inflow sources were found in shelf water downstream of the estuary, river plume water, and shelf water upstream of the estuary. Downstream is defined here with respect to the direction of coastal trapped wave propagation, which is to the right for an observer looking seaward from the estuary mouth in the northern hemisphere. Downstream of the estuary and offshore of the plume, the dynamics were quasi-geostrophic, consistent with previous simple models. The effect of this inflowing current on the geometry of the river plume front was found to be small. Novel sources of inflow were identified which originated from within the plume and upstream of the estuary.
Particle tracking is widely utilized to study transport features in a range of physical, chemical, and biological processes in oceanography. In this study, a new offline particle-tracking package, ...Tracker v1.1, is introduced, and its performance is evaluated in comparison to an online Eulerian dye, one online particle-tracking software package, and three offline particle-tracking software packages in a small, high-resolution model domain and a large coarser model domain. It was found that both particle and dye approaches give similar results across different model resolutions and domains when they were tracking the same water mass, as indicated by similar mean advection pathways and spatial distributions of dye and particles. The flexibility of offline particle tracking and its similarity against online dye and online particle tracking make it a useful tool to complement existing ocean circulation models. The new Tracker was shown to be a reliable particle-tracking package to complement the Regional Ocean Modeling System (ROMS) with the advantages of platform independence and speed improvements, especially in large model domains achieved by the nearest-neighbor search algorithm. Lastly, trade-offs of computational efficiency, modifiability, and ease of use that can influence the choice of which package to use are explored. The main value of the present study is that the different particle and dye tracking codes were all run on the same model output or within the model that generated the output. This allows some measure of intercomparison between the different tracking schemes, and we conclude that all choices that make each tracking package unique do not necessarily lead to very different results.
Abstract
The well-known Knudsen relations and the total exchange flow (TEF) analysis framework provide quantifications of exchange flow across an open boundary to the adjacent ocean in terms of bulk ...values (Knudsen theory: inflow and outflow volume or salinity) or with resolution in salinity space (TEF: profiles of volume and salt flux in salinity coordinates). In the present study, these theories are extended toward mixing of salinity, defined as the decay of salinity variance due to turbulent mixing. In addition to the advective fluxes, diffusive fluxes across the boundary are also considered now. These new Knudsen and TEF relations for mixing are derived by applying Gauss’s theorem to the salinity square and salinity variance equations. As a result of the analysis, four different Knudsen relations for the mixing in estuaries are derived. The first one is exact and considers nonperiodicity as well as nonconstancy of the inflow and outflow salinities. The other three formulations are approximate only, in the sense that either nonperiodicity or nonconstancy or both are relaxed. The simplest of those formulations has recently been derived by MacCready et al. and estimates the estuarine mixing as the product of inflow salinity, outflow salinity, and time-averaged river runoff. These four mixing estimates are systematically assessed by means of a number of idealized estuarine test cases. For periodic tidal flow, the simplest estimate still predicts the effective (physical plus numerical) mixing within an error of about 10%.
Abstract
The relationship between net mixing and the estuarine exchange flow may be quantified using a salinity variance budget. Here “mixing” is defined as the rate of destruction of ...volume-integrated salinity variance, and the exchange flow is quantified using the total exchange flow. These concepts are explored using an idealized 3D model estuary. It is shown that in steady state (e.g., averaging over the spring–neap cycle) the volume-integrated mixing is approximately given by Mixing ≅
S
in
S
out
Q
r
, where
S
in
and
S
out
are the representative salinities of in- and outflowing layers at the mouth and
Q
r
is the river volume flux. This relationship provides an extension of the familiar Knudsen relation, in which the exchange flow is diagnosed based on knowledge of these same three quantities, quantitatively linking mixing to the exchange flow.
Abstract
Subtidal adjustment of estuarine salinity and circulation to changing river flow or tidal mixing is explored using a simplified numerical model. The model employs tidally averaged, ...width-averaged physics, following Hansen and Rattray, extended to include 1) time dependence, 2) tidally averaged mixing parameterizations, and 3) arbitrary variation of channel depth and width. By linearizing the volume-integrated salt budget, the time-dependent system may be distilled to a first-order, forced, damped, ordinary differential equation. From this equation, analytical expressions for the adjustment time and sensitivity of the length of the salt intrusion are developed. For estuaries in which the up-estuary salt flux is dominated by vertically segregated gravitational circulation, this adjustment time is predicted to be TADJ = (1/6)L/u, where L is the length of the salt intrusion and u is the section-averaged velocity (i.e., that due to the river flow). The importance of the adjustment time becomes apparent when considering forcing time scales. Seasonal river-flow variation is much slower than typical adjustment times in systems such as the Hudson River estuary, and thus the response may be quasi steady. Spring–neap mixing variation, in contrast, has a period comparable to typical adjustment times, and so unsteady effects are more important. In this case, the stratification may change greatly while the salt intrusion is relatively unperturbed.
Advances in estuarine physics MacCready, Parker; Geyer, W Rockwell
Annual review of marine science,
01/2010, Letnik:
2
Journal Article
Recenzirano
Recent advances in our understanding of estuarine circulation and salinity structure are reviewed. We focus on well- and partially mixed systems that are long relative to the tidal excursion. ...Dynamics of the coupled system of width- and tidally averaged momentum and salt equations are now better understood owing to the development of simple numerical solution techniques. These have led to a greater appreciation of the key role played by the time dependency of the length of the salt intrusion. Improved realism in simplified tidally averaged physics has been driven by simultaneous advances in our understanding of the detailed dynamics within the tidal cycle and across irregular channel cross-sections. The complex interactions of turbulence, stratification, and advection are now understood well enough to motivate a new generation of physically plausible mixing parameterizations for the tidally averaged equations.
Abstract
The linkage among total exchange flow, entrainment, and diffusive salt flux in estuaries is derived analytically using salinity coordinates, revealing the simple but important relationship ...between total exchange flow and mixing. Mixing is defined and quantified in this paper as the dissipation of salinity variance. The method uses the conservation of volume and salt to quantify and distinguish the diahaline transport of volume (i.e., entrainment) and diahaline diffusive salt flux. A numerical model of the Hudson estuary is used as an example of the application of the method in a realistic estuary with a persistent but temporally variable exchange flow. A notable finding of this analysis is that the total exchange flow and diahaline salt flux are out of phase with respect to the spring–neap cycle. Total exchange flow reaches its maximum near minimum neap tide, but diahaline salt transport reaches its maximum during the maximum spring tide. This phase shift explains the strong temporal variation of stratification and estuarine salt content through the spring–neap cycle. In addition to quantifying temporal variation, the method reveals the spatial variation of total exchange flow, entrainment, and diffusive salt flux through the estuary. For instance, the analysis of the Hudson estuary indicates that diffusive salt flux is intensified in the wider cross sections. The method also provides a simple means of quantifying numerical mixing in ocean models because it provides an estimate of the total dissipation of salinity variance, which is the sum of mixing due to the turbulence closure and numerical mixing.
Abstract
A method is presented for calculating a complete, numerically closed, mechanical energy budget in a realistic simulation of circulation in a coastal–estuarine domain. The budget is ...formulated in terms of the “local” available potential energy (APE; Holliday and McIntyre 1981). The APE may be split up into two parts based on whether a water parcel has been displaced up or down relative to its rest depth. This decomposition clearly shows the different APE signatures of coastal upwelling (particles displaced up by wind) and the estuary (particles displaced down by mixing). Because the definition of APE is local in almost the same sense that kinetic energy is, this study may form meaningful integrals of reservoir and budget terms even over regions that have open boundaries. However, the choice of volume to use for calculation of the rest state is not unique and may influence the results. Complete volume-integrated energy budgets over shelf and estuary volumes in a realistic model of the northeast Pacific and Salish Sea give a new way to quantify the state of these systems and the physical forces that influence that state. On the continental shelf, upwelling may be quantified using APE, which is found to have order-one seasonal variation with an increase due to winds and decrease due to mixing. In the Salish Sea estuarine system, the APE has much less seasonal variation, and the magnitude of the most important forcing terms would take over 7 months to fully drain this energy.
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