Three-dimensional Circulation Research Articles

A Three-Layer Alternating Spinning Circulation in the South China Sea

AbstractUnderstanding of the three-dimensional circulation in the South China Sea (SCS) is crucial for determining the transports of water masses, energy, and biogeochemical substances in the regional and adjacent larger oceans. The circulation’s kinematic and dynamic natures, however, are largely unclear. Results from a three-dimensional numerical ocean circulation model and geostrophic currents, derived from hydrographic data, reveal the existence of a unique, three-layer, cyclonic–anticyclonic–cyclonic (CAC) circulation in the upper (<750 m), middle (750–1500 m), and deep (>1500 m) layers in the SCS with differing seasonality. An inflow–outflow–inflow structure in Luzon Strait largely induces the CAC circulation, which leads to vortex stretching in the SCS basin because of a lateral planetary vorticity flux in each of the respective layers. The formation of joint effects of baroclinicity and relief (JEBAR) is an intrinsic dynamic response to the CAC circulation. The JEBAR arises from the CAC flow–topography interaction in the SCS.

Water exchanges between a multi-inlet lagoon and the ocean: the role of forcing mechanisms

Understanding the influence of the main drivers controlling the circulation and the transport in coastal lagoons is a necessary step towards the description of the dynamics of their ecosystems. Thus, the influence of the main physical drivers on the water exchanges in a multi-inlet barrier island (western sector of the Ria Formosa coastal lagoon, Portugal) was investigated. Several scenarios of tide, wind, bathymetry and point source discharges were simulated using a three-dimensional circulation model and a particle-tracking model. The circulation is adequately reproduced by the hydrodynamic model, with root mean square errors of about 5–8 cm for the water levels and 5–10 cm/s for the cross-sectional averaged velocities in the western inlets and main channels. Wind has a negligible effect on the modelled water levels and cross-sectional velocities, with differences smaller than 1 % between the simulations with and without wind. However, results show that wind influences significantly the transport by affecting the residual circulation, with distinct effects depending on the wind direction. Upwelling favourable winds (with west component) increase the landward transport through the Faro-Olhao inlet and promote a larger dispersion of the water-borne material inside the lagoon. The residual circulation between the three inlets of the western sector of the lagoon is also significantly affected by the bathymetry. Results show a decrease of about 50 % of the tidal prism of the Ancao inlet between 2002 and 2011, which reduces its capacity to export waterborne material to the adjacent coastal area and increases the residence times in some areas of the lagoon, with potentially adverse effects on the ecosystems’ health. The dispersion of potential contaminants from point sources inside the lagoon to the coastal area depends on the combined effect of the tidal phase and amplitude and the location of the source. Mean residence times for the discharges from the wastewater treatment plants of the western sector ranged from 7 to 18 days. These findings improve the understanding of the influence of the physical forcings in the circulation and transport dynamics of multi-inlet coastal systems.

Quasigeostrophic Diagnosis of Mixed Layer Dynamics Embedded in a Mesoscale Turbulent Field

AbstractA quasigeostrophic model is developed to diagnose the three-dimensional circulation, including the vertical velocity, in the upper ocean from high-resolution observations of sea surface height and buoyancy. The formulation for the adiabatic component departs from the classical surface quasigeostrophic framework considered before since it takes into account the stratification within the surface mixed layer that is usually much weaker than that in the ocean interior. To achieve this, the model approximates the ocean with two constant stratification layers: a finite-thickness surface layer (or the mixed layer) and an infinitely deep interior layer. It is shown that the leading-order adiabatic circulation is entirely determined if both the surface streamfunction and buoyancy anomalies are considered. The surface layer further includes a diabatic dynamical contribution. Parameterization of diabatic vertical velocities is based on their restoring impacts of the thermal wind balance that is perturbed by turbulent vertical mixing of momentum and buoyancy. The model skill in reproducing the three-dimensional circulation in the upper ocean from surface data is checked against the output of a high-resolution primitive equation numerical simulation.

Establishment of ocean dumping area capacity assessment model

Dumping area capacity is mainly affected by the hydrodynamic process (tidal sediment, storm surge and wave, etc.) as well as the size and depth of dumping area. Based on three-dimensional ocean circulation model known as FVCOM (Finite Volume Coast and Ocean Model) and the stochastic dynamic statistical analysis model, taking advantage of dumping ground topography evolution and dumping quantity, the author aims to discuss the influence of hydrodynamic processes and dumping activity so as to built a new model of ocean dumping area capacity. With the data of depth and dumped amount in the dumping area, the changes of bottom topographic which caused by tidal current under the natural condition based on the FVCOM hydrodynamic and sediment module, the author strive to analyze the statistical relation of the changes for dumping amount, tidal current and bottom topographic. Through real data to fit revision coefficient values, which will be regarded as topographic changes reference value affected by wave and storm surges. Thus taking this evaluation as the long-term changes in the dumping capacity. In the premise of setting up the threshold of bottom topographic changes, the dumping area capacity is calculated. Take Yangtze Estuary No. 1 dumping area as an example, As the water depth reduces by 0.5 m annually, the dumping area capacity is about 6.7 million m3/a, the model results are in reasonable agreement with the actual amount. Then the model is validated in Luoyuan Bay dumping area, Shengsishangchuan Mountain dumping area, Dongding dumping area, Dongshan dumping area, and Wenzhou Port dumping area, it is turns out the results are similar to that of the actual observations.

Modeling the Effect of Waves on the Diurnal Temperature Stratification of a Shallow Lake

In this paper a three-dimensional circulation model has been applied to determine the spatial distribution of the temperature and mixed-layer depth (MLD) of a medium-sized shallow lake. In order to reproduce the vertical thermal structure of the lake, the impact of waves has been incorporated using several recently published schemes. These schemes approximate the additional mixing due to the orbital motion of waves, the turbulent kinetic energy produced by waves, or both. The reference solver is the Reynolds-Averaged Navier-Stokes equations with two-equation Mellor-Yamada 2.5 turbulence closure. The wave field is characterized by bulk parameters, e.g. significant wave height. The sensitivity to the choice of the wave mixing scheme is analyzed by means of an academic test case. The accuracy of the scheme is explored through data collected in Lake Balaton. The modeled temperature profiles and MLDs match measurements much better when the effect of non-breaking surface waves is included.

Large-Scale Atmospheric and Oceanic Control on Krill Transport into the St. Lawrence Estuary Evidenced with Three-Dimensional Numerical Modelling

ABSTRACTA three-dimensional circulation model, coupled to a Lagrangian particle drift model, is used to understand the processes leading to krill transport from the northwest Gulf of St. Lawrence (nwGSL) towards the head of the Lower St. Lawrence Estuary (LSLE), a well-known site of krill accumulation. An analysis of the circulation at the scale of the Gulf of St. Lawrence (GSL) over five years (2006 to 2010) evidenced four major findings. (i) There are two main seasonal circulation patterns, one in winter–spring and one in summer–fall, driven by local wind forcing and transport at Cabot Strait and at the Strait of Belle Isle. (ii) The freshwater runoff variability does not control the observed inflow events at the mouth of the LSLE. (iii) Extratropical storms passing over the GSL are important for the transport of krill into the LSLE through the generation of inflow events at Pointe-des-Monts. (iv) The contribution of the transport in the surface layer (where krill are found at night) during these inflow events is also important in modulating the variability of the transport of krill into the LSLE. The inflow events, combined with the presence or absence of high krill densities in the nwGSL, partly control the interannual variability of the transport of krill into the LSLE.

Three-Dimensional Structure of Optimal Nonlinear Excitation for Decadal Variability of the Thermohaline Circulation

AbstractThe decadal variability of the North Atlantic thermohaline circulation (THC) is investigated within a three-dimensional ocean circulation model using the conditional nonlinear optimal perturbation method. The results show that the optimal initial perturbations of temperature and salinity exciting the strongest decadal THC variations have similar structures: the perturbations are mainly in the northwestern basin at a depth ranging from −1500 to −3000 m. These temperature and salinity perturbations act as the optimal precursors for future modifications of the THC, highlighting the importance of observations in the northwestern basin to monitor the variations of temperature and salinity at depth. The decadal THC variation in the nonlinear model initialized by the optimal salinity perturbations is much stronger than that caused by the optimal temperature perturbations, indicating that salinity variations might play a relatively important role in exciting the decadal THC variability. Moreover, the deca...

A modeling study of physical controls on hypoxia generation in the northern Gulf of Mexico

The Louisiana shelf (LA shelf) in the northern Gulf of Mexico experiences hypoxic conditions every summer due to the combination of eutrophication and strong water column stratification. Here we use a three-dimensional circulation model coupled with a simple oxygen model to examine the physical controls on hypoxia generation on the LA shelf. The model assumes a constant oxygen utilization rate in the water column and a sediment oxygen consumption rate that depends on the bottom water oxygen concentration and temperature. Despite its simplicity, the model reproduces the observed variability of dissolved oxygen and hypoxia on the LA shelf, highlighting the importance of physical processes. Model results demonstrate that both river discharge and wind forcing have a strong effect on the distribution of the river plume and stratification, and thereby on bottom dissolved oxygen concentrations and hypoxia formation on the LA shelf. The seasonal cycle of hypoxia is relatively insensitive to the seasonal variability in river discharge, but the time-integrated hypoxic area is very sensitive to the overall magnitude of river discharge. Changes in wind speed have the greatest effect on the simulated seasonal cycle of hypoxia and hypoxic duration, while changes in wind direction strongly influence the geographic distribution of hypoxia. Given that our simple oxygen model essentially reproduces the evolution of hypoxia simulated with a full biogeochemical model and that physical processes largely determine the magnitude and distribution of hypoxia, a full biogeochemical model might not be necessary for short-term hypoxia forecasting.

Physical controls of hypoxia in waters adjacent to the Yangtze Estuary: A numerical modeling study

A three-dimensional circulation model (the Environmental Fluid Dynamic Code) was used to examine the role that physical forcing (river discharge, wind speed and direction) plays in controlling hypoxia in waters adjacent to the Yangtze Estuary. The model assumes that the biological consumption of oxygen is constant in both time and space, which allows the role of physical forcing in modulating the oxygen dynamics to be isolated. Despite of the simplicity of this model, the simulation results showed that it can reproduce the observed variability of dissolved oxygen in waters adjacent to the Yangtze Estuary, thereby highlighting the important role of changes in physical forcing in the variation of hypoxia. The scenarios tested revealed appreciable changes in the areal extent of hypoxia as a function of wind speed and wind direction. Interestingly, well-developed hypoxia was insensitive to river discharge.

The thermosphere general circulation modeling with the parametrization of radiative processes

The paper presents a new version of the global three-dimensional general circulation model of the Earth’s thermosphere (90-500 km) with high spatial resolution (2 × 2.5 × 80), including consistent calculation of radiative processes. Based on a detailed analysis of the reproduction of the various components of radiation transfer a good agreement of radiation balance with empirical data is shown in the new model. Analytical estimations and model results proved that the thermosphere global state formation is essentially determined by the ratio between the radiation heating and heat sink due to molecular parameters, as well as by the lower boundary conditions. On the base of the preliminary model identification with empirical data a satisfactory reproduction of the thermal balance and the thermospheric general circulation features is shown.

Three-Dimensional Circulation Structure of Summer Heavy Rainfall in Central North China

Abstract Based on daily rainfall observations and Japanese 25-year Reanalysis Project data during ~1981–2010, a three-dimensional circulation structure that formed before heavy summer rainfall in central north China (CNC) is revealed in this study. Composite analyses of circulation in advance of 225 heavy rain days show that the circulation structure is characterized by a remarkable upper-tropospheric warm anomaly (UTWA), which covers most of northern China with a center at ~300 hPa. Under hydrostatic and geostrophic equilibriums, the UTWA contributes to the generation of an anticyclonic (cyclonic) anomaly above (below). The anticyclonic anomaly strengthens (weakens) westerly winds to the north (south) of the warm center and pushes the high-level westerly jet to the north. The cyclonic anomaly deepens the trough upstream of CNC and intensifies lower southwesterly winds to the mideast of the warm center. As a result, the northerly stretched high-level jet produces upper divergence in its right-front side and the intensified southwesterly winds induce lower moisture convergence in its left-front side, causing heavy rainfall in CNC. Correlation analyses further confirm the close connections between UTWA and circulation in the upper and lower troposphere. The correlation coefficients between UTWA and the upper geopotential height, upper westerly jet, and lower southerly flow reach 0.95, 0.70, and 0.39, implying that the two critical factors leading to intense rainfall in CNC, the high-level jet and the low-level southerly flow, are closely connected with the UTWA. Consequently, in the future analyses and forecasts of heavy rainfall over northern China, more attention should be paid to the temperature in the upper troposphere.

Observations of the shelf circulation dynamics along Ningaloo Reef, Western Australia during the austral spring and summer

The dynamics of the three-dimensional circulation on the shelf off Ningaloo Reef in Western Australia were investigated using observations from two field experiments (Nov 2009–Jan 2010 and Sep 2010–Nov 2010). The observations revealed an onshore geostrophic flow of ~0.015ms−1 along the relatively straight Ningaloo Peninsula, coinciding with a substantial poleward increase of the along-shelf flow along this short (∼60km) section of coast. The cross-shelf transport analysis distinguished between two cross-shelf advection scenarios prevalent along this coast: 1) a transient coastal upwelling scenario when the offshore surface Ekman transport was replenished by an onshore transport, supplied mainly from the interior of the water column due to the combined effects of the stratification and steep continental slope; and 2) a downwelling scenario when offshore bottom boundary layer Ekman transport occurred, associated with a deeper pressure-gradient driven flow. A detailed along-shelf momentum balance indicated that there was a southward increase in the along-shelf pressure gradient from the northern limit of the Ningaloo Peninsula (the North West Cape), which explains the increase in the along-shelf flow down the coast. The results indicate that the Ningaloo shelf can be considered as one of the important formation regions of the poleward Leeuwin Current which, while very weak on Australia North West Shelf, is the dominant current feature south of Ningaloo and along the remaining southwestern Australia coast.

A Comparison of Bulk Estuarine Turnover Timescales to Particle Tracking Timescales Using a Model of the Yaquina Bay Estuary

The ability to determine a bulk estuarine turnover timescale that is well defined under realistic conditions is in high demand for estuarine research and management. We compare how turnover timescales vary with tidal and river forcing from idealized forcing scenarios using a three-dimensional circulation model of the Yaquina Bay estuary in order to understand the limitations and benefits of different timescale methods for future application. Using model results, we compare bulk formula approaches—the tidal prism method, freshwater fraction method, and a relatively new estuarine timescale calculation method based on the total exchange flow (TEF)—to directly calculated timescales from particle tracking in order to assess the utility of the bulk formula timescales. All of the timescales calculated had similar magnitudes during high river discharge but varied significantly at low discharge and had different dependences on tidal amplitude. Even in the application of a single estuary-averaged timescale, we did not find that any of the bulk timescales described the estuary over a realistic range of tidal and river discharge forcing. During high discharge, the Yaquina Bay timescale is on the order of 2–5 tidal cycles based on the particle tracking analysis, but during low discharge, the turnover time varies across methods and spatial considerations appear to be more important.

The Navy's Application of Ocean Forecasting to Decision Support

Abstract : The Naval Oceanographic Office (NAVOCEANO) provides daily operational global, regional, and coastal ocean model forecasts and their associated prediction products. The models utilized include three-dimensional circulation, wave and ice forecasting systems that have been developed to meet Navy requirements; the models are forced by Navy atmospheric models and constrained by Navy-developed bottom topography. In addition, NAVOCEANO acquires, quality controls, and delivers real-time ocean observations from both in situ and remote-sensing resources for assimilation by the ocean models. These observations are also used to assess model skill and develop ocean climatologies. A major supercomputing capacity is needed to run this ocean modeling suite, as well as a uniquely skilled model operations team that keeps the systems running. Dedicated ocean forecasters interpret the predictions and apply the information to Navy operations. The path from requirements to development to operations demonstrates the close links between research and development production, and operational Navy applications. This path also provides innovative future ocean modeling plans directed to improve oceanographic support to the Navy.

Hydrodynamic response of a fringing coral reef to a rise in mean sea level

Ningaloo Reef, located along the northwest coast of Australia, is one of the longest fringing coral reefs in the world extending ~300 km. Similar to other fringing reefs, it consists of a barrier reef ~1–6 km offshore with occasional gaps, backed by a shallow lagoon. Wave breaking on the reef generates radiation stress gradients that produces wave setup across the reef and lagoon and mean currents across the reef. A section of Ningaloo Reef at Sandy Bay was chosen as the focus of an intense 6-week field experiment and numerical simulation using the wave model SWAN coupled to the three-dimensional circulation model ROMS. The physics of nearshore processes such as wave breaking, wave setup and mean flow across the reef was investigated in detail by examining the various momentum balances established in the system. The magnitude of the terms and the distance of their peaks from reef edge in the momentum balance were sensitive to the changes in mean sea level, e.g. the wave forces decreased as the mean water depth increased (and hence, wave breaking dissipation was reduced). This led to an increase in the wave power at the shoreline, a slight shift of the surf zone to the lee side of the reef and changes in the intensity of the circulation. The predicted hydrodynamic fields were input into a Lagrangian particle tracking model to estimate the transport time scale of the reef-lagoon system. Flushing time of the lagoon with the open ocean was computed using two definitions in renewal of semi-enclosed water basins and revealed the sensitivity of such a transport time scale to methods. An increase in the lagoon exchange rate at smaller mean sea-level rise and the decrease at higher mean sea-level rise was predicted through flushing time computed using both methods.

Estimating ventilation time scales using overturning stream functions

A simple method for estimating ventilation time scales from overturning stream functions is proposed. The stream function may be computed using either geometric coordinates or a generalized vertical coordinate, such as potential density (salinity in our study). The method is tested with a three-dimensional circulation model describing an idealized semi-enclosed ocean basin ventilated through a narrow strait over a sill, and the result is compared to age estimates obtained from a passive numerical age tracer. The best result is obtained when using the stream function in salinity coordinates. In this case, the reservoir-averaged advection time obtained from the overturning stream function in salinity coordinates agrees rather well with the mean age of the age tracer, and the corresponding maximum ages agree very well.

A large-scale wave-current coupled module with wave diffraction effect on unstructured meshes

Based on the extended mild-slope equation, a large-scale wave module is developed. By combining the eikonal equation and the modified wave action equation, the wave model can account for diffraction in most situations such as in the lee of islands and breakwaters, and using unstructured meshes provides great flexibility for modelling the wave in the complex geomorphology of barriers and islands, also allowing for refinement of the grid resolution within computationally important domains. The numerical implementation of the module is based on the explicit second-order upwind finite-volume schemes in geographic space, the Flux-Corrected Transport (FCT) algorithm in frequency space and the implicit Crank-Nicolson method in directional space. The three-dimensional hydrodynamic module is then modified to couple with the wave model, where the wave readily provides the depth-dependent radiation stress and the wave-induced turbulence coefficient for the current fields, and the wave propagation takes into account the current-induced advection, refraction and diffraction of wave energy and the effect of water level. The applicability of the proposed model to calculate Snell’s Law, wave transformation over the breakwaters and the elliptic shoal, wave propagation over the rip current field and the undertow on a sloping beach is evaluated. Numerical results show that the present model makes better predictions of the near-shore wave propagation and complex three-dimensional (3D) near-shore circulation driven by the waves, considering analytical solutions and experimental values.

Calculation Method of Passive Residual Heat Removal Heat Exchanger and Numerical Simulation

The tube inside and outside heat transfer mechanism of Passive Residual Heat Removal Heat Exchanger (PRHR HX) was analyzed. The calculation method of this special heat exchanger under natural convection condition in In-containment Refueling Water Storage Tank (IRWST) was carried out. The single-tube coupling model three-dimensional natural circulation in the IRWST was simulated numerically using Fluent. The heat transfer and flow characteristics of the fluid in IRWST were obtained. The comparison of the results between theoretical arithmetic and numerical simulation showed that the theoretical calculation method is suitable for the heat transfer calculation of PRHR HX.

Modelling study of three-dimensional circulation and particle movement over the Sable Gully of Nova Scotia

The Sable Gully is a broad deep underwater canyon located to the east of Sable Island on the edge of the Scotian Shelf. Being the home of many marine species including the endangered Northern Bottlenose Whale, the Gully was designated as a marine protected area (MPA) in 2004. Better understanding of physical environmental conditions over this MPA is needed for sustainable ecosystem management. In this study, a multi-nested ocean circulation model and a particle tracking model are used to examine the three-dimensional (3D) circulation and movement of particles carried passively by the flow over the Sable Gully. The 3D circulation model is driven by tides, wind, and surface heat/freshwater fluxes. The model performance is assessed by comparing the results with the previous numerical tidal results and current meter observations made in the Gully. The simulated tidal circulation over the Gully and adjacent waters is relatively strong on shallow banks and relatively weak on the continental slope. Below the depth of the Gully rim ( ∼ 200 m), the tidal currents are constrained by the thalweg of the Gully and amplified toward the Gully head. The simulated subtidal circulation in the Gully has a complex spatial structure and significant seasonal variability. The simulated time-dependent 3D flow fields are then used in a particle tracking model to study the particle movements, downstream and upstream areas, and residence time of the Gully. Based on the movements of particles released at the depth of the Gully rim and tracked forward in time, the e-folding residence time is estimated to be about 7 and 13 days in February and August 2006, respectively. The Gully flanks are identified as high retention areas with the typical residence time of 10 and 20 days in February and August 2006, respectively. Tracking particles with and without tides reveals that tidal circulation reduces the value of residence time in the Gully, particularly along the Gully flanks.

Numerical Modelling of large-scale Dynamics of the White Sea

In the paper we describe the numerical model of large-scale hydro- and thermal dynamics of the White sea. The model is the model of the Polar ocean by prof. N.G. Iakovlev adapted for the shallow and small White sea. Based on the primitive equations and the finite elements, this model includes mature and effective ice sea model. We show that the numerical simulation agree well with the generally accepted three-dimensional large-scale circulation of the sea, including thermohaline fields, currents, ice drift and thickness distribution. Numerical experiments show that influence of atmospheric forcing perturbations on the dynamics of the sea is not drastic. Changes in air temperature to a few degrees lead to similar changes of the water temperature and only slightly change the distributions of the fields. Complete, no, and altering (no clouds at summer, total cover at winter) cloud cover also changes the main patterns only slightly. This also proves that the model is stable with respect to atmospheric forcing.