Flow separation, dipole formation, and water exchange through tidal straits

We investigate the formation and evolution of dipole vortices and their contribution to water exchange through idealized tidal straits. Self-propagating dipoles are important for transporting and exchanging water properties through straits and inlets in coastal regions. In order to obtain a robust dataset to evaluate flow separation, dipole formation and evolution, and the effect on water exchange, we conduct 164 numerical simulations, varying the width and length of the straits as well as the tidal forcing. We show that dipoles form and start propagating at the time of flow separation, and their vorticity originates in the velocity front formed by the separation. We find that the dipole propagation velocity is proportional to the tidal velocity amplitude and twice as large as the dipole velocity derived for a dipole consisting of two point vortices. We analyze the processes creating a net water exchange through the straits and derive a kinematic model dependent on dimensionless parameters representing strait length, dipole travel distance, and dipole size. The net tracer transport resulting from the kinematic model agrees closely with the numerical simulations and provides an understanding of the processes controlling net water exchange.

Open-Source Code-Based Tidal Modeling of Tropical and Temperate Waters

Tidal energy is the most reliable and predictable form of renewable energy capable of ensuring energy security in coastal regions of the world. Many developing countries are prone to energy self-sufficiency due to a lack of tidal data, expensive commercial tidal modeling tools, and program codes. In the present study, an open-source finite element code along with available open-source data was used to predict the tidal resource potential of sites in both temperate and coastal waters. This paper also investigates the suitability of open-source code towards accurate tidal resource prediction and provides a comparative study on tidal resource prediction of sites in both temperate and coastal waters. Based on knowledge gained from tidal experts all around the globe, the straits of the Alderney race were selected as a temperate water site because of their high tidal flow conditions and high tidal energy resource potential. Singapore was selected as a tropical water site because of its low tidal flow conditions and lack of open-source tidal resource data in the tropical belt. From the results, temperate waters such as Alderney Race experience high tidal velocity in the range of 3.5–4.5 m/s with an average power density of about 15 kW/m2 in comparison with tropical waters such as Singapore that experiences tidal velocity in the range of 1–1.5 m/s with an average power density of about 1.5 kW/m2. The thrusting force behind the coastal dynamics is mainly due to tides, their interactions, and changes in seabed topography. The seabed roughness profile creates a drag force on the flow on the velocity field. Lack of understanding of the effects of seabed friction on tidal modeling might reduce the accuracy of the model prediction. Thus, the present study also focuses on the effects of seabed roughness on tidal prediction of Alderney race straits using the open-source finite element-based 2-dimensional depth average ocean model. It can be found that an increase in seabed friction reduces the flow velocity and thus the average power density of the location due to its major energy dissipating phenomenon for the energetic ocean flow.

Flow separation, dipole formation and water exchange through tidal strait

We investigate the formation and evolution of dipole vortices and their contribution to water exchange through idealized tidal straits. Self-propagating dipoles are important for transporting and exchanging water properties through straits and inlets in coastal regions. In order to obtain a robust data-set to evaluate flow separation, dipole formation and evolution and the effect on water exchange, we conduct 164 numerical simulations, varying the width and length of the straits as well as the tidal forcing. We show that dipoles are formed and start propagating at the time of flow separation, and their vorticity originates in the velocity front formed by the separation. We find that the dipole propagation velocity is proportional to the tidal velocity amplitude, and twice as large as the dipole velocity derived for a dipole consisting of two point vortices. We analyse the processes creating a net water exchange through the straits and derive a kinematic model dependent on dimensionless parameters representing strait length, dipole travel distance and dipole size. The net tracer transport resulting from the kinematic model agrees closely with the numerical simulations and provide understanding of the processes controlling net water exchange.

The role of tides in bottom water export from the western Ross Sea

Approximately 25% of Antarctic Bottom Water has its origin as dense water exiting the western Ross Sea, but little is known about what controls the release of dense water plumes from the Drygalski Trough. We deployed two moorings on the slope to investigate the water properties of the bottom water exiting the region at Cape Adare. Salinity of the bottom water has increased in 2018 from the previous measurements in 2008–2010, consistent with the observed salinity increase in the Ross Sea. We find High Salinity Shelf Water from the Drygalski Trough contributes to two pulses of dense water at Cape Adare. The timing and magnitude of the pulses is largely explained by an inverse relationship with the tidal velocity in the Ross Sea. We suggest that the diurnal and low frequency tides in the western Ross Sea may control the magnitude and timing of the dense water outflow.

The heartbeat of a glacier: Cascading subglacial water pockets and ocean tides cause hourly to daily ice-flow variations of Priestley Glacier, Antarctica, detected with Terrestrial Radar Interferometry

<p>In Antarctica, basal melting in the ice-sheet’s interior generates subglacial water that is routed via the subglacial hydrological system towards the margins. At the grounding zone, the subglacial meltwater comes into contact with ocean water subject to tides. The mixing of the two water masses may be one reason for velocities variations on tidal timescales, providing a window into processes of basal sliding. With this goal in mind, we instrumented a flowline across the grounding zone of Priestley glacier, Antarctica, with 4 differential GNSS stations co-located with advanced phase sensitive radars (ApRES) and tiltmeters all measuring continuously over several months. Moreover, we installed a Terrestrial Radar Interferometer (TRI) overlooking the glacier from an adjacent rock outcrop. The to our knowledge first-time deployment of the TRI in Antarctica reveals a stunning picture of grounding-zone dynamics providing spatially coherent 1D flowfields every 3 hours over a time period of 10 days. This enables interpretations of velocity changes measured by GNNS in an unprecedented spatial context. We complement our on-site geophysical dataset with airborne ice-penetrating radar as well as spaceborne InSAR data using timeseries from TanDEM-X, Sentinel-1A, and the ERS satellites.</p><p>TRI and GNSS stations jointly detect tidal velocity fluctuations (> 50 % around the mean) which decay landwards with increasing distance from the grounding line. Triple differences in satellite interferometry reveal transient bull’s eye patterns far upstream of the grounding line quantifying localized surface lowering together with adjacent surface uplift. We interpret this as a result from abruptly migrating subglacial water pockets cascading over obstacles in the basal topography. The TRI also shows such bull’s eye patterns pulsating in our highly resolved time series. Moreover, all GNSS stations and the TRI detect a short-lived acceleration event (~100 % horizontal speedup over 2 hours) paired with spatially coherent surface uplift (~15 cm). Magnitude and duration of this event suggests operation of hydraulic jacking, a mechanism explaining short-lived speed-ups with pressure variations in a linked-cavity system. However, usually this is pre-conditioned to the existence of significant surface meltwater entering the subglacial hydrological system, which is not the case at our study site. Our joint observations with multiple sensors and instruments therefore  provide unique observations to further develop our understanding of basal sliding, particularly it's dependency on upstream water supply and ocean tides.</p>

A baroclinic tidal forecasting model for the Mediterranean Sea - First validation results

<p>In the framework of the Copernicus Marine Environment Monitoring Service (CMEMS) Mediterranean</p><p>Analysis and Forecasting Physical System (MedFS), a specific modeling upgrade has been carried out</p><p>by including the main lunisolar tides.</p><p>Mediterranean tides, even if characterized by small amplitudes, play an important role on the dynamics</p><p>of the Mediterranean sea and the introduction of tides in the hydrodynamic numerical model simulations</p><p>represent the first step in the development of a numerical forecasting model that considers explicitly the</p><p>tidal dynamics and the mesoscales.</p><p>MedFS is an operational system that produces weekly analysis and daily 10-days forecasts of the main</p><p>physical fields with a resolution of around 4.5km over the whole Mediterranean basin including the</p><p>Atlantic Ocean adjacent area (Clementi et al., 2018).</p><p>Baroclinic high resolution numerical experiments have been performed including a tidal potential and</p><p>forcing the model at the Atlantic boundaries with tidal elevation downscaled from a global model</p><p>FES2014 and tidal velocity derived from the TUGOm (http://sirocco.omp.obsmip.</p><p>fr/ocean_models/tugo) ocean hydrodynamic model. The experiments have been carried out</p><p>including the 8 most relevant tidal constituents in the Mediterranean Sea, namely M2, S2, K1, O1, K2,</p><p>N2, P1 and Q1.</p><p>In this work, first results of baroclinic tidal model experiments are presented together with their</p><p>validation with respect to insitu and satellite data as well as comparing with available literature studies.</p><p>In particular the harmonic analysis of tidal amplitudes and phases highlight the model ability to</p><p>correctly represent the tide gauges observations in the whole basin and in the areas of large tidal signal.</p>

Estuarine Exchange Flow Variability in a Seasonal, Segmented Estuary

Small estuaries in Mediterranean climates display pronounced salinity variability at seasonal and event time scales. Here, we use a hydrodynamic model of the Coos Estuary, Oregon, to examine the seasonal variability of the salinity dynamics and estuarine exchange flow. The exchange flow is primarily driven by tidal processes, varying with the spring–neap cycle rather than discharge or the salinity gradient. The salinity distribution is rarely in equilibrium with discharge conditions because during the wet season the response time scale is longer than discharge events, while during low flow it is longer than the entire dry season. Consequently, the salt field is rarely fully adjusted to the forcing and common power-law relations between the salinity intrusion and discharge do not apply. Further complicating the salinity dynamics is the estuarine geometry that consists of multiple branching channel segments with distinct freshwater sources. These channel segments act as subestuaries that import both higher- and lower-salinity water and export intermediate salinities. Throughout the estuary, tidal dispersion scales with tidal velocity squared, and likely includes jet–sink flow at the mouth, lateral shear dispersion, and tidal trapping in branching channel segments inside the estuary. While the estuarine inflow is strongly correlated with tidal amplitude, the outflow, stratification, and total mixing in the estuary are dependent on the seasonal variation in river discharge, which is similar to estuaries that are dominated by subtidal exchange flow.

Role and Impact of Hydrograph Shape on Tidal Current-Induced Scour in Physical-Modelling Environments

For physical model tests, the time-varying characteristics of tidal currents are often simplified by a hydrograph following a shape of a unidirectional current or by resolving the tidal velocity signal into discrete steps of constant flow velocity. The influence of this generalization of the hydrograph’s shape on the scouring process in tidal currents has not yet been investigated systematically, further increasing the uncertainty in the prediction of scour depth and rate. Therefore, hydraulic model tests were carried out to investigate and quantify the influence of the hydrograph shape on the scouring processes under tidal currents. Several different hydrographs including those with continuously changing velocities, constant unidirectional currents, square-tide velocities and stepped velocity time series were analyzed. Results show that the scouring process in tidal currents is characterized by concurrent sediment backfilling and displacement which can only be reproduced by hydrographs that incorporate a varying flow direction. However, if only a correct representation of final scour depths is of interest, similar scour depths as in tidal currents might be achieved by a constant, unidirectional current, provided that a suitable flow velocity is selected. The effective flow work approach was found capable to identify such suitable hydraulic loads with reasonable practical accuracy.

Regional-scale paleobathymetry controlled location, but not magnitude, of tidal dynamics in the Late Cretaceous Western Interior Seaway, USA

Despite extensive outcrop and previous sedimentologic study, the role of tidal processes along sandy, wave- and river-dominated shorelines of the North American Cretaceous Western Interior Seaway remains uncertain, particularly for the extensive mid-Campanian (ca. 75–77.5 Ma) tidal deposits of Utah and Colorado, USA. Herein, paleotidal modeling, paleogeographic reconstructions, and interpretations of depositional process regimes are combined to evaluate the regional-scale (hundreds to thousands of kilometers) basin physiographic controls on tidal range and currents along these regressive shorelines in the “Utah Bight”, southwestern Western Interior Seaway. Paleotidal modeling using a global and astronomically forced tidal model, combined with paleobathymetric sensitivity tests, indicates the location of stratigraphic units preserving pronounced tidal influence only when the seaway had a deep center (∼400 m) and southern entrance (>100 m). Maximum tidal velocity vectors under these conditions suggest a dominant southeasterly ebb tide within the Utah Bight, consistent with the location and orientation of paleocurrent measurements in regressive, tide-influenced deltaic units. The modeled deep paleobathymetry increased tidal inflow into the basin and enhanced local-scale (tens to hundreds of kilometers) resonance effects in the Utah Bight, where an amphidromic cell was located. However, the preservation of bidirectional, mudstone-draped cross-stratification in fine- to medium-grained sandstones requires tides in combination with fluvial currents and/or local tidal amplification below the maximum resolution of model meshes (∼10 km). These findings suggest that while regional-scale controls govern tidal potential within basins, localized physiography exerts an important control on the preservation of tidal signatures in the geologic record.