Coastal Ocean Circulation Model Research Articles

Hurricanes enhance coral connectivity but also superspread coral diseases.

Climate change poses an existential threat to coral reefs. A warmer and more acidic ocean weakens coral ecosystems and increases the intensity of hurricanes. The wind-wave-current interactions during a hurricane deeply change the ocean circulation patterns and hence potentially affect the dispersal of coral larvae and coral disease agents. Here, we modeled the impact of major hurricane Irma (September 2017) on coral larval and stony coral tissue loss disease (SCTLD) connectivity in Florida's Coral Reef. We coupled high-resolution coastal ocean circulation and wave models to simulate the dispersal of virtual coral larvae and disease agents between thousands of reefs. While being a brief event, our results suggest the passage of hurricane Irma strongly increased the probability of long-distance exchanges while reducing larval supply. It created new connections that could promote coral resilience but also probably accelerated the spread of SCTLD by about a month. As they become more intense, hurricanes' double-edged effect will become increasingly pronounced, contributing to increased variability in transport patterns and an accelerated rate of change within coral reef ecosystems.

Adjoint Data Assimilation of the Flow on the Southern Flank of Georges Bank: March–June 1999

An adjoint data assimilation method was incorporated into the ECOM-si coastal ocean circulation model and applied to assimilate the flow field on the southern flank of Georges Bank from March to June 1999. The model was driven by tidal forcing consisting of ten tidal constituents at the open boundary, observed winds, and surface heat fluxes. Numerical experiments were conducted following a strategy to understand the critical issues affecting the efficiency and accuracy of the assimilated flow field. The adjoint data assimilation method significantly improved the computational accuracy of subtidal currents, especially for the along-isobath velocity. The integration window length and iteration number were two parameters affecting the assimilation convergence rate toward the observations. In such a nonlinear dynamical system, using a window length close to the M2 tidal period could make the adjoint model difficult to converge, no matter how many iterations were made. Reducing the time interval for the window length speeded up the convergence rate, but it was paid out with the sacrifice of statistical confidence. This assimilation experiment used a 6-h window length, which led to a faster convergence rate in the first ten iterations. The best-assimilated fields that satisfied the error criteria were obtained as the iteration number increased.

Offshore Sea Levels Measured With an Anchored Spar‐Buoy System Using GPS Interferometric Reflectometry

AbstractConventional tide gauges are usually housed along the coast. Satellite altimetry works well in the open ocean but poorly near the coast due to signal contamination by land returns. These limitations lead to an observational gap in the transition zone between the coast and open ocean. Using data collected by a GPS installed on top of an anchored spar‐buoy in Tampa Bay, we retrieved water levels through a combination of precise positioning and interferometric reflectometry. Individual water level retrievals agree with a nearby acoustic tide gauge (19.5 km distance) at ∼15 cm level. Amplitude and phase of the major tidal constituents are well recovered by the GPS spar‐buoy measurements. Over a 2.9‐year period, agreement of de‐tided daily mean sea levels measured by the GPS spar‐buoy and the nearby acoustic tide gauge is 4.4 cm. When sea level data measured by the GPS spar‐buoy are included in the local coastal ocean circulation model, low‐frequency error propagated from the open boundary is significantly reduced.

The impact of surface currents on the wave climate in narrow fjords

This study investigates the effect of surface currents on wind-generated waves in a complex coastal system with narrow fjords. The simulations are based on a phase-averaged wave model forced with surface currents from a high-resolution coastal ocean and fjord circulation model, and high-resolution winds from a nested atmospheric model. Wave simulations with and without ocean forcing are evaluated by comparing integrated wave parameters and modelled spectra with observations from five wave buoys. The comparison covers three winter seasons (2017–2020) and a case study. The wind sea part of the spectrum is better simulated at all locations when using the current forcing. At the most sheltered location, where wind sea dominated the wave climate, the wave height estimates improved by 12 percentage points when including current forcing. Spectral moments and the shape of the average spectra are also improved at most of the locations when current forcing is applied. The effect of wave–current interactions was found to be more pronounced at inner locations where the relative difference of spectral bandwidth is up 5%, the difference in directional spreading is greater than 5 degrees during strong surface currents, and the relative difference in peak frequency is exceeding 10%. Our results are consequential for narrow, deep and sheltered water bodies, but are not expected to carry over to shallow water areas.

Ensemble 4DVAR (En4DVar) data assimilation in a coastal ocean circulation model. Part II: Implementation offshore Oregon–Washington, USA

The ensemble four-dimensional variational (En4DVar) data assimilation (DA) system introduced in Part I (Pasmans and Kurapov, 2019) is tested in the coastal waters offshore Oregon and Washington, U.S. West coast, during the spring and summer of 2011. The background error covariance B is derived from the forecast ensemble. Satellite sea-surface temperature (SST), sea-surface height (SSH), and daily-averaged radial surface currents from high-frequency radars (HFRs) are assimilated. The performance of the En4DVar system is compared with a “traditional” 4DVAR system using a static B. It is found that the presence of the Columbia River plume has a profound impact on the ensemble B. Near the plume front the SST–SSS covariance can be up to a factor 20 larger in magnitude than in the static B. This introduces large spatial and temporal variability in the ensemble B. The En4DVar system is more successful than the 4DVAR with the static B preserving the temperature–salinity properties when compared to glider data. The En4DVar system also produces more accurate forecasts and analyses for temperature in the subsurface below 30 m at a buoy location on the continental shelf. In comparisons with other surface and subsurface observations En4DVar shows consistent, albeit not significant, improvement over traditional 4DVAR. Large surface temperature–salinity covariances in combination with the episodic occurrence of large-scale errors in the SST observations lead to erroneous freshening in the centre of the model domain. Adding constraints on the surface salinity corrections based on the prior model reduces this effect.

On the automatic and a priori design of unstructured mesh resolution for coastal ocean circulation models

This study investigates the design of unstructured mesh resolution and its impact on the modeling of barotropic tides along the United States East Coast and Gulf Coast (ECGC). A discrete representation of a computational ocean domain (mesh design) is necessary due to finite computational resources and an incomplete knowledge of the physical system (e.g., shoreline and seabed topography). The selection of mesh resolution impacts both the numerical truncation error and the approximation of the system’s physical domain. To increase confidence in the design of high-resolution coastal ocean meshes and to quantify the efficacy of current mesh design practices, an automated mesh generation approach is applied to objectively control resolution placement based on a priori information such as shoreline geometry and seabed topographic features. The simulated harmonic tidal elevations for each mesh design are compared to that of a reference solution, computed on a 10.8 million vertex mesh of the ECGC region with a minimum shoreline resolution of 50 m. Our key findings indicate that existing mesh designs that use uniform resolution along the shoreline and slowly varying resolution sizes on the continental shelf inefficiently discretize the computational domain. Instead, a targeted approach that places fine resolution in narrow geometric features, along steep topographic gradients, and along pronounced submerged estuarine channels, while aggressively relaxing resolution elsewhere, leads to a mesh with an order of magnitude fewer vertices than the reference solution with comparable accuracy (within 3% harmonic elevation amplitudes in 99% of the domain).

Ensemble of 4DVARs (En4DVar) data assimilation in a coastal ocean circulation model, Part I: Methodology and ensemble statistics

The ocean state off Oregon-Washington, U.S. West coast, is highly variable in time. Under these conditions the assumption made in traditional 4-dimensional variational data assimilation (4DVAR) that the prior model (background) error covariance is the same in every data assimilation (DA) window can be limiting. A DA system based on an ensemble of 4DVARs (En4DVar) has been developed in which the background error covariance is estimated from an ensemble of model runs and is thus time-varying. This part describes details of the En4DVar method and ensemble statistics verification tests. The control run and 39 ensemble members are forced by perturbed wind fields and corrected by DA in a series of 3-day windows. Wind perturbations are represented as a linear combination of empirical orthogonal functions (EOFs) for the larger scales and Daubechies wavelets for the smaller scales. The variance of the EOF expansion coefficients is based on estimates of the wind field error statistics derived using scatterometer observations and a Bayesian Hierarchical Model. It is found that the variance of the wind errors relative to the natural wind variability increases as the horizontal spatial scales decrease. DA corrections to the control run and ensemble members are calculated in parallel by the newly developed, cost-effective cluster search minimization method. For a realistic coastal ocean application, this method can generate a 30% wall time reduction compared to the restricted B-conjugate gradient (RBCG) method. Ensemble statistics are generally found to be consistent with background error statistics. In particular, ensemble spread is maintained without inflating. However, sea-surface height background errors cannot be fully reproduced by the ensemble perturbations.

OceanMesh2D 1.0: MATLAB-based software for two-dimensional unstructured mesh generation in coastal ocean modeling

Abstract. OceanMesh2D is a set of MATLAB functions with preprocessing and post-processing utilities to generate two-dimensional (2-D) unstructured meshes for coastal ocean circulation models. Mesh resolution is controlled according to a variety of feature-driven geometric and topo-bathymetric functions. Mesh generation is achieved through a force balance algorithm to locate vertices and a number of topological improvement strategies aimed at improving the worst-case triangle quality. The placement of vertices along the mesh boundary is adapted automatically according to the mesh size function, eliminating the need for contour simplification algorithms. The software expresses the mesh design and generation process via an objected-oriented framework that facilitates efficient workflows that are flexible and automatic. This paper illustrates the various capabilities of the software and demonstrates its utility in realistic applications by producing high-quality, multiscale, unstructured meshes.

3D Modal Solution for Tidally Induced Lagrangian Residual Velocity with Variations in Eddy Viscosity and Bathymetry in a Narrow Model Bay

In this study, we examine the results obtained by the Finite-volume Coastal Ocean Circulation Model (FVCOM) regarding the effects of eddy viscosity and bathymetry on the three-dimensional (3D) Lagrangian residual velocity (LRV) in a narrow bay. The results are cast in terms of two nondimensional numbers: the ratio of friction to local acceleration (δ) and the ratio of the minimum depth over shoals to the maximum depth in the channel (e). The ratio δ depends on the eddy viscosity and mean depth. For a given eddy viscosity, when e > 0.5, the along-estuary LRV tends to be vertically sheared and when e < 0.5, the exchange is laterally sheared. When e << 1, the structure of the 3D, depth-integrated, and breadth-averaged LRV changes only slightly as δ increases. For e values between 0.33 and 0.5, the structure of the 3D LRV is mainly laterally sheared. In the same e range, the 3D and depth- integrated LRV exhibit reversed structures from high to low δ values. In addition, the breadth-averaged LRV weakens the typical twolayered circulation when δ decreases. When e is 1, the two-layered vertical structure reverses direction, and a three- layered vertical structure develops in the outer bay as δ decreases.

Why Gliders Appreciate Good Company: Glider Assimilation in the Oregon‐Washington Coastal Ocean 4DVAR System With and Without Surface Observations

AbstractGliders are low‐power autonomous underwater vehicles used to obtain oceanic measurements in vertical sections. Assimilation of glider temperature and salinity into coastal ocean circulation models holds the potential to improve the ocean subsurface structure estimate. In this study, the impact of assimilation of glider observations is studied using a four‐dimensional variational (4DVAR) data assimilation and forecast system set offshore of Oregon and Washington on the U.S. West Coast. Four test cases are compared: (1) no assimilation, (2) assimilation of glider temperature and salinity data alone, (3) assimilation of the glider data in combination with the surface observations including satellite sea surface temperature, sea surface height, and high‐frequency radar surface velocities, and (4) assimilation of the surface data alone. It is found that the assimilation of glider observations alone creates unphysical eddies in the vicinity of the glider transect. As a consequence, the forecast errors in the surface velocity and temperature increase compared to the case without data assimilation. Assimilation of surface and subsurface observations in combination prevents these features from forming and reduces the errors in the forecasts for the subsurface fields compared to the other three experiments. These improvements persisted in 21‐day forecasts run after the last data assimilation cycle.

INTERACTIONS OF WAVES WITH IDEALIZED HIGH-RELIEF BOTTOM ROUGHNESS

Interactions between waves and high-relief bottom roughness were investigated using Large Eddy Simulations of oscillatory flow over an infinite array of regularly spaced hemispheres. Simulation results were analyzed using a spatially- and phase-averaged momentum balance to provide insight into how flow-topography interactions affect wave-driven oscillating flows. Phase-averaging was applied first, and then spatial averaging was applied over volumes with horizontal length scales greater than the size of a single solid obstacle but fine enough in the vertical direction that the vertical structure of the dynamics was resolved. Spatial averaging of the momentum equation results in terms that represent drag and inertial forces, and a dispersive stress term that represents a vertical momentum flux induced by the spatial heterogeneity of the phase-averaged flow. These new terms require parameterization in coastal ocean wave and circulation models.

A Hydraulic Model for Multiple-Bay-Inlet Systems on Barrier Islands

In this study, we apply the traditional hydraulic engineering approach to model an inter-connected multiple-bay-inlet system that can represent the Great South Bay-Moriches Bay system on Long Island, New York. We show that the hydraulic model captures the essential physics of the system, despite its apparent simplicity in mathematical expressions. The model gives good estimates of bay tidal transmissions, including the tidal ranges, phase lags, and the flood-ebb asymmetry behavior in Moriches Bay. The hydraulic modeling results compare well with the simulations from a 3D coastal ocean circulation model, in particular the changes in bay tides due to the breach of Old Inlet by Hurricane Sandy. The modeled inlet discharge rates are in good agreement with the observations.

Towards wide-swath high-resolution mapping of total ocean surface current vectors from space: Airborne proof-of-concept and validation

Two-dimensional high-resolution maps of total surface current vectors obtained for the first time with an airborne demonstrator of the innovative Wavemill instrument concept are validated against HF radar data and compared with output from the POLCOMS high-resolution coastal ocean circulation model. Wavemill is a squinted along-track interferometric SAR system optimized for ocean surface current vector retrieval that operates at moderate incidence angles (∼30°) and is compatible with spaceborne implementation. This paper represents the first comprehensive validation of the current retrieval capabilities of squinted along-track SAR interferometry in support of its development as a future European Space Agency Earth Explorer mission.Wavemill airborne data were acquired in October 2011 in Liverpool Bay off the west coast of Great Britain in light southerly wind (5.5m/s) and maximum tidal ebbing flow (0.7m/s) conditions. Contributions to the measured SAR interferometric phase by surface gravity waves, known as the Wind-wave induced Artefact Surface Velocity (WASV), were removed using our best estimate of wind conditions and the (Mouche et al., 2012) empirical correction derived from Envisat ASAR. Validation of the 1.5km resolution Wavemill current vectors against independent current measurements from HF radar gives very encouraging results, with Wavemill biases and precisions typically better than 0.05m/s and 0.1m/s for surface current speed, and better than 10° and 7° for current direction.The sensitivity of the current retrieval to the wind vector used to compute the WASV is estimated. A ± 1m/s error (bias) in wind speed has minimal impact on the quality of the retrieved currents. In contrast, the choice of wind direction is critical: a bias of ± 15° in the direction of the wind vector degrades the accuracy of the airborne current speed against the HF radar by about ± 0.2m/s. This highlights the need for future instruments to provide calibrated SAR Normalised Radar Cross Section data to support retrieval of wind and current vectors simultaneously.Comparisons of POLCOMS surface currents with HF radar data indicate that the model reproduces well the overall temporal evolution of the tidal current (correlation of spatial fields against HF radar over two tidal cycles of 0.9) but that the model features a systematic 1-h delay in the timing of the maximum ebbing flow in eastern parts of the domain near the Mersey Bar Light buoy. At the maximum ebb flow, the model underestimates the current speed (bias of −0.2m/s) with respect to the HF radar and Wavemill data at the time of the flights. Both the HF radar and Wavemill data reflect much greater snapshot spatial variability of the ocean surface current field than is present in the model, resulting in poor correlation of instantaneous spatial fields (< 0.5) between POLCOMS and the HF radar data. The Wavemill data reveal high spatial variability of ocean surface currents at fine scales, which are not visible in the 4km resolution HF radar data. Wavemill detects several strong (1–1.5m/s) localized current jets associated with deeper bathymetry channels in shallow waters (< 10 m) that are too narrow or too close to land to be observed by the HF radar. The study confirms the value of synoptic wide-swath maps of high-resolution ocean surface current vectors for coastal applications and to validate and develop high-resolution ocean circulation models.

A parallel domain decomposition algorithm for coastal ocean circulation models based on integer linear programming

This paper presents a new parallel domain decomposition algorithm based on integer linear programming (ILP), a mathematical optimization method. To minimize the computation time of coastal ocean circulation models, the ILP decomposition algorithm divides the global domain in local domains with balanced work load according to the number of processors and avoids computations over as many as land grid cells as possible. In addition, it maintains the use of logically rectangular local domains and achieves the exact same results as traditional domain decomposition algorithms (such as Cartesian decomposition). However, the ILP decomposition algorithm may not converge to an exact solution for relatively large domains. To overcome this problem, we developed two ILP decomposition formulations. The first one (complete formulation) has no additional restriction, although it is impractical for large global domains. The second one (feasible) imposes local domains with the same dimensions and looks for the feasibility of such decomposition, which allows much larger global domains. Parallel performance of both ILP formulations is compared to a base Cartesian decomposition by simulating two cases with the newly created parallel version of the Stevens Institute of Technology’s Estuarine and Coastal Ocean Model (sECOM). Simulations with the ILP formulations run always faster than the ones with the base decomposition, and the complete formulation is better than the feasible one when it is applicable. In addition, parallel efficiency with the ILP decomposition may be greater than one.

Calibrating the Loss Coefficient of a Porous Plate

AbstractMarine structures are sometimes represented in coastal ocean circulation models as a porous plate that exerts drag on the flow and dissipates energy. Such a representation requires that a loss coefficient be specified to reduce the flow speed to some prescribed fraction of the upstream incident flow. To provide guidance and a framework for this problem, the case of a uniform channel that is partially blocked by a porous structure is considered. Drawing on existing results from actuator disc theory, simple analytical relations are presented for the dependence of the flow speed through a porous plate on the loss coefficient. It is shown that limitations on representing structures with high hydrodynamic loading using an actuator disc in an unbounded region can be avoided by allowing for weak geometrical blockage by lateral boundaries.

Three-dimensional water quality model based on FVCOM for total load control management in Guan River Estuary, Northern Jiangsu Province

Guan River Estuary and adjacent coastal area (GREC) suffer from serious pollution and eutrophicational problems over the recent years. Thus, reducing the land-based load through the national pollutant total load control program and developing hydrodynamic and water quality models that can simulate the complex circulation and water quality kinetics within the system, including longitudinal and lateral variations in nutrient and COD concentrations, is a matter of urgency. In this study, a three-dimensional, hydrodynamic, water quality model was developed in GREC, Northern Jiangsu Province. The complex three-dimensional hydrodynamics of GREC were modeled using the unstructured-grid, finite-volume, free-surface, primitive equation coastal ocean circulation model (FVCOM). The water quality model was adapted from the mesocosm nutrients dynamic model in the south Yellow Sea and considers eight compartments: dissolved inorganic nitrogen, soluble reactive phosphorus (SRP), phytoplankton, zooplankton, detritus, dissolved organic nitrogen (DON), dissolved organic phosphorus (DOP), and chemical oxygen demand. The hydrodynamic and water quality models were calibrated and confirmed for 2012 and 2013. A comparison of the model simulations with extensive dataset shows that the models accurately simulate the longitudinal distribution of the hydrodynamics and water quality. The model can be used for total load control management to improve water quality in this area.

Numerical study of seasonal circulation and variability over the inner shelf of the northern South China Sea

This study examines seasonal circulation, hydrography, and associated spatial variability over the inner shelf of the northern South China Sea (NSCS) using a nested-grid coastal ocean circulation model. The model external forcing consists of tides, atmospheric forcing, and open boundary conditions based on the global ocean circulation and hydrography reanalysis produced by the Hybrid Coordinate Ocean model. Five numerical experiments are conducted with different combinations of external forcing functions to examine main physical processes affecting the seasonal circulation in the study region. Model results demonstrate that the monthly mean circulation in the study region features the Guangdong Coastal Current (GCC) over coastal waters and the South China Sea Warm Current (SCSWC) in the offshore deep waters. The GCC produced by the model flows nearly southwestward in winter months and northwestward in summer months, which agrees with previous studies. The SCSWC flows roughly northeastward and is well defined in summer months. In winter months, by comparison, the SCSWC is superseded by the southwestward strong wind-driven currents. Analysis of model results in five different experiments demonstrates that the monthly mean circulation over coastal and inner shelf waters of the NSCS can be approximated by barotropic currents forced by the southwestward monsoon winds in winter months. In summer months, by comparison, the monthly mean circulation in the study region is affected significantly by baroclinic dynamics associated with freshwater runoff from the Pearl River and advection of warm and saline waters carried by the SCSWC over the NSCS.

Influences of Channel Deepening and Widening on the Tidal and Nontidal Circulations of Tampa Bay

A high-resolution, three-dimensional coastal ocean circulation model is applied to Tampa Bay to determine the effects that channel deepening and widening may have on the estuary’s tidal and nontidal circulations. The model domain includes all of Tampa Bay, Sarasota Bay, the Intra-coastal Waterway, and the adjoining rivers. With resolution as high as 20 m, the model resolves the main shipping channels, the bridge causeways, and all important inlet mass conveyances. Model evaluations are made against in situ data and a previously published, relatively lower-resolution version. By deepening and widening the shipping channels for the purpose of accommodating larger vessels, our results suggest an increase in tidal range and a decrease in tidal phase from the mouth to the head of the bay. Along with these tidal effects, our results also suggest an increase in the nontidal circulation, resulting in an increase in salinity and attendant consequences to the estuary’s ecology.

On the Flushing of Tampa Bay

A high-resolution, three-dimensional coastal ocean circulation model is used to estimate the bulk residence time distributions for the entirety of Tampa Bay and for five of its sub-regions using an Eulerian passive tracer technique. With a resolution as high as 20 m, the model domain includes all of the important mass conveyances by the channels and inlets, plus the impediments to flow by bridge causeways and natural constrictions. The non-tidal circulation is found to be the primary agent for the flushing of Tampa Bay. Tides alone have a minor effect. Exceptions pertain to within a tidal excursion from the mouth of the bay and sub-regions with multiple inlets, where differential flows related to tidal phase differences aid in flushing. By resolving the main shipping channels, we also address the effect of channel deepening and widening on the flushing of Tampa Bay. Bulk residence time is found to decrease due to an increase in the non-tidal circulation.

X-band COSMO-SkyMed wind field retrieval, with application to coastal circulation modeling

Abstract. In this paper, X-band COSMO-SkyMed© synthetic aperture radar (SAR) wind field retrieval is investigated, and the obtained data are used to force a coastal ocean circulation model. The SAR data set consists of 60 X-band Level 1B Multi-Look Ground Detected ScanSAR Huge Region COSMO-SkyMed© SAR data, gathered in the southern Tyrrhenian Sea during the summer and winter seasons of 2010. The SAR-based wind vector field estimation is accomplished by resolving both the SAR-based wind speed and wind direction retrieval problems independently. The sea surface wind speed is retrieved by means of a SAR wind speed algorithm based on the azimuth cut-off procedure, while the sea surface wind direction is provided by means of a SAR wind direction algorithm based on the discrete wavelet transform multi-resolution analysis. The obtained wind fields are compared with ground truth data provided by both ASCAT scatterometer and ECMWF model wind fields. SAR-derived wind vector fields and ECMWF model wind data are used to construct a blended wind product regularly sampled in both space and time, which is then used to force a coastal circulation model of a southern Tyrrhenian coastal area to simulate wind-driven circulation processes. The modeling results show that X-band COSMO-SkyMed© SAR data can be valuable in providing effective wind fields for coastal circulation modeling.