OS4.1

The Global Extreme Sea Level Analysis (GESLA) dataset contains, in its recently released version 3, a total of 5199 tide gauge records of hourly (or higher) temporal resolution, globally distributed and totalling more than 91000 years of data (www.gesla.org). This represents twice the number of observations compared to the former version of the database. The tide gauge records have been compiled from multiple data providers and so they have different levels of quality controls. Here we describe a set of tools to homogenise and quality control sea level observations from raw GESLA files, including adjustments of datum jumps and time shifts in the time series. We apply these tools to estimate tidal constituents from the extended in-situ dataset. The results are used to identify the river influences on coastal tide gauges and to map the spatial patterns of mean tidal ranges along densely monitored coastlines.

How to cite: Marcos, M., Haigh, I. D., Talke, S. A., Hart-Davis, M., Dettmering, D., Woodworth, P. L., and Hunter, J. R.: The new GESLA-3 tide gauge data set and its quality control for tidal studies, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-3944, https://doi.org/10.5194/egusphere-egu22-3944, 2022.

How to cite: Bradshaw, E., Matthews, A., Williams, S., and Hibbert, A.: A new service providing sea level height data using GNSS sensors from around the globe, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-9396, https://doi.org/10.5194/egusphere-egu22-9396, 2022.

How to cite: Williams, J., Matthews, A., and Bradshaw, E.: Using citizen science to digitise 3 million hand-written tide-gauge data entries, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-740, https://doi.org/10.5194/egusphere-egu22-740, 2022.

The Global Tide and Surge Model (GTSM) is a depth-averaged hydrodynamic model, developed by Deltares. GTSM can be used to dynamically simulate water levels and currents, that arise from tides and storm surges. The model is based on Delft3D Flexible Mesh software and has a spatially varying resolution which increases towards the coast. Previous studies with this model used GTSMv3.0 and focused for instance on operational forecasting, reanalysis and climate projections and estimation of return periods (Muis et al., 2020; Dullaart et al., 2021), satellite altimetry (Bij de Vaate, 2021), changes in tides due to sea level rise and various others.

Significant improvements in model performance were made in the newest GTSMv4.1, released in 2021. This model with increased resolution and improved representation of physical processes was calibrated by applying bathymetry and friction correction (Wang et al., 2021). From GTSMv3.0 to GTSMv4.1, the model performance showed great improvements with a 37% reduction of the root-mean-squared-error between modelled and observed tides from 17.8 cm to 11.3 cm.

The model development is an ongoing and continuous effort. The current developments are to improve the grid+bathymetry, representation of the sea-land interface, improving the spatial distribution of internal tide energy dissipation and the inclusion of other baroclinic processes like steric and radiational tides. Preliminary results show improvements in several areas. Furthermore, improving geometry representation by cutting parts of coastal cells with a landboundary often shows to improve the model performance just as significant as a resolution increase, while saving computational cost.

How to cite: Veenstra, J., Muis, S., and Verlaan, M.: Developments of the Global Tide and Surge Model, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-9505, https://doi.org/10.5194/egusphere-egu22-9505, 2022.

Low-frequency non-astronomical changes of tides are among the most puzzling signals in the world ocean. Although the relevance of these signals in the order of a few cm is gradually being appreciated in the context of coastal flooding or de-aliasing of satellite gravimetry observations, a detailed quantitative understanding of the causative mechanisms has been lacking. Among the suspected forcing factors are fluctuations and trends in relative sea level, basin geometry (associated with, e.g., melting Antarctic ice-shelves), bed roughness, and ocean stratification. Here, we use a high-resolution general circulation model to spatially map the influence of stratification changes on the global M₂ tide, on time scales from years out to decades. We conduct global tidal simulations in annually changing density structures, as drawn from hydrographic profiles and other external datasets (e.g., an eddying ocean reanalysis) from 1993 to present day. We perform internal-tide permitting simulations (1/12° horizontal grid spacing, 50 vertical layers) to resolve the relevant physics, particularly low-mode barotropic-to-baroclinic energy conversion at topographic features and vertical mixing in shallow water. Atmospheric forcing is omitted to constrain the model’s density distribution to the prescribed initial hydrography. We validate the resulting annual M₂ amplitude changes against estimates from harmonically analyzed tide gauge series, distributed across the globe. Particular emphasis in our analysis is given to the tropical Pacific and the South China Sea, where the seesawing of stratification between positive and negative phases of ENSO (El Niño-Southern Oscillation) is expected to introduce spatially coherent amplitude modulations of ±1 cm on interannual time scales.

How to cite: Opel, L. and Schindelegger, M.: Modeling the impact of contemporary ocean stratification changes on the global M2 tide, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-4695, https://doi.org/10.5194/egusphere-egu22-4695, 2022.

Satellite altimetry observations have provided a significant contribution to the understanding of global sea surface processes, particularly allowing for advances in the accuracy of ocean tide estimations. Accurate estimations of ocean tides are valuable for the understanding of sea surface processes from along-track satellite altimetry. Ocean tide models have done a suitable job in providing these estimations, however, difficulties remain in the handling of minor tidal constituents. The estimation of minor tides from altimetry-derived products proves difficult due to the relatively small signals of these tides and due to the temporal sampling of the altimetry missions meaning a long time series of observations is required. This is generally solved by models and tidal prediction software by using admittance theory to infer these minor constituents from the more well-known and better estimated major constituents. In this presentation, the results of a recent study that looked at the estimation of several minor constituents directly from tide models compared to the inferred version of these tides are presented. The model used for the direct estimations and the inferences is a regional version of the Empirical Ocean Tide model (EOT) which is a data-constrained model derived from multi-mission satellite altimetry. The resultant estimations from these two approaches are compared to two global numerical tide models (TiME and FES2014) and in situ tide gauge observations (from the TICON dataset). Based on the study of eight tidal constituents, a recommendation of directly estimating four tides (J1, L2, μ2 and ν2) and inferring four tides (2N2, ϵ2, MSF and T2) is given to optimise the ocean tidal correction. Following on from this, a new approach of merging tidal constituents from different tide models to produce the ocean tidal correction for satellite altimetry that benefits from the strengths of the respective models is presented. This concept allows for the benefit of using data-constrained tide models in the estimation of the major constituents as well as the use of numerical models in providing a greater number of minor constituents, to be combined to provide a more optimised estimation of the full tidal signal.

How to cite: Hart-Davis, M., Sulzbach, R., Dettmering, D., Thomas, M., Schwatke, C., and Seitz, F.: The assessment of minor tidal constituents in ocean models for optimising the ocean tidal correction, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-2478, https://doi.org/10.5194/egusphere-egu22-2478, 2022.

The Sea of Azov is the most isolated and shallow sea of the World Ocean. Longterm hourly data from 14 coastal tide gauges were used to study the features of tides in the Sea of Azov. Spectral analysis showed well-defined spectral peaks at tidal diurnal and semidiurnal frequencies. Harmonic analysis of tides for individual annual sea level series with consecutive vector averaging over the entire observation period was applied to estimate mean amplitudes and phases of 11 tidal constituents. The amplitude of the major diurnal harmonics is generally greater than the semidiurnal ones. The amplitude of the diurnal radiational constituent S₁ changes from 6 cm at the head of the Taganrog Bay to 0.5 cm in the Kerch Strait, while the amplitude of the main semidiurnal gravitational harmonic M₂ inside the sea varies from 1.0 cm in the southeastern part of the Sea of Azov, to 0.38 cm at Mysovoye. The tidal form factor within the Sea of Azov changes significantly from the diurnal form in the north to the mixed, mainly semidiurnal near the Kerch Strait. The maximum theoretical tidal range of 19.5 cm were found at the head of the Taganrog Bay, and the lowest was noted in the Kerch Strait, 4.9 cm. The assumption about the predominantly radiational genesis of diurnal tides is confirmed by the seasonal variations of their spectrum. Radiational tides in the Sea of Azov may be initiated by sea breeze winds, which is best expressed in summer.

How to cite: Korzhenovskaia, A., Medvedev, I., and Arkhipkin, V.: Tidal sea level oscillations in the Sea of Azov, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-8346, https://doi.org/10.5194/egusphere-egu22-8346, 2022.

Approximately 70% of the global dissipation of the barotropic tide occurs in the waters of the continental margins, due to bottom friction on the shelves and internal tide generation at the continental slopes. Here we are interested in the latter process, and how it depends upon the presence of submarine canyons, which are a ubiquitous feature of continental slopes. Whilst there have been modeling studies of internal tide generation at particular canyons (e.g., Monterey), our emphasis is on understanding the effects of canyon geometry more generally, given the diversity of canyons that exist across the globe.  

To do this, we study idealised canyon configurations cutting through idealised continental slopes, enabling us to define and then explore a relevant parameter space (canyon length, width, depth, etc.). For forcing by a prescribed barotropic tide, taking the form of a Kelvin wave with predominantly alongshore flow, we investigate both the amplitude and direction of the implied radiating internal tides, and generate scaling laws for how the tidal dissipation varies across parameter space.

Such a study would be challenging and extremely time consuming with traditional ocean circulation models, because of the small length scales of both the canyons and the internal tides. For efficiency, we thus use the multi-modal linear modelling strategy of Griffiths and Grimshaw (2007), but solved with cutting-edge numerics in the form of a Discontinuous Galerkin Finite Element methodology. We have generated high-quality multi-scale triangular meshes to resolve the canyons, and can deploy a range of test-function orders and numerical fluxes therein. This methodology is a key part of this study.

How to cite: Elmes, J., Griffiths, S., and Bokhove, O.: Internal Tide Generation by Submarine Canyons, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-1644, https://doi.org/10.5194/egusphere-egu22-1644, 2022.

The energy budget of the global ocean circulation highlights the importance of winds and tides as main sources of energy. As wind forcing acts at the ocean surface, tidal potential affects the entire water column and, in regions of rough topography, it generates energy conversion from barotropic to baroclinic high frequency modes. An intercomparison is computed between experiments with and without tidal forcing, using a global ocean general circulation model in two different configurations, respectively mesoscale-permitting and mesoscale-resolving ones. Regardless of the resolution, the contribution of tides to the mean kinetic energy is negligible on the global scale, while it enhances the eddy kinetic energy, especially on continental shelves and rough bottom topography sites, where internal waves are generated before being dissipated or radiated away. The interaction between these waves and mesoscale features is enhanced in the higher-resolution experiments, and their effects on the mean circulation are analysed in two regions where the tidal activity is well documented: the North-West Atlantic Ocean and the Indonesian region. We investigate the impact of internal tides presence on the modelled tidal amplitude, and we include a topographic wave drag as an additional term of internal wave dissipation.

How to cite: Borile, F., Masina, S., Iovino, D., Pinardi, N., and Cessi, P.: Tidal effects in a global general circulation model: comparison between coarse and high resolution configurations, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-10432, https://doi.org/10.5194/egusphere-egu22-10432, 2022.

Internal tides (ITs) are internal waves which oscillate at the tidal frequencies. ITs may cross entire ocean basins and along the way, they may be redirected, break, and dissipate. The latter is due to changes in stratification, bottom turbulence, wave-wave interactions, and of interest in this study, the scattering of ITs by balanced flow. Mesoscale wave-vortex interactions are characterized by low Rossby numbers. With the aid of satellite altimetry, the effects of mesoscale eddies on ITs has been used successfully to map low mode IT propagation.  In the submesoscale, these interactions become more complex, due to strong non-linearities, a partial breakdown of geostrophic balance, and intermediate scales for both balanced flows and ITs, which are hard to observe with current methods. However, the next generation of satellite altimetry, the Surface Water and Ocean Topography mission, will have fine enough resolution to begin to capture the submesoscale, which makes it an exciting time to explore wave-vortex interactions in this regime. We use the one-layer shallow water model to run idealized numerical simulations of a single wave mode propagating through a (cyclo)geostrophic vortex. By varying the Rossby number, which controls the strength of the vortex, and varying the relative scale of the vortex size to IT wavelength, we observe the IT energy redistribution at the lee side of a submesoscale vortex. We find that high Rossby numbers and relatively small waves will induce sharper deflections in wave propagation, which we quantify with energy flux calculations. By applying complex demodulation, we can filter the incoming plane wave to reveal the characteristic pattern of an isolated vortex scatter, which consists of three beams, two slightly skewed beams from the edge of the vortex, and one strongly skewed beam from the middle.

How to cite: Uncu, J. and Grisouard, N.: Internal Tide Scattering by an Isolated Cyclogeostrophic Vortex, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-10852, https://doi.org/10.5194/egusphere-egu22-10852, 2022.