An official website of the United States government
Abstract The salinity structure in an estuary is controlled by time‐dependent mixing processes. However, the locations and temporal variability of where significant mixing occurs is not well‐understood. Here we utilize a tracer variance approach to demonstrate the spatial and temporal structure of salinity mixing in the Hudson River Estuary. We run a 4‐month hydrodynamic simulation of the tides, currents, and salinity that captures the spring‐neap tidal variability as well as wind‐driven and freshwater flow events. On a spring‐neap time scale, salinity variance dissipation (mixing) occurs predominantly during the transition from neap to spring tides. On a tidal time scale, 60% of the salinity variance dissipation occurs during ebb tides and 40% during flood tides. Spatially, mixing during ebbs occurs primarily where lateral bottom salinity fronts intersect the bed at the transition from the main channel to adjacent shoals. During ebbs, these lateral fronts form seaward of constrictions located at multiple locations along the estuary. During floods, mixing is generated by a shear layer elevated in the water column at the top of the mixed bottom boundary layer, where variations in the along channel density gradients locally enhance the baroclinic pressure gradient leading to stronger vertical shear and more mixing. For both ebb and flood, the mixing occurs at the location of overlap of strong vertical stratification and eddy diffusivity, not at the maximum of either of those quantities. This understanding lends a new insight to the spatial and time dependence of the estuarine salinity structure.
more »
« less
Mixing in a Salinity Variance Budget of the Salish Sea is Controlled by River Flow
Abstract A salinity variance framework is used to study mixing in the Salish Sea, a large fjordal estuary. Output from a realistic numerical model is used to create salinity variance budgets for individual basins within the Salish Sea for 2017–19. The salinity variance budgets are used to quantify the mixing in each basin and estimate the numerical mixing, which is found to contribute about one-third of the total mixing in the model. Whidbey Basin has the most intense mixing, due to its shallow depth and large river flow. Unlike in most other estuarine systems previously studied using the salinity variance method, mixing in the Salish Sea is controlled by the river flow and does not exhibit a pronounced spring–neap cycle. A “mixedness” analysis is used to determine when mixed water is expelled from the estuary. The river flow is correlated with mixed water removal, but the coupling is not as tight as with the mixing. Because the mixing is so highly correlated with the river flow, the long-term average approximation M = Q r s out s in can be used to predict the mixing in the Salish Sea and Puget Sound with good accuracy, even without any temporal averaging. Over a 3-yr average, the mixing in Puget Sound is directly related to the exchange flow salt transport.
more »
« less
- Award ID(s):
- 1736242
- PAR ID:
- 10384533
- Date Published:
- Journal Name:
- Journal of Physical Oceanography
- Volume:
- 52
- Issue:
- 10
- ISSN:
- 0022-3670
- Page Range / eLocation ID:
- 2305 to 2323
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
-
Tide and salinity data collected at minute intervals over multiple semidiurnal tides were used to investigate the source of water (e.g., seawater, river, groundwater and rain) and their relative timing in mixing at the mouth of a river, a tidal creek at mid-estuary and a tidal creek at the shoreline at the head of a tropical mangrove estuary. Our objectives were to document the temporal changes in tide induced water level changes and salinity at each location and to use the relationship between salinity and water level to elucidate the sources of water and the timing of different sources of water in the hydrologic mixing processes. The data trends in tide vs. salinity (T-S) plots for the river mouth revealed mixing with seawater during rising tides and freshwater diluted seawater (brackish) drainage from the mangrove forest during ebb tides. In the mangrove creek at mid-estuary, the data trends in the T-S plots for rising tides initially showed constant salinity, followed by sharp rises in salinity to peak tide caused by seawater intrusion. The salinity decreased precipitously at the start of tidal ebbing due to influx of freshwater (rain) diluted brackish water from the mangrove forest. The data trends in the T-S plots for the tidal creek at the shoreline located at the estuary head showed constant salinity which decreased only near peak rising tide because of river dilution. During tidal ebbing, the salinity further decreased from groundwater influx before increasing to background salinity, which stayed constant to low tide. Establishing T-S patterns for multiple locations in mangrove estuaries over sub-tidal to tidal scales define the expected salinity variations in seawater-freshwater mixing which can be used to (1) establish baseline hydrologic and salinity (hydrochemical) conditions for temporal and spatial assessments and (2) serve to guide short to long-term sampling regimes for scientific studies and estuarine ecosystem management.more » « less
-
The two-way exchange of water and properties such as heat and salinity as well as other suspended material between estuaries and the coastal ocean is important to regulating these marine habitats. This exchange can be challenging to measure. The Total Exchange Flow (TEF) method provides a way to organize the complexity of this exchange into distinct layers based on a given water property. This method has primarily been applied in numerical models that provide high resolution output in space and time. The goal here is to identify the minimum horizontal and vertical sampling resolutions needed to measure TEF depending on estuary type. Results from three realistic hydrodynamic models were investigated. These models included three estuary types: bay (San Diego Bay: data/SDB_*.mat files), salt-wedge (Columbia River: data/CR_*.mat files), and fjord (Salish Sea: data/SJF_*.mat files). The models were sampled using three different mooring strategies, varying the number of mooring locations and sample depths with each method. This repository includes the Matlab code for repeating these sampling methods and TEF calculations using the data from the three estuary models listed above.more » « less
-
Abstract Estuaries in the northern California current system (NCCS) experience seasonally reversing wind stress, which is expected to impact the origin and properties of inflowing ocean water. Wind stress has been shown to affect the source of estuarine inflow by driving alongshelf currents. However, the effects of vertical transport by wind‐driven Ekman dynamics and other shelf and slope currents on inflow are yet to be explored. Variations in inflow to two NCCS estuarine systems, the Salish Sea and the Columbia River estuary, were studied using particle tracking in a hydrodynamic model. Particles were released in a grid extending two degrees of latitude north and south of each estuary every two weeks of 2017 and tracked for sixty days. Inflow was identified as particles that crossed the estuary mouths. Wind stress was compared with initial horizontal and vertical positions and physical properties of shelf inflow particles. Inflow to the Salish Sea came from Vancouver Island and Washington slope water upwelled through canyons during upwelling‐favorable wind stress, and from Washington slope water or Columbia River plume water during downwelling‐favorable wind stress. Inflow to the Columbia River estuary came from Washington shelf bottom water during upwelling‐favorable wind stress and Oregon shelf surface water during downwelling‐favorable wind stress. For both estuaries, upwelling‐favorable wind stress direction was significantly correlated with a denser and deeper shelf inflow source north of the estuary mouth. These results may help predict the source and properties of inflow to estuaries in other regions with known wind or shelf current patterns.more » « less
-
Abstract A realistic numerical model is used to study the circulation and mixing of the Salish Sea, a large, complex estuarine system on the United States and Canadian west coast. The Salish Sea is biologically productive and supports many important fisheries but is threatened by recurrent hypoxia and ocean acidification, so a clear understanding of its circulation patterns and residence times is of value. The estuarine exchange flow is quantified at 39 sections over 3 years (2017–2019) using the Total Exchange Flow method. Vertical mixing in the 37 segments between sections is quantified as opposing vertical transports: the efflux and reflux. Efflux refers to the rate at which deep, landward‐flowing water is mixed up to become part of the shallow, seaward‐flowing layer. Similarly, reflux refers to the rate at which upper layer water is mixed down to form part of the landward inflow. These horizontal and vertical transports are used to create a box model to explore residence times in a number of different sub‐volumes, seasons, and years. Residence times from the box model are generally found to be longer than those based on simpler calculations of flushing time. The longer residence times are partly due to reflux, and partly due to incomplete tracer homogenization in sub‐volumes. The methods presented here are broadly applicable to other estuaries.more » « less
- Free Publicly Accessible Full Text
-
Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1175/JPO-D-21-0227.1
