Displays

Sea-level rise (SLR) can modify not only total water levels, but also tidal dynamics. Several studies have investigated the effects of SLR on the tides of the western European continental shelf (mainly the M2 component). Idier et al. (2017) further investigate this issue using a modelling-based approach, considering uniform SLR scenarios from −0.25 m to +10 m above present-day sea level. Assuming that coastal defences are constructed along present-day shorelines, the patterns of change in high tide levels (annual maximum water level) are spatially similar, regardless of the magnitude of sea-level rise (i.e., the sign of the change remains the same, regardless of the SLR scenario) over most of the area (70%). These changes are generally proportional to SLR, as long as SLR remains smaller than 2 m. Depending on the location, they can account for +/−15% of regional SLR. Changes in high tide levels are much less proportional to SLR when flooding is allowed, in particular in the German Bight. However, some areas (e.g., the English Channel) are not very sensitive to this option, meaning that the effects of SLR would be predictable in these areas, even if future coastal defence strategies are ignored.

In the present work, we focus on the mechanisms driving these tide changes, especially the bed friction damping, the resonance properties and the reflection at the coast, i.e., local and non-local processes. Additional simulations are done to quantify the effect of these mechanisms on tide changes.

Reference: Idier D., Paris F., Le Cozannet G., Boulahya F., Dumas F. (2017) Sea-level rise impacts on the tides of the European Shelf. Continental Shelf Research, 137, 56-71.

How to cite: Idier, D., Paris, F., Le Cozannet, G., Boulahya, F., and Dumas, F.: Sea-level rise impacts on the tides of the European Shelf: mechanisms analysis, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2835, https://doi.org/10.5194/egusphere-egu2020-2835, 2020.

The Reykjanes Ridge is a key topographic structure stretching south of Iceland and located at the crossroad of the Atlantic Meridional Overturning Circulation upper and lower limbs. It has been inferred to host significant mixing and water mass transformation, yet the mechanisms at play remain obscure. Using data from an array of 7 moorings deployed for two years over the Reykjanes Ridge in a cross-ridge direction, we computed internal waves’ energy density and energy fluxes in the dominating wavebands, i.e., near-inertial and semi-diurnal (tidal) bands to assess the contribution of several mechanisms at fuelling a route to energy dissipation and mixing. Internal tide fluxes are dominating the energy fluxes by an order of magnitude right on top of the ridge and follow a clear spring-neap cycle; but rapidly fade away and become decoherent O(100) km away from the ridge, suggesting a strong scattering by mesoscale turbulence and seafloor topography. Near-inertial energy fluxes and density are surface-intensified and follow a seasonal cycle, with a winter intensification due to storms and intense low-pressure weather systems. The level of near-inertial energy density is roughly explained by the local wind power input, and rapid decay of energy with depth suggests that most of the dissipation occurs in the surface layers, thus is not important for deep water mass transformation. 

How to cite: Vic, C., Ferron, B., Salaün, I., Thierry, V., and Mercier, H.: Tidal and near-inertial energy density and energy fluxes over the Reykjanes Ridge, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10007, https://doi.org/10.5194/egusphere-egu2020-10007, 2020.

The impact of Arctic sea ice decline on future global tidal and storm surge extreme water levels is unknown. Regional studies show that the impact can be substantial; causing increased erosion and posing higher risks to fragile Arctic ecosystems in low-lying areas. Since Arctic tides and surges influence global water levels, consequences of Arctic sea ice decline will be noticed across the globe. In the ongoing FAST4Nl project, an Arctic Total Water Level model will be used to quantify this impact. The model will be developed as an extension of the operational Global Tide and Surge Model (GTSM) and includes the effect of sea ice on tides.

Here we present the results of a study on the seasonal variability of the M₂ tide with respect to differences in sea ice cover. The effect of sea ice on the M₂ amplitude was modelled for minimal and maximal sea ice configurations. In addition, tidal harmonic analysis was performed on a global tide gauge data set, supplemented by SAR altimeter derived water levels from the Arctic region. The high along-track resolution of SAR altimeters (300 m) enables to derive water levels from leads in the sea ice. Here, the retrieved sea surface heights within a given region were stacked, in order to obtain a sufficiently large data set for analysis of the predominantly ice-covered areas. This allowed to gain insight in the seasonal modulation of both local and global tides and directly relate these processes to variations in sea ice.

How to cite: Bij de Vaate, I., Vasulkar, A., Slobbe, C., and Verlaan, M.: The impact of Arctic sea ice cover on seasonal modulation of the M2 tide, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-16421, https://doi.org/10.5194/egusphere-egu2020-16421, 2020.

In this presentation we combine several model and observational data sets to better understand the dissipation of the diurnal and semidiurnal internal tide in the global ocean, which is relevant for maintaining the global overturning circulation. We compute depth-integrated internal tide dissipation rates from a realistically-forced global HYbrid Coordinate Ocean Model (HYCOM) simulation with a horizontal resolution of 4 km (1/25 degrees) and 41 layers. We also compute dissipation rates from altimetry in two ways: 1) from the low-mode flux divergence away from topography and 2) by fitting exponential decay curves along low-mode internal tide beams. The internal-tide sea-surface height amplitude is computed with a least-squares harmonic analysis over a 20+ year altimetry data set. Hence, the altimetry-inferred dissipation rates both reflect the tidal dissipation and the energy scattered from the stationary to the nonstationary internal tide. To account for the dissipation of the nonstationary tide, we apply a spatially-varying correction factor to the stationary dissipation inferred from altimetry.  This correction factor is computed from a global 8-km HYCOM simulation with a duration of 6 years, from which the stationary and nonstationary internal tides can be easily isolated. We compare the simulated and the corrected altimetry-inferred dissipation rates with dissipation rates from finescale and microstructure observations. Preliminary results show that the simulated dissipation is up to a factor of two larger than the depth-integrated dissipation rates inferred from finescale methods, but smaller than the dissipation rates from microstructure.

How to cite: Buijsman, M., Kaur, H., Zhao, Z., Waterhouse, A., and Whalen, C.: The dissipation of the internal tide inferred from a global ocean model, altimetry, and in-situ observations, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12500, https://doi.org/10.5194/egusphere-egu2020-12500, 2020.

The Free Core Nutation (FCN) is a retrograde mode related to the slight misalignment of the rotation axis of the fluid outer core and the elastic mantle, with a period of about 430 sidereal days in the celestial frame. In the Earth-fixed reference frame, the (complex) frequency of the Free Core Nutation (FCN) is inside the diurnal tidal band and causes a resonant response (Free Core Resonance, FCR) of some diurnal tidal waves to the tide-generating forces.
The FCN is usually investigated through its effects on gravity tides and Earth nutations. Here we analyse about 7 years of discontinuous strain records from two 90-m long laser interferometers (strainmeters) operating under the Gran Sasso (Italy) massif and about 4.6 years of discontinuous strain records from two 70-m-long laser interferometers operating the Central Pyrenees (Spain).
Starting from the expressions for the vector displacements due to diurnal and semi-diurnal solid tides, we express  extension along any azimuthal direction in terms of three complex parameters (related to areal strain and the two shear strain components), which are functions of the latitude-dependent Love and Shida numbers. Those three complex parameters are affected by the FCR through three complex resonance strengths.
We find that we can infer 4 model parameters from the inversion of our data, i. e. from the comparison between amplitudes and phases of the measured and theoretical diurnal tides close to the resonance: the FCN period, the FCR quality factor, the imaginary part of one of the three resonance strengths, and the real part of another resonance strength. However, local deformation is distorted with respect to regional deformation because of siting effects. Coupling between local extension (measured by the interferometers) and regional deformation can be described by three coupling coefficients per interferometer, thus introducing 12 additional unknown in the inversions.
We minimize misfit between amplitudes and phases of the measured and theoretical tidal strain jointly for all the interferometers by sampling the 4D model parameter space, while optimal coupling coefficients for each interferometer are computed through a simple matrix inversion at each sampled point.
Theoretical strain tides is corrected for the effects of the water load oscillations caused by ocean tides. We use FES2014 and TPXO9 ocean models, while the appropriate Earth model for different ocean load areas is chosen basing on the widths of the continental shelves nearby the stations and the inversion misfits.
Although we analyse records from two stations only and the amount of data is relatively small, our results for the FCN period and (to some extent) the FCR quality factor are robust and comparable to those obtained from gravity tides and  nutations. Moreover, we obtain reliable values of the resonance strengths and robust estimates of the coupling coefficients for all the interferometers.

How to cite: Amoruso, A. and Crescentini, L.: Free Core Resonance parameters from diurnal strain tides recorded by the Gran Sasso (Italy) and Canfranc (Spain) underground geodetic interferometers, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10392, https://doi.org/10.5194/egusphere-egu2020-10392, 2020.

Tides always behaves different rising and falling durations, which can mostly attribute to the shallow-water effect and interactions among tidal constituents. The duration asymmetry may lead to an inequality in flood/ebb tidal current magnitudes, affecting the net sediment transport. Tidal duration asymmetry has time-dependent characteristics. We deducted a general framework for identifying the time-variability in tidal duration asymmetry. The application to the global tides showed that the fortnightly variability in tidal asymmetry is universal and that duration asymmetry can be stronger during neap tide than during spring tide. Then the framework is applied to the tides in the Changjiang Estuary. Prominent seasonal variation in tidal asymmetry is revealed, mainly relate to the river-tide interaction. Application to the tides in the Yangshan Harbor sea area revealed that the local-scale tidal asymmetry can be changed strongly by a large coastal engineering.

How to cite: Wenyun, G., Dehai, S., Leicheng, G., Jianzhong, G., Pingxing, D., and Xiaohua, W.: The time-varying characteristics in tidal duration asymmetry, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4387, https://doi.org/10.5194/egusphere-egu2020-4387, 2020.

Nuisance flooding (NF) or high tide flooding describes minor nondestructive flooding which can nonetheless cause substantial negative socio-economic impacts to coastal communities. The frequency of NF events has increased and accelerated over the past decades along the U.S. coast, leading to changes ranging from 300% to 900%. This is mainly a result of sea level rise reducing the gap between high tidal datum and flood thresholds. While long-term relative sea level rise is the main driver for the increased number of NF events, other factors such as variability in the Gulf stream, the storm climate, and infragravity waves can also contribute. Another important driver that is often overlooked is related to changes in coastal and estuary tides, through secular trends in the amplitudes of major tidal constituents. In this presentation we assess the role of tidal changes in modulating the frequency of NF events along the U.S. coastline. We analyze hourly records from 49 U.S. tide gauges for which the National Weather Service has defined NF thresholds. We find that (1) overall across all tide gauges the number of NF days has increased since 1950 due to changes in coastal tides, adding up to 100 NF days in recent years (on top of the increase due to relative sea level rise), (2) more tide gauges experience an increase in NF events than a decrease due to changes in tides, (3) tide gauges in major estuaries which have undergone major anthropogenic alterations experience the strongest changes; in Wilmington (Cape Fear estuary), for example, 10-40% of NF events in recent years can be attributed to tidal changes. 

How to cite: Li, S., Wahl, T., Jay, D., Talke, S., and Liu, L.: The effects of tidal changes on the frequency of nuisance flooding events in the United States, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12115, https://doi.org/10.5194/egusphere-egu2020-12115, 2020.

The quality of global ocean tide models has increased drastically over the last decades due to the availability of dense open-ocean observations from satellite altimetry. In regions of poor altimetry coverage (e.g., polar seas and coastal areas) and for minor tides with a small signal-to-noise ratio, however, reliable estimates from unconstrained global numerical models are still (and will remain) critically important. We will present in this contribution recent results from the purely-hydrodynamic, barotropic tidal model TiME (Weis et al., 2008) that benefit from a newly introduced rotated grid avoiding the singularity at the North Pole; a revised scheme for dynamic feedbacks of self-attraction and loading; and revised bathymetry data-sets that also include water column height modifications in cavities underneath the Antarctic ice-shelves.

By focussing exemplarily on the M₂ tide, we will demonstrate the individual impact of all those changes on the simulated water height variations. It will be shown that the effects of ice-shelf cavities extend well beyond the Southern Ocean and affect even amphidromic systems in the Northern Hemisphere. We will also emphasize the ability of unconstrained numerical models as TiME to explicitly simulate minor tidal lines, thereby allowing to thoroughly test (and subsequently improve) admittance-based methods currently employed for the processing of satellite gravimetry data from the GRACE and GRACE-FO missions.

How to cite: Sulzbach, R., Dobslaw, H., and Thomas, M.: New unconstrained global ocean tide solutions for satellite gravimetry including minor tides, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10207, https://doi.org/10.5194/egusphere-egu2020-10207, 2020.

We present the results of our studies of singular value decomposition (SVD) of the forward operator in tidal analysis. Using the resolution matrix and the ratio between singular values, we distinguish significant contributions that compose the tidal signal and we study cross-talk within and between tidal groups. Using all harmonics from the tidal catalogue we investigate the resolution matrix properties with decreasing amplitude of harmonics. We demonstrate the loss of resolution even for harmonics of large amplitude with decreasing time-series length. Our further investigation shows the cross-talk from atmospherically induced gravity variation into a tidal signal (expected and unexpected, e.g. S1, Fi1, Sig1). We investigate the ability to determine the ratio of gravimetric factors of degree 2 and degree 3 tides from the specific tidal gravity signal recordings.

The main interest of tidal analysis is the accurate and precise determination of tidal parameters, which are amplitude (gravimetric) factor and phase lag, the quantities describing the Earth response to the tidal forcing. Tidal catalogues define the tide generating potential in terms of harmonics. Widely used software, like ETERNA or Baytap-G, uses a-priori grouping of harmonics which is based on reasonable considerations like the Rayleigh criterion of spectral resolution. Wave grouping is a model parameterisation used to make the analysis problem overdetermined by using assumptions regarding the model parameters (e.g. credo of smoothness, known free-core resonance parameters, known ratio between response to degree 2 and degree 3 forcing). If those assumptions are incorrect, this can lead to artefacts which might go unnoticed. This presents a limitation for example in the search for causes of temporal variation of tidal parameters, as reported recently. SVD of the unparameterised problem allows us to investigate these limitations.

In our analysis, SVD is a factorisation of a linear regression matrix. The regression matrix consists of tidal harmonics in-phase and quadrature signal for rigid Earth tide (tidal forcing to Earth surface). We compute time series for each harmonic present in Tamura tidal catalogue by using a modified version of "Predict" (ETERNA package). Resulting values can be, but do not need to be, grouped prior to SVD analysis. Other than with conventional programs, wave groups can not only be defined along the frequency axis. They can as well be used to separate harmonics of degree 2 and degree 3. SVD allows us to study the significance of tidal harmonics, cross-talk between harmonics or groups and matrix null space. Thus, we can discriminate the parameters with small singular value, which do not significantly contribute to the predicted tidal data or are noise-sensitive.

How to cite: Ciesielski, A. and Forbriger, T.: Resolution and significant contributions of tidal forcing in flexible harmonic grouping computed using Singular Value Decomposition, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-17758, https://doi.org/10.5194/egusphere-egu2020-17758, 2020.

Combined tide and surge models are very useful tools to issue warnings for storm surges as well as for assessment of the potential impacts of sealevel rise. Over the past decade, a Global Tide and Surge Model (GTSM) has been developed by Deltares with improvements in physics, grid resolution and skill in the each new version. The uncertainties in bathymetry and friction are currently a major part of the remaining model uncertainty. Improved estimates of these parameters would be desirable, but the required computing speed and memory storage are limiting the possibilities at the moment. Here, we propose an efficient coarse grid parameter estimation scheme for the high resolution GTSM to estimate the bathymetry. OpenDA software is combined with GTSM using DUD algorithm (Does not Use Derivative) making use of the FES2014 dataset as observations in deep water. Even though parallel computing is implemented for model simulation, calibration of the fine grid model directly would still require too much computer time, for instance, e.g. it takes 9 hours on 20 cores to simulate 45 days and calibration typically requires many of these simulations. Therefore, a coarse-to-fine strategy is developed by replacing the fine grid with coarse grid in parameter estimation iterations to reduce the computing cost by 67%. Moreover, via a sensitivity analysis we are able to reduce the parameter dimension from O(10⁶) to O(10²) which leads to a  further reduction of the required computation and memory. The results of the estimation and model validation demonstrate the parameter estimation scheme to improve the accuracy of the model by approximately 30% with affordable computational and storage demands.

How to cite: wang, X., Verlaan, M., and Lin, H. X.: Efficient Calibration of a Global Tide and Surge Model, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5446, https://doi.org/10.5194/egusphere-egu2020-5446, 2020.

Concern over increased flooding and the need for earlier and more reliable risk forecasts motivate the continued development of operational forecasts of coastal water level. We report here on results from a year long ensemble of total water level forecasts calculated using a dynamical ocean model forced with ensemble atmospheric forcing and tidal boundary conditions. We focus on the east coast of Canada. The domain includes the Gulf of St. Lawrence, the Labrador Shelf, the Scotian Shelf, and the Gulf of Maine. The water level ensemble is made of a control and 20 perturbed members. Individual forecasts are produced twice daily for 16 days.

The novelty of the present study is in the exploration of perturbations of the ocean contributions. In addition to examining how uncertainty in atmospheric forcing maps into flood risk, we also explore the feasibility, and impact, of perturbing the ocean tides. We use a recent case study to demonstrate our findings.

How to cite: Bernier, N., Huziy, O., Thompson, K., Wang, P., Pouliot, B., and Pell, S.: Ensemble Water Level Prediction System: Improving the Representation of Model Uncertainty , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-22142, https://doi.org/10.5194/egusphere-egu2020-22142, 2020.

The study is dedicated to the tidal dynamics in the Sylt-Rømø Bight with a focus on the non-linear processes. The FESOM-C model was used as the numerical tool, which works with triangular, rectangular or mixed grids and is equipped with a wetting/drying option. As the model’s success at resolving currents largely depends on the quality of the bathymetric data, we have created a new bathymetric map for an area based on recent studies of Lister Deep, Lister Ley, and the Højer and Rømø Deep areas. This new bathymetric product made it feasible to work with high resolution grids (up to 2 m in the wetting/drying zone). As a result, we were able to study the tidal energy transformation and the role of higher harmonics in the domain in detail. The tidal ellipses, maximum tidally-induced velocities, energy fluxes and residual circulation maps were constructed and analysed for the entire bight. Additionally, tidal asymmetry maps were introduced and constructed. The full analysis was performed on two grids with different structures and showed a convergence of the results as well as fulfillment of the energy balance. The tidal residual circulation and asymmetric tidal cycles largely define the circulation pattern, transport and accumulation of sediment and the distribution of bedforms in the bight, therefore the results are necessary and useful benchmarks for further studies in the area, including baroclinic and sediment dynamics investigations.

How to cite: Fofonova, V., Androsov, A., Sander, L., Kuznetsov, I., Amorim, F., Hass, H. C., Mielck, F., and Wiltshire, K. H.: Tidal asymmetry in the Sylt-Romo Bight, south-eastern North Sea, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-1359, https://doi.org/10.5194/egusphere-egu2020-1359, 2020.

The harmonic representation of inequalities (HRoI) is a technique for tidal analysis and prediction. The HRoI has been used at BSH for over six decades to calculate heights and times of high and low waters for German tide tables. In its original form, it is tailored to predict the vertices of semi-diurnal tides. A more generalized version of the HRoI allows analysing the full tidal curve at equal fractions of the mean lunar day. We compare results from the HRoI with other tidal analysis techniques, e.g. the common harmonic method, for locations in the German Bight. The study includes several tide gauges in the rivers Ems, Weser and Elbe. Short durations of rise and rapid water level changes after low water often characterize the tide curves in these rivers and pose challenges to tidal predictions.

How to cite: Boesch, A. and Jandt-Scheelke, S.: A comparison study of tidal prediction techniques for applications in the German Bight, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-1640, https://doi.org/10.5194/egusphere-egu2020-1640, 2020.

Analyses of water level measurements in the Gulf of Maine and Bay of Fundy suggest interannual changes in the tidal M₂ amplitude of significant size (∼1–5 cm) and remarkable spatial coherence. These signals are of unknown origin and cannot be caused by the observed sea-level fluctuations of less than 10 cm. Here we use a regional ocean model setup to link interannual M₂ variations to changes in stratification, which may alter the surface tide through processes of turbulent mixing and baroclinic wave scattering as the tide propagates into and across the gulf. We run short (20-day) simulations at 1/30° horizontal resolution, omit atmospheric forcing, and prescribe background temperature and salinity fields at annual intervals starting in 1993. Tidal velocities are introduced along with geostrophic currents at open boundaries and are not varied in between runs. M₂ amplitudes inferred from these experiments exhibit a year-to-year variability of 1–2 cm throughout the Gulf of Maine, mostly reflecting the sign (but not always the magnitude) of measured amplitude changes at tide gauges Boston, Portland, and Eastport. In particular, we are able to reproduce the 3-cm drop in the M₂ time series from 1993 to 1998 and the subsequent increase by 1.5 cm until the year 2000. Over the period 1993–2013 and at all three stations, our simulations explain 22–29% of the locally observed M₂ variance, with linear correlations ranging from 50 to 56%. Although model sensitivities and the exact mechanisms underlying these signals are yet to be worked out, our study provides appreciable evidence that varying stratification may indeed be a significant driver for the gulf's tidal changes on interannual and perhaps secular time scales.

How to cite: Kotzian, D., Schindelegger, M., Green, M., and Stolzenberger, S.: Interannual stratification changes affect tides in the Gulf of Maine, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8382, https://doi.org/10.5194/egusphere-egu2020-8382, 2020.

Taiwan is an island entirely surrounded by oceans, so living and economics are significantly influenced by the oceans. The electronic navigational chart system is extremely important for improving the safety of marine navigation and ocean depth is the essential data for electronic charts. Sea surface variations affected by ocean tide and sea level change are the main error sources in hydrographic surveys since the traditional tidal correction only using tide gauge stations, ignoring geographically non-uniform ocean tides and sea level anomalies around Taiwan. In this research, we evaluate two factors impacting the accuracy of hydrographic surveys, including ocean tides and seasonal sea level variations, using tide gauge records, satellite altimeter data and ocean tide models around Taiwan, and also analyze the accuracy of the ocean tide models around Taiwan. In addition, sea level anomalies are strongly influenced by climate changes in recent years. An understanding of seasonal sea level cycle and its spatial and temporal changes are importance because its temporal changes can result in the variation of the frequency and magnitude of coastal hazards. Therefore, we will apply the Ensemble Empirical Mode Decomposition to sea level data to assess the stability of the long-term seasonal sea level fluctuations with time.

How to cite: Lan, W.-H., Kuo, C.-Y., Lin, S.-F., and Lu, C.-H.: Impact of sea level variations on hydrographic survey around Taiwan, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-13544, https://doi.org/10.5194/egusphere-egu2020-13544, 2020.

The city of Venice (Northern Italy), together with its lagoon, is a historic, cultural and artistic heritage of inestimable value. One of its peculiarities consists in the recurrent storm surge phenomena, referred to as acqua alta. Sea level rise and local subsidence made their frequency to increase dramatically with respect to the past, causing severe damages to the lagoon and in particular to the city centre, as during the exceptional high tide verified on November 12, 2019.
Here we show the analysis of the historical time series of tidal maxima and minima recorded in the Venetian lagoon, covering the period 1872-2018. It is the longest and most complete historical series of the Venetian area and one of the longest records of the entire Mediterranean region. During this period, the relative sea level height has increased of about 30 cm with respect to the reference level, while the average number of acqua alta events – evaluated over a 40-year time interval - has passed from about 4 to 70 per year. These events usually occur during the fall season (from October to December), even if a not negligible number has been also recorded during winter. Therefore, we analyse the October-March average annual time series with advanced spectral analysis methods, like Monte Carlo Singular Spectrum Analysis (MC-SSA), to extract and reconstruct the significant variability modes characterizing the record. They are the increasing long-term trend and components with multidecadal, decadal and interannual periods. The trend results from the superposition on the global eustacy of the local subsidence affecting the Venetian lagoon, which is due to both natural causes and human activities. We also discuss the possible linkage of the other significant spectral components to large scale climatic patterns. In particular, the decadal-scale oscillation is one of the most important variability modes affecting Northern Italian hydrology.
Finally, we apply simple statistical methods (autoregressive models and feed-forward neural networks) to forecast the long-term evolution of sea level over the next ten years. In this contribution, we illustrate results from this state of the art two-fold statistical prediction system that provides robust predictions of sea level in the Venetian lagoon for the next decade and discuss them in the light of current longer-term projections of future sea level rise. Finally, we will test the predictive skill of the applied methods using tidal measurements recorded during 2019, to verify if our predictions are able to describe tidal variability characterizing the current year.       

How to cite: Rubinetti, S., Taricco, C., Zanchettin, D., Arnone, E., and Rubino, A.: Sea level rise in the Venetian lagoon inferred from the 150-year-long tidal record, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-1046, https://doi.org/10.5194/egusphere-egu2020-1046, 2020.

Information on extremes of the sea-level is obtained from tide-gauge
records.  Such records may have gaps.

Estimates of potential changes in the size and/or frequency of sea-level
extremes are hampered by long gaps, or when just the high extremes are
missing due, e.g. to equipment failure.

Methods used for filling such gaps can be based on having multiple
records from gauges near each other; but what to do if there is
only one record? This problem can typically occur when old tide-gauge
records are used -- the use of multiple recorders at the same place is
more wide-spread today. However, especially older and therefore longer
records hold the key to obtaining long-baseline insights into the temporal
evolution of extreme tides and thus impacts of e.g. climate change.

In this work, we review and assess methods for gap filling. We asses using
the 'known truth' method, i.e. by applying realistic gaps to complete
gauge records and reconstructing and then comparing errors calculated as
the diffrence between modelled and actual values.  We compare a simple
harmonic model fit method to various spline methods as well as Neural
network and deep learning approches.  We also test a hybrid method
which uses not just tide-gauge data but also air pressure readings
from a meteorological station near the tide-gauge.

We then attempt to fill in the missing maxima of the Esbjerg, Denmark
hourly tide-gauge record since 1889. Particularly, before 1910 the maxima
above 300 cm are missing (Bijl, et al., 1999), and we try to fill these in.

How to cite: Thejll, P.: Review and Assessment of gap-filling methods from tide-gauges: Maxima missing at the Esbjerg, Denmark station, before 1910., EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4487, https://doi.org/10.5194/egusphere-egu2020-4487, 2020.

Tide gauges provide a vital component in coastal flooding alert systems, and as a record of past events. They are used to record short duration extremes such as tsunamis, storm surges lasting a few hours, regular tides, and long term changes in relative sea-level. Globally, there is far more tide gauge data in existence than is available in the public domain for research. A significant factor obstructing the release of data is that quality control of tide-gauge records is still carried out with a great deal of manual inspection, and is therefore labour-intensive. Automated systems must carefully distinguish between spikes due to instrumental error and genuine rare extreme events; and between damaged instruments and still water. The National Oceanography Centre automatic quality control software aims to enable analysis of any high-frequency tide-gauge record around the world with minimal manual intervention or parameter selection. We demonstrate the implementation in Matlab and discuss the successes and challenges of the software.

How to cite: Williams, J. and Matthews, A.: Automating Tide Gauge Quality Control, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-1561, https://doi.org/10.5194/egusphere-egu2020-1561, 2020.

Recent research of coupled tidal and tectonic modelling has found that during periods in an ocean’s Wilson cycle, (i.e. during dispersal, and subsequent convergence of oceans due to plate tectonic movement), oceans occasionally become resonant for the semi-diurnal component of the tide (M₂). This results in an approximately 20-Million-year long period of enhanced tidal dissipation in the resonant ocean (assuming continental plate drift rates of ~5 cm yr^-1). This resonant “Super-tide” has been simulated in numerical tidal models for both past and future tectonic scenarios, and they show that the current tides are among the most energetic found.

Here we use an established tidal model to analyse the conditions required for open ocean tidal resonance. Our conceptual “Earths” consist of two or more simplified oceans, which are shaped to represent conceptual versions of oceans of the past, present, and future: triangular (Tethys ocean), circular (Pacific and Arctic oceans), rectangular (Southern and Indian oceans), and rhomboid shaped (North, and South Atlantic Ocean). Each scenario was conducted using ocean bathymetry ranging from a “bathtub” ocean (a uniformly deep flat abyssal plane from coast to coast), to a continental shelf with no abyssal bathymetry, to a “realistic” ocean with ocean shelves, ridges, and subduction zones. The global ocean land ratio and ocean volume was conserved to present-day in most conceptual scenarios however, to investigate the maximum tidal dissipation possible on Earth, some scenarios deviated from the ocean volume and global coverage. In every scenario, ocean width is progressively increased relative to the predominant ocean boundaries, simulating plate tectonic opening of each ocean.

The aim of the work was to assess the frequency of the occurrence of resonance in the open ocean, and the upper limit for tidal dissipation of the semi-diurnal tide on Earth. We found that super-tides are common in the results with their dissipative strength varying from weaker than present day to five times present day.

The occurrence of tidal resonances in modelled conceptual oceans further confirms the link between tectonics and tidal evolution. These super-tidal periods of markedly increased tidal dissipation alter the ocean’s energy budget, nutrient dispersal and the carrying capacity of coastal and oceanic ecosystems.

How to cite: Davies, H., Green, J. A. M., and Duarte, J. C.: Back to the future 3: Analysing tectonically induced tidal resonance with conceptual models, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-596, https://doi.org/10.5194/egusphere-egu2020-596, 2020.

The global mean sea-level decrease of 120 – 130 m during the Last Glacial Maximum (LGM; 26 – 19 kyr BP) is thought to have substantially altered semidiurnal tidal dynamics in the glacial North Atlantic. This more than doubled global open ocean tidal dissipation in comparison to present day and increased the amount of energy available for diapycnal mixing which is important for driving the global meridional overturning circulation. Reconstructions of the glacial ocean have generally suggested a more sluggish Atlantic meridional overturning circulation (AMOC) during the LGM together with weaker mixing. Here, we investigate the impact of tidal dissipation changes on the LGM AMOC and the carbon cycle using the intermediate complexity ocean model UVic coupled to the biogeochemistry model MOBI forced with three different LGM dissipation estimates. The simulations are constrained with LGM δ¹³C and radiocarbon data from sediments. Our results suggest that our simulations, as previously inferred, most closely agree with a weakened LGM AMOC (8 – 9 Sv), and importantly, that the agreement is consistent with increased LGM tidal mixing. These results firstly imply that a weakened AMOC state can occur with stronger tidal mixing without hampering the agreement with the sediment isotope data. Secondly, this work highlights the importance of considering tidal dissipation changes when modelling the paleo-ocean.

How to cite: Wilmes, S.-B., Green, M., and Schmittner, A.: The impact of tidal dissipation changes on the Last Glacial Maximum AMOC, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9930, https://doi.org/10.5194/egusphere-egu2020-9930, 2020.

The intracratonic Paraná Basin and its western extension - Chaco-Paraná Basin - are located in the south-central portion of South America and cover an area of about  ~1.8 million km², including portions in Brazil, Argentina, Uruguay, and Paraguay. The Early Permian epicontinental sea was shallow and likely connected with the Panthalassa in the southern portion of Uruguay (Lavina, 1992). The transgressive sedimentary succession of the Guatá Group is composed of coastal plain and shallow marine deposits (Rio Bonito Formation), in complex associations due to base level fluctuations and irregular deglaciation paleotopography, and offshore-transitional deposits (Palermo Formation) (Lavina and Lopes, 1987). The Rio Bonito Formation is mostly preserved within paleovalleys carved by glaciers and tectonic. The tidal-influence in this formation occurs throughout the succession and are mainly characterized by medium- to coarse-grained arkosic and quartz sandstones with uni- and bidirectional cross-bedding, herring-bone cross-bedding, tidal bundles, reactivation surfaces, mud drapes, and double mud drapes (Fritzen et al., 2019; Lopes and Lavina, 2001). Besides the tidal sedimentological aspects, the conditions that governed tide in this epicontinental sea are poorly understood. In this work, we present a theoretical perspective on the behavior of tides in the Paraná Basin epicontinental sea during the Early Permian. Mathematical models were applied to test the existence of amphidromic points in the basin, to verify the possibility of resonance, as well as to test the tidal amplification inside two paleovalleys. The obtained results were compared to Hudson Bay, considered here a modern analog. According to the paleogeography, paleolatitude (Southern Hemisphere), depositional records and insights from the modern analog, the studied Early Permian epicontinental sea likely had bear more than one clockwise-rotation amphidromic system. Resonant effects may also have affected circulation, especially at sea depth below 100 m. In the simulated scenarios, tidal amplification in both valleys was variable but concentrated between micro to mesotidal amplitudes. This is the first contribution to the understanding of the tidal behavior of the Early Permian epicontinental sea.

References:

Fritzen, M.R., Cagliari, J., Candido, M., Lavina, E.L., 2019. Tidal bar cyclicity record in the Lower Permian (Rio Bonito Formation of the Paraná Basin). Sedimentary Geology 381, 76-83.

Lavina, E.L., 1992. Geologia sedimentar e paleogeografia do Neopermiano e Eotriassico (intervalo Kanzaniano - Scythiano) da Bacia do Parana. Ph.D. Thesis, UFRGS University.

Lavina, E.L., Lopes, R.C., 1987. A Transgressão Marinha do Permiano Inferior e a Evolução Paleogeográfica do Supergrupo Tubarão no Estado do Rio Grande do Sul. Paula-Coutiana 1, 51-103.

Lopes, R.C., Lavina, E.L., 2001. Estratigrafia de sequências nas Formações Rio Bonito e Palermo (Bacia do Paraná), na região carbonífera do Jacuí, Rio Grande do Sul, in: Severiano Ribeiro, H.J.P. (Ed.), Estratigrafia de sequências: fundamentos e aplicações. Edunisinos, São Leopoldo, pp. 391-419.

How to cite: Candido, M., Cagliari, J., and Lavina, E. L.: Tidal circulation in an Early Permian epicontinental sea: insights from a mathematical modeling approach, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11923, https://doi.org/10.5194/egusphere-egu2020-11923, 2020.