Abstract. As three-dimensional (3-D) aquatic ecosystem models are used more frequently for operational water quality forecasts and ecological management decisions, it is important to understand the relative strengths and limitations of existing 3-D models of varying spatial resolution and biogeochemical complexity. To this end, 2-year simulations of the Chesapeake Bay from eight hydrodynamic-oxygen models have been statistically compared to each other and to historical monitoring data. Results show that although models have difficulty resolving the variables typically thought to be the main drivers of dissolved oxygen variability (stratification, nutrients, and chlorophyll), all eight models have significant skill in reproducing the mean and seasonal variability of dissolved oxygen. In addition, models with constant net respiration rates independent of nutrient supply and temperature reproduced observed dissolved oxygen concentrations about as well as much more complex, nutrient-dependent biogeochemical models. This finding has significant ramifications for short-term hypoxia forecasts in the Chesapeake Bay, which may be possible with very simple oxygen parameterizations, in contrast to the more complex full biogeochemical models required for scenario-based forecasting. However, models have difficulty simulating correct density and oxygen mixed layer depths, which are important ecologically in terms of habitat compression. Observations indicate a much stronger correlation between the depths of the top of the pycnocline and oxycline than between their maximum vertical gradients, highlighting the importance of the mixing depth in defining the region of aerobic habitat in the Chesapeake Bay when low-oxygen bottom waters are present. Improvement in hypoxia simulations will thus depend more on the ability of models to reproduce the correct mean and variability of the depth of the physically driven surface mixed layer than the precise magnitude of the vertical density gradient.
Inlet wave-current dynamics and interactions are vital to the physical exchanges in a lagoon-inlet-coastal ocean system. A wave-current coupled model was calibrated and validated against observational data, and then applied to investigate the complex dynamics in the Maryland Coastal Bays during Hurricane Irene (2011). With the inclusion of wave-current interactions, skill in simulating the maximum total water surface elevation was improved under hurricane conditions. Major processes of wave-current interactions include the radiation stressinduced setup and current, and water depth variation-induced wave breaking. Wave-induced bottom friction and sea surface roughness are of secondary importance to nearshore dynamics. Further investigations reveal that tidal currents and ocean swells dominate inlet circulation and wave dynamics, respectively. Physical dynamics within the paired inlets are regulated by local winds, wave-current interactions, and unique inlet characteristics. However, wave dynamics in the lagoon and behind inlets are dominated by local winds and modulated by the shallow bathymetry. With the hypothetical closure of any inlet, wave-current dynamics and interactions behind the corresponding inlet are strongly altered, whereas they are weakly influenced from a remote one. Occasionally, the circulation near the narrow Ocean City Inlet area is influenced moderately by artificially shutting down the relatively wider Chincoteague Inlet. The finding from this work on the Maryland Coastal Bays can be beneficial to understanding similar lagoon-inlet-coastal ocean systems elsewhere. Longuet-Higgins and Stewart (1964), who proposed that two-dimensional (2D), depth-averaged wave radiation stress is responsible for generating the wave-induced setup and longshore currents in the surf zone. This proposal was verified by observations in the shallow regions of North Carolina (Lentz et al., 1999; Lentz and Raubenheimer, 1999), Kaneohe Bay, Hawaii (Lowe et al., 2009), and the Red Sea (Lentz et al., 2016). Perrie et al. (2003) determined that the conversion of excessive momentum fluxes from waves to surface currents begins to take effect during intense storms. Recently, Mellor (2005, 2013, 2015) extended the work of Longuet-Higgins and Stewart (1964) and derived vertically dependent equations of radiation stress. Subsequently, the significance of this newly developed three-dimensional (3D) radiation stress was recognized in shallow-water dynamics from idealized numerical experiments (
Interbasin exchange and interannual variability in Lake Erie's three basins are investigated with the help of a three-dimensional unstructured-grid-based Finite Volume Coastal Ocean Model (FVCOM). Experiments were carried out to investigate the influence of grid resolutions and different sources of wind forcing on the lake dynamics. Based on the calibrated model, we investigated the sensitivity of lake dynamics to major external forcing, and seasonal climatological circulation patterns are presented and compared with the observational data and existing model results. It was found that water exchange between the western basin (WB) and the central basin (CB) was mainly driven by hydraulic and density-driven flows, while density-driven flows dominate the interaction between the CB and the eastern basin (EB). River-induced hydraulic flows magnify the eastward water exchange and impede the westward one. Surface wind forcing shifts the pathway of hydraulic flows in the WB, determines the gyre pattern in the CB, contributes to thermal mixing, and magnifies interbasin water exchange during winter. Interannual variability is mainly driven by the differences in atmospheric forcing, and is most prominent in the CB.
The exchange processes between the Maryland Coastal Bays system (MCBs) and their adjacent coastal ocean were simulated using a three-dimensional unstructured-grid based hydrodynamic model, which was validated by observed data including water level, current velocity and salinity. Idealized experiments were then carried out to investigate the impact of wind forcing on water exchange and salt flux. Through these experiments, the exchanges between the MCBs and coastal ocean were investigated at two inlets (Ocean City Inlet and Chincoteague Inlet). Given that winds and tides are two key external forces known to impact estuarine dynamics, the effect of each individual force on the exchange processes was studied to evaluate the corresponding influence on the inlet dynamics. It was found that wind forcing significantly impacts the inlet dynamics: the effect of wind directions on exchange processes under strong wind speeds is substantial; for example, northwesterly winds push flux to the southern part of the bays, while southwesterly winds pile up flux towards northern Chincoteague Bay. The effect of wind forcing on the exchange dynamics becomes stronger with the augmentation of its speed. Meanwhile, tidal forcing is the major driver of exchange dynamics at weak wind speeds (e.g., 3 m/s), and its effect on exchange process gradually weakens with stronger wind speeds (e.g., 7 m/s, 15 m/s). In addition, sensitivity tests elucidated that closing either inlet results in a significant impact on the water elevation, current velocity and salinity nearby the relevant cutoff inlet areas.
High‐turbidity events (HTEs) are common phenomena in shallow‐water environments that can alter ecological interactions. The relative contributions of river input (external loading) vs. resuspension (internal loading) to the occurrence, duration, and influenced areas of HTEs are not fully understood in most systems, owing to the lack of long‐term, source‐specified sediment maps. Using a Finite Volume Community Ocean Model‐based wave‐current forced sediment model, we investigated sediment dynamics in the shallow, river‐dominated Western Lake Erie during ice‐free cycles (April–November) of 2002–2012. Results indicated that wind waves predominated sediment dynamics in the offshore areas, with river discharges causing substantial inshore to offshore gradients. Owing to varying wind waves and river discharges, both the mean and extreme sediment dynamics had distinctive seasonal variations. The basin was turbid during spring and fall, with frequent (> 15%), broad (O [102–103 km2]), and more persistent (means of 3.2/4.4 d during spring/fall) HTEs caused mainly by resuspension events. During summer, the basin was clearer with occasional (< 1%), small (O [1–102 km2]), and short (mean of 1.5 d) HTEs near the mouths generated by pulsing river loadings. Although river loading rarely induced basin‐wide HTEs, they were important during floods, enlarging the high‐turbidity areas by 11.3%. Overall, by delineating the drivers of HTEs in Western Lake Erie, this study furthered the understanding of sediment dynamics in shallow ecosystems and provides a basis for investigating the ecological impact of sediments from different sources in river‐ and wave‐energetic systems.
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