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Atlantic halibut are a “Species of Concern” in US waters and little is known about their movements and stock structure. Recent stock assessments drew attention to the paucity of information for assessing and managing this stock. To investigate movement patterns and stock structure, halibut were tagged off Massachusetts and Maine within US waters of the Gulf of Maine region using pop-up satellite archival tags and data storage tags. A hidden Markov model (HMM) geolocation method previously developed for other groundfish species was adapted to estimate the movement tracks of the tagged halibut (n = 25) based on the tag-recorded depth and temperature. Total distance travelled based on geolocation ranged from 36 to 1701 km, whereas straight line distance between tagging and end locations ranged from 0.4 to 440.7 km. Estimated movement rates varied between 2.7 and 10 km day−1. Two tagged halibut made long-distance movements to putative spawning habitat in the Northeast Channel off Georges Bank. Thirteen (13) out of 25 geolocated individuals were estimated to have reached Canadian waters. Geolocation results revealed home range, return movement, and seasonal migration movement patterns exhibited by the tagged halibut. The HMM geolocation method could be a useful tool in providing information on halibut movements that can inform stock assessment and management decisions.

Geolocation is increasingly employed to reconstruct the movements of demersal fishes using data retrieved from electronic archival tags. However, geolocation methods commonly suffer from limitations such as low horizontal resolution of locations, flawed land boundary treatment, and extensive computation time. We addressed these issues using a state-space approach based on the particle filter (PF), and developed a geolocation package with graphics processing unit (GPU) acceleration. Our method focused on application to demersal fish and utilizes comparison of the tag-recorded depth and temperature to the same variables from an unstructured grid regional oceanographic model. A rigorous boundary treatment scheme was implemented to handle regions with complex coastline geometry. Validation exercises using stationary mooring tags and double-electronic-tagged (archival and acoustic tags) Atlantic cod in the Gulf of Maine resulted in

We employed numerical wave models, GIS, and stable isotope analyses of otolith material to identify interspecific differences in habitat and resource use among juveniles of two sympatric and morphologically indistinct bonefishes, A. goreensis and A. vulpes in littoral zones of The Bahamas. Both species occurred in similar water temperatures; however, A. goreensis juveniles occupied habitats characterized by greater wave-driven flow velocities and closer proximity to coral reefs than A. vulpes. Likewise, A. goreensis was present across a broader range of flow environments and sampling stations than A. vulpes, which was typically confined to sheltered, low-flow habitats. The results of stable isotope analyses were consistent with the species’ relationships with environmental parameters, providing support for differential habitat and/or resource utilization. Otolith δ18O did not differ significantly between species, suggesting they experience comparable thermal regimes. However, δ13C varied substantially, with the otoliths of A. goreensis depleted in 13C relative to A. vulpes by approximately 1‰, potentially signifying a greater reliance on pelagic carbon sources by the former, in agreement with observed distinctions in habitat use. In linear models, otolith δ13C was negatively correlated with ambient flow velocity and positively related to distance from coral reef habitats, and these relationships did not vary across species. After accounting for the effects of these variables, species-specific differences in otolith δ13C remained, indicating that other unknown factors contributed to the observed disparities. Collectively, our findings suggest that niche partitioning between A. goreensis and A. vulpes is likely mediated by their differential abilities to compete across various flow environments, likely as a result of divergent behavioral and/or physiological adaptation.

The coastal waters of Massachusetts, USA encompass tidal phenomena that generate flows of sufficient magnitude for commercially viable power extraction. We examine the tidal power resource of the Massachusetts coastal region with two high-resolution hydrodynamic tidal models: a regional model encompassing the coastal waters of southeastern New England and a local domain model of Cape Cod Canal. Both models have been subject to comprehensive skill assessment using available surface elevation and ADCP measurements. Based on the model results, we identify five high-energy sites (Cape Cod Canal, Muskeget Channel, Quicks Hole, Robinson Hole and Woods Hole) for evaluation of the maximum extractable tidal power. The power extraction at these sites is modeled using linear momentum actuator disk theory applied to a cross-channel array of turbines. Of the sites evaluated, Muskeget Channel has the greatest resource, with an estimated maximum extractable power of 24 MW. The estimated total power available from all five sites is 44 MW. These estimates agree within 21% with predictions from analytical approaches at all sites. Potential applications for the models include: providing developers with an initial assessment of the resource, guiding observation programs for further study of the resource, and facilitating optimization of turbine array design.

Models developed to geolocate individual fish from data recorded by electronic tags often require major modification to be applied to new regions, species, or tag types due to variability in oceanographic conditions, fish behavior, and data resolution. We developed a model for geolocating Atlantic cod (Gadus morhua) off New England that builds upon an existing hidden Markov model (HMM) framework and addresses region- and species-specific challenges. The HMM framework contains a likelihood model that compares tag-recorded environmental data (depth, temperature, tidal characteristics) with those derived from an oceanographic model and a behavior model that constrains the horizontal movement of the fish. Validation experiments were performed on stationary tags, double-electronic-tagged fish (archival and acoustic tags), and simulated tracks. Known data, including fish locations and activity metrics, showed good agreement with those estimated by the modified approach and improvements in performance of the modified method over the original. The modified geolocation approach will be applicable to additional species and regions to obtain valuable movement information that is not typically available for demersal fishes.

Social information obtained from heterospecifics can enhance individual fitness by reducing environmental uncertainty, making it an important driver of mixed-species grouping behavior. Heterospecific groups are well documented among fishes, yet are notably more prevalent among juveniles than more advanced life stages, implying that the adaptive value of joining other species is greater during this developmental period. We propose this phenomenon can be explained by the heightened ecological relevance of heterospecifically produced cues pertaining to predation risk and or resources, as body-size uniformity inherent in early ontogeny yields greater overlap in predator and prey guild membership across juveniles of disparate taxa. To evaluate the putative role of information in shaping juvenile fish assemblages, we employed a joint species distribution model (JSDM), identifying nonrandom relationships among fishes collected in 785 seine hauls within the shallow littoral zones of a subtropical island. After accounting for species-environment relationships, which explained 39% of observed covariation in the abundance of 11 taxa, we detected high rates of positive association (84% of significant correlations) predominantly between mutual foraging guild members, consistent with assemblage patterns predicted to evolve under widespread interspecific information use. Affiliations occurred primarily between species characterized by neutral (i.e., noninteracting) or negative (i.e., predator-prey) relationships in later life stages, supporting the notion that heightened niche overlap due to body size homogeneity acted to increase the pertinence of information among juveniles. Taxa exerted varying degrees of influence on assemblage structure; however Eucinostomus spp., a gregarious generalist with exceptional information-production potential, had an effect several times that of all other species combined, further evidencing the likely role of information in motivating observed relationships. Co-occurrence and qualitative behavioral data inferred from remote underwater video surveys reinforced these conclusions. Collectively, these results suggest that positive interactions linked to information exchange can be among the principal factors organizing juvenile fish assemblages at local scales, highlighting the role of ontogeny in mediating the relevance and exploitation of information across species.

Movement patterns of marine fishes can have considerable impacts on their population dynamics. A thorough understanding of fish movements is therefore required for informing stock identification, stock assessment, and fishery management. This study investigated the seasonal movements and connectivity of a spring-spawning component of Atlantic cod (Gadus morhua) in the western Gulf of Maine. From 2010 through 2013, spawning cod were sampled within an inshore spawning closure and tagged with conventional tags (n = 2368), acoustic transmitters (n = 106), and archival data storage tags (n = 266). Acoustic receivers were deployed on three inshore spawning sites to test for connectivity among sites. Data from archival tags were used to describe seasonal habitat occupancy and movement patterns via geolocation to statistical areas. Tagging data indicated that cod were primarily residential in the western Gulf of Maine, moving inshore to spawn during the spring (April–July), followed by an offshore migration to their feeding grounds for summer and fall. Cod generally inhabited waters from 45 to 175 m, with the deep offshore basins (>150 m) serving as overwintering habitat. Occupied water temperatures ranged from 4.0 to 13.3 °C, with the coldest temperatures experienced from March through July and the warmest temperatures experienced from September through January. Results provided evidence of spawning site fidelity and connectivity among spawning sites, with some fish visiting multiple spawning sites within or between years. The movements observed during and after the spring-spawning season serve as important mechanisms influencing metapopulation dynamics in the Gulf of Maine region, including both fine- and broad-scale population structure. The improved understanding of cod movement patterns will assist fishery managers in developing management plans, including spawning protection measures, and help to address remaining uncertainties with respect to cod population structure in the Gulf of Maine and other regions.

Winter flounder, Pseudopleuronectes americanus, from southern New England and the Gulf of Maine were historically considered to be obligate estuarine spawners. However, recent experiments and observations document that winter flounder in the Gulf of Maine also utilize coastal waters for spawning. An individual-based modeling approach was used to investigate the transport of winter flounder larvae from three hypothesized coastal spawning grounds in the Gulf of Maine. Transport success rates were greatest for larvae released from Ipswich Bay, intermediate for Stellwagen Bank and least successful for those released from Jeffreys Ledge. There was substantial interannual variability in larval transport and geographic patterns of potential connectivity. Furthermore, the date of spawning had an important influence on transport success. Model results suggest that certain coastal spawning grounds used by winter flounder may serve as an important source of larvae to estuaries and nearshore nursery areas. The potential influx of coastal spawned larvae could have implications for the resilience, productivity and gene flow in local populations. Model results provide further support for the conclusion that winter flounder in the Gulf of Maine may not be solely dependent upon estuaries for spawning. Results also suggest that coastal spawning groups should be considered explicitly in the management of winter flounder, and protected under Essential Fish Habitat regulations.

Lake Bardawil is an important hypersaline lake located in Egypt on the coast of the Sini Peninsula adjacent to the Mediterranean Sea. The lake is host to several industries which provide critical contributions to the regional economy including farmed and wild-caught fisheries and salt extraction. The lake also has significant ecological importance, serving as a rest stop and overwintering location for a numerous waterfowl. To date, only limited studies of circulation and water properties have been performed leaving lake managers without the information needed to make strategic decisions needed to mitigate long-term threats to the lake stemming from regional infrastructure projects, expansion of industry, and natural processes such as inlet shoaling. The present chapter presents a numerical model of Lake Bardawil which can be used to study dynamic processes and predict the outcome of management actions. The approach predicts the three-dimensional circulation using an unstructured grid approach which enables resolution of the complex coastline and wide range of spatial scales associated with the lake. In a validation study, the model was found to reproduce the annual variation and magnitude of monthly averaged salinity at ten measurement stations but significantly overpredict salinity at the two stations in the shallow far western section of the lake. The model demonstrates that evaporation, wind forcing, and tidal exchanges all play important roles in lake forcing. The present work represents a critical step toward the longer-term goal of establishing an operational circulation model for Lake Bardawil which can be employed as a tool by managers to assist and accelerate the decision-making process.

The Finite-Volume Community Ocean Model (FVCOM) is configured to evaluate the potential impact of the proposed Muskeget Tidal Energy Project on circulation and sediment transport in the surrounding region. The extraction of tidal kinetic energy from the water column is modeled by augmenting the momentum equations with additional drag terms parameterized using local flow velocities and parameters specific to the installed turbine farm. Model-computed power output compares well with estimates based on velocities derived from a shipboard acoustic Doppler current profiler (ADCP). Total extracted power from the proposed installations during a spring ebb tide represents roughly 9% of the natural power in the deep section of the channel and 30% of the natural tidal dissipation in the turbine installation region. Due to this low level of extraction, turbine installations at the proposed transects result in relatively minor differences in the tidal current magnitude (2.5%), water level (0.8%), sediment flux (0.6%), and bed level (9%). Computations also indicate that the proposed installation generates minimal impacts to the tidal harmonics (3.3% change in amplitude and 1-min delay in phase) and tide-induced depth-averaged residual currents (2.8%). Model-computed extraction at increased levels is associated with greater perturbations to the natural conditions.

Surficial sediment stability was estimated on Georges Bank, in the Great South Channel and on eastern Nantucket Shoals (36,699 km 2 ) by determining where benthic shear stresses derived from an ocean model matched or exceeded the critical shear stress of the observed surficial sediments. The shear stress resulting from M 2 and S 2 semi-diurnal tides was estimated with the Finite-Volume Community Ocean Model. Mixed-sediment critical shear stress levels were calculated for sediment compositions ranging from sand to boulder-dominated using 67,400 underwater video quadrats sampled from 1999 to 2010. Stresses matched or exceeded the sediment critical levels in 16,926 km 2 (46%) of the study area, and were inversely related to water depth ( r 2 =69.1%). In depths >50 m (10,953 km 2 ) all sediments were stable due to weak flow (≤0.4 N m −2 ). In the shallower higher flow areas (>0.4 N m −2 , 25,716 km 2 ) only sediments containing gravel remained stable. The largest stresses occurred on Nantucket Shoals and central and northeastern Georges Bank (≥2 N m −2 ); in these areas only sand with cobbles or sediments dominated by gravel remained stable. Outcrops of these stable sediments were surrounded by highly unstable areas with stresses 2 to 9 times higher than the sediment critical levels. This analysis identifies the locations which likely remain stable even under the high shear stresses typical of Georges Bank, the Great South Channel and eastern Nantucket Shoals. Further, we provide the map products needed to begin investigating the influences of natural sediment disturbance on the spatial and temporal patterns of the benthos including the resilience of stable versus unstable areas to anthropogenic disturbances.

Calanus spp. copepods play a key role in the Arctic pelagic ecosystem. Among four congeneric species of Calanus found in the Arctic Ocean and its marginal seas, two are expatriates in the Arctic ( Calanus finmarchicus and Calanus marshallae ) and two are endemic ( Calanus glacialis and Calanus hyperboreus ). The biogeography of these species likely is controlled by the interactions of their life history traits and physical environment. A mechanistic understanding of these interactions is critical to predicting their future responses to a warming environment. Using a 3-D individual-based model that incorporates temperature-dependent and, for some cases, food-dependent development rates, we show that (1) C. finmarchicus and C. marshallae are unable to penetrate, survive, and colonize the Arctic Ocean under present conditions of temperature, food availability, and length of the growth season, mainly due to insufficient time to reach their diapausing stage and slow transport of the copepods into the Arctic Ocean during the growing season or even during the following winter at the depths the copepods are believed to diapause. (2) For the two endemic species, the model suggests that their capability of diapausing at earlier copepodite stages and utilizing ice-algae as a food source (thus prolonging the growth season length) contribute to the population sustainability in the Arctic Ocean. (3) The inability of C. hyperboreus to attain their first diapause stage in the central Arctic, as demonstrated by the model, suggests that the central Arctic population may be advected from the surrounding shelf regions along with multi-year successive development and diapausing, and/or our current estimation of the growth parameters and the growth season length (based on empirical assessment or literature) needs to be further evaluated. Increasing the length of the growth season or increasing water temperature by 2 °C, and therefore increasing development rates, greatly increased the area of the central Arctic in which the Arctic endemics could reach diapause but had little effect on the regions of successful diapause for the expatriate species.

Tian, R. C., Chen, C., Stokesbury, K. D. E., Rothschild, B. J., Cowles, G. W., Xu, Q., Hu, S., Harris, B. P., and Marino II, M. C. 2009. Dispersal and settlement of sea scallop larvae spawned in the fishery closed areas on Georges Bank. – ICES Journal of Marine Science, 66: 2155–2164. Three fishery closed areas in the Georges Bank (GB) region were implemented in 1994 to protect depleted groundfish stocks for population replenishment. However, the drift and ultimate destination of larvae spawned in the closed areas have not been analysed specifically within the framework of ocean currents. To assess the efficiency of the closed areas as population replenishment sources, we conducted a simulation-based analysis on the dispersal and settlement of sea scallop larvae spawned in the closed areas from 1995 to 2005 using circulation fields computed by the Finite-Volume Coastal Ocean Model, scallop survey data, and a population dynamics model. Closed area I located in the Great Southern Channel (GSC) had a persistently high rate of larval retention (86% on average). For closed area II located on eastern GB, a considerable quantity of larvae was dispersed out of the domain. For the Nantucket Lightship Closed Area located on Nantucket Shoals, larvae consistently drifted away from the region during the 11 years simulated. Our simulation revealed three high-retention regions that are the most suitable for closed-area selection and rotational fishery management in terms of larval supply to the GB–GSC region.

The previous larval-trajectory modeling studies on Georges Bank were assessed through process-oriented Lagrangian-tracking comparison experiments using the high-resolution Gulf of Maine/Georges Bank Finite-Volume Coastal Ocean Model (GOM-FVCOM). The results indicate that in a strong nonlinear system such as Georges Bank, the passive tracer movement is driven by a fully three-dimensional Lagrangian flow field that varies in space and time due to large tidal excursion and steep bottom topography. The particle-tracking methods developed based on the assumption of weak nonlinearity of the flow field are not applicable to Georges Bank. The results of previous larval transport studies driven by circulation fields constructed under the weak-nonlinearity assumption need to be interpreted with caution. In the present work, the influence of model physical setups on sea scallop larval dispersal and settlement on Georges Bank and adjacent shelf regions is examined. Distinct differences in the spatial distribution of the passive larvae predicted by the model under various physical conditions suggest that a fully nonlinear model driven by realistic spatially and temporally varying forcing should be employed for Lagrangian-based studies of fishery population dynamics on Georges Bank.

The dispersion and settlement of sea scallop larvae spawned on Georges Bank (GB) and in the Great Southern Channel (GSC) were explored using an individual-based population dynamics model. The model consisted of 4 pelagic life stages (egg, trochophore, veliger, and pediveliger) and 3 benthic life stages (juvenile, young adult, and adult). It was driven by the 1995 to 2005 hindcast flow field predicted by the Finite Volume Coastal Ocean Model (FVCOM) in the Gulf of Maine (GOM), with spawning stocks specified by field survey data. In 1998, 2001, 2004, and 2005, a large amount of larvae drifted southward along the shelf break to the Middle Atlantic Bight (MAB). The potential for long-distance southward transport of larvae was dependent on the upstream flow conditions on the Nova Scotian Shelf, climate forcing, and the timing and location of spawning on GB. The model also predicts considerable larval exchange between the GB and the GSC subpopulations, with 83% of larvae settled in the GSC being spawned on GB, and 46% of larvae settled on GB being spawned in the GSC on average from 1995 to 2005.

The structured-grid surface wave model SWAN (Simulating Waves Nearshore) has been converted into an unstructured-grid finite-volume version (hereafter referred to as FVCOM-SWAVE) for use in coastal ocean regions with complex irregular geometry. The implementation is made using the Flux-Corrected Transport (FCT) algorithm in frequency space, the implicit Crank–Nicolson method in directional space and options of explicit or implicit second-order upwind finite-volume schemes in geographic space. FVCOM-SWAVE is validated using four idealized benchmark test problems with emphasis on numerical dispersion, wave-current interactions, wave propagation over a varying-bathymetry shallow water region, and the basic wave grow curves. Results demonstrate that in the rectangular geometric domain, the second-order finite-volume method used in FVCOM-SWAVE has the same accuracy as the third-order finite-difference method used in SWAN. FVCOM-SWAVE was then applied to simulate wind-induced surface waves on the US northeast shelf with a central focus in the Gulf of Maine and New England Shelf. Through improved geometric fitting of the complex irregular coastline, FVCOM-SWAVE was able to robustly capture the spatial and temporal variation of surface waves in both deep and shallow regions along the US northeast coast.

Process-oriented studies with the unstructured-grid, three-dimensional Finite-Volume Coastal Ocean Model (FVCOM) of Georges Bank were used to examine the importance of physical processes on the cross-isobath transport of nutrients onto the Bank. Starting from idealized vertical profiles of NO 3 constructed from summertime climatologic fields, the nutrient field was integrated in time using a conservative tracer equation with both homogenous and stratified initial hydrography and both tide and wind forcing. The model results reveal that: a) nutrient fluxes are spatially inhomogeneous, with the greatest nutrient flux generated by tidal pumping into surface waters along the edge of the Bank's northern flank; b) a surface nutrient maximum occurs on the northeast flank as a result of advection along the northern edge and bifurcation of the flow as waters circulate clockwise and spread laterally around the eastern portion of the Bank; c) advection enriches nutrient concentrations downstream and around the Bank, generating a donut-shaped pattern of elevated nutrients; and d) waters on the top of the Bank, especially in the southwest portions, experience the lowest nutrient flux rates. The length of time required to reach a quasi-equilibrium state of nutrient distribution over the Bank is controlled primarily by tidal advection, with cross-frontal fluxes modulated by stratification, surface wind stress, and the initial nutrient concentration in the Gulf of Maine source waters.

A design method for shape optimization in incompressible turbulent viscous flow has been developed and validated for inverse design. The gradient information is determined using a control-theory based algorithm. With such an approach, the cost of computing the gradient is negligible. An additional adjoint system must be solved which requires the cost of a single steady state flow solution. Thus, this method has an enormous advantage over traditional finite-difference based algorithms. The method of artificial compressibility is utilized to solve both the flow and adjoint systems. An algebraic turbulence model is used to compute the eddy viscosity. The method is validated using several inverse wing design test cases. In each case, the program must modify the shape of initial wing such that its pressure distribution matches that of the target wing. Results are shown for the inversion of both finite thickness wings as well as zero thickness wings which can be considered a model of yacht sails.

In this work we will employ an unstructured grid, coupled wave-current-sediment model to study the influence of wave-induced near bottom stresses on the sediment transport and morphological change within the Skagit River delta and Skagit Bay in Western Washington. The resulting coupled wave-current model will resolve the influence of external processes, including tidal forcing, buoyant river discharge, fluvial sediment supply and wind on tidal flat sediment transport. It will be used to evaluate the capabilities of state-of-the-art open source sediment models and to examine dynamic processes influencing net sediment transport over tidal flats and channel networks including convergence fronts; tidal asymmetries; buoyancy forcing; spatial and temporal variations in bed stress; and interactions between channel networks and adjacent tidal flats. Our project is designed to meet the following objectives: couple the existing high-resolution hydro-sediment model of Skagit Bay with a phase-averaged surface wave model, work closely with field measurement programs on the North Fork Flats (S. Elgar and B. Raubenheimer [WHOI]) and South Fork Flats (R. Geyer, P. Traykovski, and D. Ralston [WHOI]) on model-observation comparisons and validation; and use the coupled model to characterize the wave-current regime in Skagit Bay and the spatial distribution of wave-induced bottom shear stresses and their role in the large-scale morphodynamics of the flats. Both of these models were developed with support from the ONR Tidal Flats DRI (N00014-08-1-1115).

[1] The unstructured-grid, Finite-Volume Community Ocean Model (FVCOM) was used to simulate the tides in the Gulf of Maine (GoM) and New England Shelf (NES) for homogeneous and summer stratified conditions. FVCOM captures the near-resonant nature of the semidiurnal tide and energy flux in the GoM and the complex dynamics governing the tide in the NES. Stratification has limited impact on tidal elevation, but can significantly modify the tidal current profile. Internal tides are energetic in the stratified regions over steep bottom topography, but their contribution to the total tidal energy flux is only significant over the northeast flank of Georges Bank. The model suggests that the tidal flushing-induced eddy east of Monomoy Island is the dynamic basis for the locally observed phase lead of the M2 tide. The southward propagating tidal wave east of Cape Cod encounters the northeastward propagating tidal wave from the NES south of Nantucket Island, forming a zone of minimum sea level along a southeast-oriented line from Nantucket Island. These two waves are characterized by linear dynamics in which bottom friction and advection are negligible in the momentum balance, but their superposition leads to a strong nonlinear current interaction and large bottom stress in the zone of lowest sea elevation.

[1] The generation, propagation, and dissipation processes of large-amplitude nonlinear internal waves in Massachusetts Bay during the stratified season were examined using the nonhydrostatic Finite-Volume Coastal Ocean Model (FVCOM-NH). The model reproduced well the characteristics of the high-frequency internal waves observed in Massachusetts Bay in August 1998. The model experiments suggested that internal waves over Stellwagen Bank are generated by the interaction of tidal currents with steep bottom topography through a process of forming a large-density front on the western slope of the bank by the release of an initial density perturbation near ebb-flood transition, nonlinear steepening of the density front into a deep density depression, and disintegrating of the density depression into a wave train. Earth's rotation tends to transfer the cross-bank tidal kinetic energy into the along-bank direction and thus reduces the intensity of the density perturbation at ebb-flood transition and density depression in the flood period. The internal wave packet propagates as a leading edge feature of the internal tidal wave, and the faster propagation speed of the high-frequency internal waves in Massachusetts Bay is caused by Earth's rotation. The model experiments suggested that bottom friction can significantly influence the cross-bank scale of the density perturbation and thus the density depression during wave generation and the dissipation during the wave's shoaling. Inclusion of vertical mixing using the Mellor-Yamada level 2.5 turbulence closure model had only a marginal effect on wave evolution. The model results support the internal wave theory proposed by Lee and Beardsley (1974) but are in disagreement with the lee-wave mechanism proposed by Maxworthy (1979).

[1] The unstructured grid finite volume coastal ocean model (FVCOM) system has been expanded to include nonhydrostatic dynamics. This addition uses the factional step method with both split mode explicit and semi‐implicit schemes. The unstructured grid finite volume method, combined with a correction of the final free surface from its intermediate value with inclusion of nonhydrostatic effects, efficiently reduces numerical damping and thus ensures second‐order accuracy of the solutions with local/global volume conservation. Numerical experiments have been made to fully validate the nonhydrostatic FVCOM, including surface standing and solitary waves in idealized flat‐ and sloping‐ bottomed channels in homogeneous conditions, the density adjustment problem for lock exchange flow in a flat‐bottomed channel, and two‐layer internal solitary wave breaking on a sloping shelf. The model results agree well with the relevant analytical solutions and laboratory data. These validation experiments demonstrate that the nonhydrostatic FVCOM is capable of resolving complex nonhydrostatic dynamics in coastal and estuarine regions.

[1] Twin experiments were made to compare the reduced rank Kalman filter (RRKF), ensemble Kalman filter (EnKF), and ensemble square-root Kalman filter (EnSKF) for coastal ocean problems in three idealized regimes: a flat bottom circular shelf driven by tidal forcing at the open boundary; an linear slope continental shelf with river discharge; and a rectangular estuary with tidal flushing intertidal zones and freshwater discharge. The hydrodynamics model used in this study is the unstructured grid Finite-Volume Coastal Ocean Model (FVCOM). Comparison results show that the success of the data assimilation method depends on sampling location, assimilation methods (univariate or multivariate covariance approaches), and the nature of the dynamical system. In general, for these applications, EnKF and EnSKF work better than RRKF, especially for time-dependent cases with large perturbations. In EnKF and EnSKF, multivariate covariance approaches should be used in assimilation to avoid the appearance of unrealistic numerical oscillations. Because the coastal ocean features multiscale dynamics in time and space, a case-by-case approach should be used to determine the most effective and most reliable data assimilation method for different dynamical systems.

[1] The finite volume coastal ocean model (FVCOM) is configured to study the interannual variability of circulation in the Gulf of Maine (GoM) and Georges Bank. The FVCOM-GoM system incorporates realistic time-dependent surface forcing derived from a high-resolution mesoscale meteorological model (MM5) and assimilation of observed quantities including sea surface temperature and salinity and temperature fields on the open boundary. An evaluation of FVCOM-GoM model skill on the New England shelf is made by comparison of computed fields and data collected during the Coastal Mixing and Optics (CMO) Program (August 1996–June 1997). Model mean currents for the full CMO period compare well in both magnitude and direction in fall and winter but overpredict the westward flow in spring. The direction and ellipticity of the subtidal variability correspond but computed magnitudes are around 20% below observed, partially due to underprediction of the variability by MM5. Response of subtidal currents to wind-forcing shows the model captures the directional dependence, as well as seasonal variability of the lag. Hydrographic results show that FVCOM-GoM resolves the spatial and temporal evolution of the temperature and salinity fields. The model-computed surface salinity field compares well, except in May when there is no indication of the fresh surface layer from the Connecticut River discharge noted in the observations. Analysis of model-computed results indicates that the plume was unable to extend to the mooring location due to the presence of a westward mean model-computed flow during that time that was stronger than observed. Overall FVCOM-GoM captures well the dynamics of the mean and subtidal flow on the New England shelf.

[1] The unstructured-grid Finite-Volume Coastal Ocean Model (FVCOM) is evaluated using three idealized benchmark test problems: the Rossby equatorial soliton, the hydraulic jump, and the three-dimensional barotropic wind-driven basin. These test cases examine the properties of numerical dispersion and damping, the performance of the nonlinear advection scheme for supercritical flow conditions, and the accuracy of the implicit vertical viscosity scheme in barotropic settings, respectively. It is demonstrated that FVCOM provides overall a second-order spatial accuracy for the vertically averaged equations (i.e., external mode), and with increasing grid resolution the model-computed solutions show a fast convergence toward the analytic solutions regardless of the particular triangulation method. Examples are provided to illustrate the ability of FVCOM to facilitate local grid refinement and speed up computation. Comparisons are also made between FVCOM and the structured-grid Regional Ocean Modeling System (ROMS) for these test cases. For the linear problem in a simple rectangular domain, i.e., the winddriven basin case, the performance of the two models is quite similar. For the nonlinear case, such as the Rossby equatorial soliton, the second-order advection scheme used in FVCOM is almost as accurate as the fourth-order advection scheme implemented in ROMS if the horizontal resolution is relatively high. FVCOM has taken advantage of the new development in computational fluid dynamics in resolving flow problems containing discontinuities. One salient feature illustrated by the three-dimensional barotropic winddriven basin case is that FVCOM and ROMS simulations show different responses to the refinement of grid size in the horizontal and in the vertical.

[1] Physical mechanisms for the summertime offshore detachment of the Changjiang Diluted Water (CDW) into the East China Sea are examined using the high-resolution, unstructured-grid, Finite-Volume Coastal Ocean Model (FVCOM). The model results suggest that isolated low salinity water lens detected west of Cheju Island can be formed by (1) a large-scale adjustment of the flow field to the Changjiang discharge and (2) the detachment of anticyclonic eddies as a result of baroclinic instability of the CDW front. Adding the Changjiang discharge intensifies the clockwise vorticity of the subsurface current (originating from the Taiwan Warm Current) flowing along the 50-m isobath and thus drives the low-salinity water in the northern coastal area of the Changjiang mouth offshore over a submerged plateau that extends toward Cheju Island. Given a model horizontal resolution of less than 1.0 km, the CDW front becomes baroclinically unstable and forms a chain of anticyclonic and cyclonic eddies. The offshore detachment of anticyclonic eddies can carry the CDW offshore. This process is enhanced under northward winds as a result of the spatially nonuniform interaction of wind-induced Ekman flow and eddy-generated frontal density currents. Characteristics of the model-predicted eddy field are consistent with previous theoretical studies of baroclinic instability of buoyancy-driven coastal density currents and existing satellite imagery. The plume stability is controlled by the horizontal Ekman number. In the Changjiang, this number is much smaller than the criterion suggested by a theoretical analysis.

Cowles, G.W., 2013. A block-structured adaptive mesh refinement solver for morphodynamic modeling. An adaptive solver for two-dimensional horizontal morphodynamic modeling studies is presented. The method employs an established augmented Riemann solver to compute unsteady, two-dimensional flows over arbitrary topography. An Exner equation is used to model the evolution of the bed using several common load formulas. The morphodynamic coupling is stabilized using an upwind approach. The scheme is implemented in a highly scalable block-structured adaptive mesh refinement framework. The resulting solver is well balanced and resolves the dry state while maintaining nonnegative depth. Idealized tests include comparison with exact Riemann solutions for dam break problems and an analytical solution for the migration of a one-dimensional sinusoidal bedform. Practical tests include the migration of a Gaussian perturbation and the evolution of flood and ebb shoals in a tidal inlet. The solver is benchmarked...