Tidal Propagation Research Articles

Effects of Radial Sand Ridges on tidal process in the South Yellow Sea

The South Yellow Sea continental shelf, with a wide area of shallow waters, is located at the intersection of the rotating tidal wave system in the west of the South Yellow Sea and the tidal wave system in the East China Sea. The South Yellow Sea has a unique radiation sandbar terrain, and there are large currents of sand ridges and current channels. Most of the current researches focus on the development and evolution mechanisms of Radial Sand Ridges. However, there are few studies on the influence of tidal waves on terrain in this region. In this paper, the three-dimensional numerical model FVCOM (Finite Volume Coastal Ocean Model) was used to simulate the tidal propagation in the South Yellow Sea and was verified with measurements. This paper compares the changes in the tidal wave of the South Yellow Sea with and without the effect of Radial Sand Ridges to analyze the topographic effect. The changes of tidal wave shape, tidal current velocity, the redistribution of tidal energy and the distribution of tidal mixed zone were analyzed. This research has important implications for the prediction of tides, the development of fisheries and the maintenance of the ecological environment of the South Yellow Sea. The results show that the shape of tidal wave is distorted by the radial sand ridges: mainly, the duration of tidal ebbing is longer than that of tidal flooding. The distortion becomes distinct as the tidal wave approaches the shore, implying that the closer the tidal wave gets to the shore, the greater the nonlinear effect will be. The presence of the Radial Sand Ridges makes the flow rate of the radial inlet larger than 1 m/s, and the flow rate of both the rising tide and the ebbing tide is greater than that of the Waste Yellow River Estuary and the South Yangtze Estuary. At the same time, the existence of the special terrain enlarges the tidal energy flux, speeds up the tidal energy dissipation, and also makes the critical values of the tidal mixing front closer to the shore, namely the tidal mixing zone becoming shoreward. The Radial Sand Ridges also turns down the value of nearshore-stratification parameter. The sea-water blending is more severe in this area, compared to an idealized case with a flat shelf covering the South Yellow Sea. The range of influence of Radial Sand Ridges on tidal current velocity, tidal energy flux, tidal energy dissipation and tidal front position is relatively uniform. It has affected from the eastern part of the Shandong peninsula (around 122°E) across the Radial Sand Ridges to the southern Yangtze River estuary.

Observations of semidiurnal internal tides on the Patagonian Shelf

The Patagonian Shelf in the western South Atlantic is known for strong semidiurnal tides and intense seasonal stratification. Both observations and models reveal substantial cross-isobath tidal energy fluxes at the shelf break between 44 and 41°S. These conditions are potentially favorable for internal wave generation, but global tidal models do not show significant baroclinic energy radiation from this region. A possible explanation for the lack of offshore radiation is explored through analyzing the data from two moorings deployed along the Patagonian shelf break. At both mooring sites, the velocity spectra reveal strong baroclinic oscillations at the semidiurnal frequency with a vertical structure resembling the first mode. Unlike barotropic tides with strong cross-isobath polarization, the baroclinic tidal ellipses are less polarized. Internal tide propagation is delineated by estimating a proxy for the baroclinic energy flux, which is the product of band-passed baroclinic velocity near the bottom and temperature at mid-depth. The proxy vectors show that internal tide propagation is highly sensitive to the strength of the Malvinas Current propagating northward over the continental slope. When the mean current becomes stronger, the proxy vectors turn southward, pointing upstream of the Malvinas Current direction. It is concluded that the presence of an energetic western boundary current reduces the offshore radiation of internal tides, likely due to the wave refraction on the mean current.

Internal Tide Generation at the Vitória‐Trindade Ridge, South Atlantic Ocean

AbstractSea surface height data from multiple satellite altimetric missions are analyzed in order to investigate the surface manifestation of internal tides generated at the Vitória‐Trindade Ridge (VTR), in the western South Atlantic Ocean. The VTR is formed by a sequence of submerged seamounts disposed in a west‐east alignment around 20.5°S, extending for over 1,000 km from the Brazilian continental margin to the Trindade and Martim Vaz islands. Point‐wise harmonic analysis of the along‐track sea surface height data, combined with a two‐dimensional plane wave fitting at selected subregions, depicts well‐organized northbound and southbound wave beams radiating from the VTR, and propagating as far as ~1,000 km from the ridge. Energy convergence is observed at a focal point located circa 300 km south of the VTR, associated with a lens effect due to the arc‐shaped ridge. Wavenumber spectra show highest energy associated with wavelengths of 140 and 70 km, close to the theoretical values expected for the first and second baroclinic modes, respectively. Energy fluxes for the mode 1 principal lunar (M2) constituent reach values of 2 to 3 kW/m for the southward beams and 1 kW/m north of the VTR. Judging from its surface signature, the VTR appears to be the most significant internal tide generation site in the western South Atlantic.

ВНУТРЕННИЙ ПРИЛИВ НА ПЛАТО САНТОС

We analyze internal tides over the Santos Plateau in the South Atlantic. The study is based on the measurements of temperature and currents on the line of moorings normal to the South America coast in the latitudinal band between 28°S and 31°S. The moorings were deployed at distances between 25 and 888 km (from the 500 m isobath) southeast of the continental slope. The instruments were set at 900 m. Numerical modeling revealed the properties of internal tides (amplitudes, wavelength, and decay of amplitudes) along the line of their propagation from the continental slope. The amplitudes of internal tide decrease from the continental slope in the direction to the Vema Channel from 36 m to 15 m. The densities of the energy of tidal internal waves were calculated from the semidiurnal tidal components of current and temperature time series taking into account the vertical gradients of temperature and the Brunt-Vaisala frequency. Decay of the energy of internal tide occurs according to an inverse power law. Numerical modeling of the generation and propagation of internal tides reveals that beams of internal tides are formed near the continental slope. The energy decay based on modeling is similar to the measured data.

Fully integrated modeling of surface water and groundwater in coastal areas

This paper presents an improved model based on a three-dimensional non-hydrostatic wave model NHWAVE to simulate the interactions between the surface water and the groundwater affected by tides or waves in coastal areas. With the model, both the surface water flow and the groundwater flow are calculated based on the well-balanced Volume-averaged Reynolds-averaged Navier-Stokes equations. The spatially varying porosity and hydraulic conductivity are used to identifiy the domains for the surface water and the groundwater. The model is calibrated and validated using a wide range of laboratory measurements reported in the literature, involving the tide propagation through a sandy embankment, the tide-induced groundwater table fluctuation in a sandy beach, and the wave setup in a sloping sandy beach. The interactions between the surface water and the groundwater are analyzed and the influencing factors on the groundwater flow are discussed. The phase lag between the surface water elevation and the groundwater table fluctuation is mainly influenced by the hydraulic conductivity of the porous media. The wave attenuation in the groundwater is proportional to its propagation distance. The computed wave setup elevationin the permeable beach is smaller than in the impermeable beach. It is shown that the fully integrated model is capable of simulating the interactions between the surface water and the groundwater affected by tides or waves in coastal areas. It can be an efficient tool to study the tide and wave dynamics in the permeable sandy beach.

Modeling of Atmospheric Tides with Account of Diurnal Variations of Ionospheric Conductivity

A semiempirical model of ionospheric conductivity for calculating the coefficients of ionic friction and magnetic twisting is described. The influence of diurnal variations of the ionic friction and magnetic twisting in the upper atmosphere on the formation of latitudinal and altitudinal structures of various components of atmospheric tides is considered. It is shown that taking into account diurnal variations of the ionic friction and magnetic twisting leads to an additional generation of non-migrating tides in the thermosphere due to the nonlinear interaction of migrating tides with these variations.

Influence of MeteOcean processes on MSYM sea level predictions in the Malacca Straits

The South China Sea region, and particularly the Malacca and Singapore Straits, are known by the complex tidal dynamics, which is influenced by the tidal propagation from Pacific and Indian Oceans. In spite of the dynamic complexity, the region is very relevant economically, especially concerning the growing oil drilling activities. To give support to accidental oil spill prevention and response, an operational oil spill forecast system was developed for the Strait of Malacca. The hydrodynamic system validation revealed good results, in general. However, besides all the modeling efforts, some discrepancies between observed and predicted sea levels were identified, mainly during neap tide and for specific tide-gauges. Therefore, the main aim of this study consists in researching the origin of these discrepancies by comparing predictions with available data and exploring their relation with the MeteOcean processes of the region. Initially sea level data for eight tide gauges was explored to get a general overview of the local tidal dynamics, and then the model performance for astronomic tide was assessed. Analysis of meteorological tides was also performed for three tide-gauges located in the Singapore and Malacca Strait, which are under the influence of the northeast or southwest monsoons. The results show that the differences between the observed and predicted sea levels in Singapore Strait are usually due to discrepancies in the meteorological tide induced by the surface wind stress acting over the Taiwan-Singapore axis, while in the Malacca Strait are mainly related with model limitations in reproducing the astronomical tide.

Long-Term Design of Mangrove Landfills as an Effective Tide Attenuator under Relative Sea-Level Rise

A mangrove ecosystem is an important option in Ecosystem based Disaster Risk Reduction (Eco-DRR). The effectiveness of an artificial mangrove landfill in reducing tidal amplitudes was studied by performing a coupled numerical model that simulated wave propagation and soil consolidation. The constructed model simulated the propagation of tide over an artificial landfill that was subjected to land subsidence, sea-level rise, vegetation growth, and sediment deposition. A case study analysis confirmed that the tidal amplitudes are reduced if the initial elevation of the landfill is appropriately considered to achieve an equilibrium state of the landfill over its lifetime. Sediment deposition may be the only dependable source to sustain the surface elevation of a mangrove with relative sea-level rise. Sediment deposition is important to promote vegetation growth, which in turn contributes to sedimentation by enhancing a tranquil hydrodynamic environment. An insufficient initial elevation of the landfill will result in less effective protection against tidal propagation after it substantially subsides.

Tidal dynamics in the inter-connected Mediterranean, Marmara, Black and Azov seas

In this study we investigated the tidal dynamics in a system of inter-connected land-locked basins formed by the Mediterranean, the Marmara, the Black and the Azov seas (MMBA system). Through the application of an unstructured grid hydrodynamic model to a unique domain representing the whole MMBA system, we simulated the tidal propagation and transformation inside each basin and in the straits connecting them. The model performance was evaluated against amplitudes and phases of major tidal constituents from 77 tidal gauges. The numerical results provided a description of the characteristics of the principal semi-diurnal, diurnal and long-term tides over the entire system. Even if the narrow straits act as a barrier for the tidal sea surface oscillations, our numerical results demonstrated that the along-strait interface slope produces water fluxes between the adjacent basins of the same order of magnitude of the climatological transports estimated by several authors. The long-term tidal modulations of the water exchange between the Mediterranean and the Black seas resulted to be non negligible and may partially explain the monthly and fortnightly flow variability observed in the Dardanelles and Bosphorus straits.

Modeling analysis of tidal bore formation in convergent estuaries

Despite the recognized impact of tidal bores on estuarine ecosystems, the large scale mechanism of bore formation in convergent alluvial estuaries is still under investigation. So far, field data exist only for a small number of estuaries, while numerical simulations employ the shallow water equations mainly focusing on the small-scale and local processes. In this work, firstly we apply the fully nonlinear weakly dispersive Serre–Green–Naghdiequations to simulate the tide propagation in a convergent estuary of idealized form, verifying that the local dispersion effects, responsible for the appearance of the secondary waves, do not influence the tidal bore onset, which only results from the large scale processes of amplification/damping and distortion of the incoming wave. In a second part, we numerically investigate (225 runs) the estuarine parameter space in order to identify the physical conditions that lead to tidal bore generation. In this parameter space, we determine a critical curve which divides estuaries according to tidal bore occurrence. As a result of this investigation we have shown that bore formation is controlled by the competition between two physical processes: (a) the knee-shaped distortion of the tidal wave, with flood dominance and eventually bore inception; (b) the dissipation of the tidal wave, which is unfavorable to bore formation. We also provide evidence that amplification due to topographic convergence is not a necessary condition for tidal bore generation and that there exist estuaries which display both wave damping and bore development. Finally, the validity of the results has been also assessed in the presence of freshwater river discharge, showing that for low river discharge, its effect on estuarine dynamics can be neglected.

The Lifecycle of Semidiurnal Internal Tides over the Northern Mid-Atlantic Ridge

AbstractThe life cycle of semidiurnal internal tides over the Mid-Atlantic Ridge (MAR) sector south of the Azores is investigated using in situ, a high-resolution mooring and microstructure profiler, and satellite data, in combination with a theoretical model of barotropic-to-baroclinic tidal energy conversion. The mooring analysis reveals that the internal tide horizontal energy flux is dominated by mode 1 and that energy density is more distributed among modes 1–10. Most modes are compatible with an interpretation in terms of standing internal tides, suggesting that they result from interactions between waves generated over the MAR. Internal tide energy is thus concentrated above the ridge and is eventually available for local diapycnal mixing, as endorsed by the elevated rates of turbulent energy dissipation ε estimated from microstructure measurements. A spring–neap modulation of energy density on the MAR is found to originate from the remote generation and radiation of strong mode-1 internal tides from the Atlantis-Meteor Seamount Complex. Similar fortnightly variability of a factor of 2 is observed in ε, but this signal’s origin cannot be determined unambiguously. A regional tidal energy budget highlights the significance of high-mode generation, with 81% of the energy lost by the barotropic tide being converted into modes >1 and only 9% into mode 1. This has important implications for the fraction (q) of local dissipation to the total energy conversion, which is regionally estimated to be ~0.5. This result is in stark contrast with the Hawaiian Ridge system, where the radiation of mode-1 internal tides accounts for 30% of the regional energy conversion, and q < 0.25.

Changes in Bottom Water Physical Properties Above the Mid‐Atlantic Ridge Flank in the Brazil Basin

AbstractWarming of abyssal waters in recent decades has been widely documented around the global ocean. Here repeat hydrographic data collected in 1997 and 2014 near a deep fracture zone canyon in the eastern Brazil Basin are used to quantify the long‐term change. Significant changes are found in the Antarctic Bottom Water (AABW) within the canyon. The AABW in 2014 was warmer (0.08 ± 0.06 ), saltier (0.01 ± 0.005), and less dense (0.005 ± 0.004 ) than in 1997. In contrast, the change in the North Atlantic Deep Water has complicated spatial structure and is almost indistinguishable from zero at 95% confidence. The resulting divergence in vertical displacement of the isopycnals modifies the local density stratification. At its peak, the local squared buoyancy frequency ( ) near the canyon is reduced by about 20% from 1997 to 2014. Similar reduction is found in the basinwide averaged profiles over the Mid‐Atlantic Ridge flank along 25 W in years 1989, 2005, and 2014. The observed changes in density stratification have important implications for internal tide generation and dissipation.

Modelling and Simulation of an Artificial Tide Generation System

In this paper we consider a small-scale experimental apparatus that allows to reproduce and understand phenomena in large-scale typical lagoonal environments and to draw inferences about channel network ontogeny and evolution. In particular, the device reproducing the artificial tide is the core of the experimental apparatus and it is controlled in real-time to satisfactorily reproduce the natural phenomena. The experimental apparatus has an intrinsic complexity and it represents an example of a multi-domain physical system (electrical, mechanical, and hydraulic). In order to design and to asses suitable control strategies, we develop a Matlab-based simulation environment which is able to reproduce the behaviour of the artificial tide generation system. In particular, we use a hybrid modelling technique which integrates casual and acausal approaches. Finally, the dynamic model is calibrated and validated by using real experimental data. The developed model represents a good trade-off between accuracy and complexity and it is useful for control system development purposes.

Effects of meteorological factors on the temporal distribution of red tides in Tolo Harbour, Hong Kong

Red tides represent a major environmental issue in coastal waters globally. However, few studies have examined the relationship between red tides and meteorological factors. Thus, we used a 32-year time-series of frequent red tide events in Tolo Harbour and Channel, to study their relationship with meteorological factors. Most red tides are dominated by dinoflagellates in March, while most diatom red tides in May. Dinoflagellate and diatom red tides respond differently to different meteorological factors. Warming air temperatures in spring favor the generation of dinoflagellate red tides, while precipitation hinders them. The optimum temperature range is approximately 17–23°C and 26–29°C for dinoflagellate and diatom red tides, respectively. Moderate northeasterly winds promote the formation of dinoflagellate red tides. Dinoflagellate red tides are not hindered by cloudy weather and occur in sunlight of varying brightness, whereas diatoms red tides require a certain amount of bright sunlight.

Propagation of the Semidiurnal Internal Tide: Phase Velocity Versus Group Velocity

AbstractThe superposition of two waves of slightly different wavelengths has long been used to illustrate the distinction between phase velocity and group velocity. The first‐mode M2 and S2 internal tides exemplify such a two‐wave model in the natural ocean. The M2 and S2 tidal frequencies are 1.932 and 2 cycles per day, respectively, and their superposition forms a spring‐neap cycle in the semidiurnal band. The spring‐neap cycle acts like a wave, with its frequency, wave number, and phase being the differences of the M2 and S2 internal tides. The spring‐neap cycle and energy of the semidiurnal internal tide propagate at the group velocity. Long‐range propagation of M2 and S2 internal tides in the North Pacific is observed by satellite altimetry. Along a 3,400 km beam spanning 24°–54°N, the M2 and S2 travel times are 10.9 and 11.2 days, respectively. For comparison, it takes the spring‐neap cycle 21.1 days to travel over this distance. Spatial maps of the M2 phase velocity, the S2 phase velocity, and the group velocity are determined from phase gradients of the corresponding satellite observed internal tide fields. The observed phase and group velocities agree with theoretical values estimated using the World Ocean Atlas 2013 annual‐mean ocean stratification.

Hydrodynamic modelling of Port Foster, Deception Island, Antarctica

AbstractOver the last decade, the Antarctic continent has been the object of intensive scientific programmes. However, the emphasis of these studies rarely focuses on the Antarctic as a source of potential elements such as mercury. The release of mercury to the environment is known to occur at Deception Island, associated with volcanic activity. In this study, a 3D hydrodynamic model was used to assess the role of water circulation on the dispersion of released mercury. Sea level variation and tidal circulation data were obtained. Residence time was calculated using two different approaches. Internal tide generation in summer and winter were recognized and the barotropic tidal components obtained. Lagrangian tracers were used to depict particle circulation (simulating particulate mercury) in a three month summer simulation for barotropic and baroclinic conditions. The results show that particles accumulate in the northern and western parts of the bay. It is acknowledged that the results of the 3D model are associated with a non-negligible uncertainty, which can only be reduced with an ongoing commitment to monitoring. The findings of this study indicate that mercury accumulation is occurring in Port Foster (Deception Island), which is a potential threat to the local ecosystem.

SOMAR-LES: A framework for multi-scale modeling of turbulent stratified oceanic flows

A new multi-scale modeling technique, SOMAR-LES, is presented in this paper. Localized grid refinement gives SOMAR (the Stratified Ocean Model with Adaptive Resolution) access to small scales of the flow which are normally inaccessible to general circulation models (GCMs). SOMAR-LES drives a LES (Large Eddy Simulation) on SOMAR’s finest grids, forced with large scale forcing from the coarser grids. Three-dimensional simulations of internal tide generation, propagation and scattering are performed to demonstrate this multi-scale modeling technique. In the case of internal tide generation at a two-dimensional bathymetry, SOMAR-LES is able to balance the baroclinic energy budget and accurately model turbulence losses at only 10% of the computational cost required by a non-adaptive solver running at SOMAR-LES’s fine grid resolution. This relative cost is significantly reduced in situations with intermittent turbulence or where the location of the turbulence is not known a priori because SOMAR-LES does not require persistent, global, high resolution. To illustrate this point, we consider a three-dimensional bathymetry with grids adaptively refined along the tidally generated internal waves to capture remote mixing in regions of wave focusing. The computational cost in this case is found to be nearly 25 times smaller than that of a non-adaptive solver at comparable resolution. In the final test case, we consider the scattering of a mode-1 internal wave at an isolated two-dimensional and three-dimensional topography, and we compare the results with Legg (2014) numerical experiments. We find good agreement with theoretical estimates. SOMAR-LES is less dissipative than the closure scheme employed by Legg (2014) near the bathymetry. Depending on the flow configuration and resolution employed, a reduction of more than an order of magnitude in computational costs is expected, relative to traditional existing solvers.

Climate Process Team on Internal Wave-Driven Ocean Mixing.

Diapycnal mixing plays a primary role in the thermodynamic balance of the ocean and, consequently, in oceanic heat and carbon uptake and storage. Though observed mixing rates are on average consistent with values required by inverse models, recent attention has focused on the dramatic spatial variability, spanning several orders of magnitude, of mixing rates in both the upper and deep ocean. Away from ocean boundaries, the spatio-temporal patterns of mixing are largely driven by the geography of generation, propagation and dissipation of internal waves, which supply much of the power for turbulent mixing. Over the last five years and under the auspices of US CLIVAR, a NSF- and NOAA-supported Climate Process Team has been engaged in developing, implementing and testing dynamics-based parameterizations for internal-wave driven turbulent mixing in global ocean models. The work has primarily focused on turbulence 1) near sites of internal tide generation, 2) in the upper ocean related to wind-generated near inertial motions, 3) due to internal lee waves generated by low-frequency mesoscale flows over topography, and 4) at ocean margins. Here we review recent progress, describe the tools developed, and discuss future directions.

Evidence of Tropospheric 90 Day Oscillations in the Thermosphere

AbstractIn the last decade evidence demonstrated that terrestrial weather greatly impacts the dynamics and mean state of the thermosphere via small‐scale gravity waves and global‐scale solar tidal propagation and dissipation effects. While observations have shown significant intraseasonal variability in the upper mesospheric mean winds, relatively little is known about this variability at satellite altitudes (∼250–400 km). Using cross‐track wind measurements from the Challenging Minisatellite Payload and Gravity field and steady‐state Ocean Circulation Explorer satellites, winds from a Modern‐Era Retrospective Analysis for Research and Applications/Thermosphere‐Ionosphere‐Mesosphere‐Electrodynamics General Circulation Model simulation, and outgoing longwave radiation data, we demonstrate the existence of a prominent and global‐scale 90 day oscillation in the thermospheric zonal mean winds and in the diurnal eastward propagating tide with zonal wave number 3 (DE3) during 2009–2010 and present evidence of its connection to variability in tropospheric convective activity. This study suggests that strong coupling between the troposphere and the thermosphere occurs on intraseasonal timescales.

Internal tides in a dendritic submarine canyon

Submarine canyons are a common geomorphological feature along continental slopes worldwide and often found to be ‘hotspots’ of internal tide activity. However, the majority of well-studied submarine canyons are simple linear incisions or have meandering morphology; internal tide energetics in branching (dendritic) canyons has not previously been investigated. Here we present a high-resolution (500-m) numerical modelling study of the internal tide within Whittard Canyon, a large, dendritic submarine canyon system that incises the Celtic Sea continental slope. A modified version of the Princeton Ocean Model is used to simulate the M2 (semidiurnal) internal tide in the Whittard Canyon region, verified against a hydrographic dataset collected by an autonomous ocean glider. Much of the internal tide energy entering Whittard Canyon originates to the southeast, along the Celtic Sea shelf break. Internal tide generation also occurs within the canyon itself, but is in part compensated by areas of negative energy conversion. Depth-integrated internal tide energy fluxes exceed 8kWm−1 in the eastern limb of the canyon. The internal tide is topographically steered through the major limbs and along-canyon energy flux is bottom intensified, suggesting topographic focusing. The down canyon extent of bottom intensification closely corresponds to the point that along-canyon slope becomes near-critical to the semidiurnal internal tide. Energetically, the multiple limbs of Whittard Canyon behave differently, some are net sources of internal tide energy whilst others are net sinks. Internal tide energy dissipation also varies between the canyon limbs; bulk dissipation rates are 2.1-7.7×10-8Wkg-1. In addition, the effect of bathymetric resolution on internal tide generation and propagation is investigated by progressively smoothing the model domain. Decreasing the bathymetric resolution reduces internal tide generation and energy dissipation in both Whittard Canyon and the model domain as a whole, however, internal tide energy flux into the canyon is not consistently changed. At least 1.5-km resolution bathymetry is required to adequately resolve the semidiurnal internal tide field in this region of complex topography.