OS4.1

EOT20 is the latest in a series of empirical ocean tide (EOT) models derived using residual tidal analysis of multi-mission satellite altimetry at DGFI-TUM. The amplitudes and phases of seventeen tidal constituents are provided on a global 0.125-degree grid based on empirical analysis of eleven satellite altimetry missions. The EOT20 model shows significant improvements compared to the previous iteration of the global model (EOT11a) throughout the ocean, particularly in the coastal and shelf regions, due to the inclusion of more recent satellite altimetry data as well as more missions, the use of the updated FES2014 tidal model as a reference to estimated residual signals, the inclusion of the ALES retracker and improved coastal representation. In the validation of EOT20 using tide gauges and ocean bottom pressure data, these improvements in the model compared to EOT11a are highlighted with the root-square sum (RSS) of the eight major tidal constituents improving by ~3 cm for the entire global ocean with the major improvement in RSS (~3.5 cm) occurring in coastal regions (<1 km to the coast). Compared to the other global ocean tidal models, EOT20 shows a clear improvement of ~0.4 cm in RSS compared to the closest model (FES2014) in the global ocean. Compared to the FES2014 model, the RSS improvement in EOT20 is mainly seen in the coastal region (~0.45 cm) while in the shelf and open ocean regions these two models only vary in terms of RSS by ~0.005 cm. The significant improvement of EOT20, particularly in the coastal region, provides encouragement for the use of the EOT20 model as a tidal correction of satellite altimetry in coastal sea level research. 

How to cite: Hart-Davis, M., Dettmering, D., Piccioni, G., Schwatke, C., Passaro, M., and Seitz, F.: EOT20: A new global empirical ocean tide model derived from multi-mission satellite altimetry., EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-2037, https://doi.org/10.5194/egusphere-egu21-2037, 2021.

As grid resolutions of operational ocean models are becoming finer and approach closer to the coast, the importance of inclusion of tidal forcing in high resolution operational ocean forecasting systems has increasingly been recognized. In the current work,  we present a 3D general ocean circulation model of ocean tides in the pan-Arctic region at ~3km horizontal grid resolution and 50 hybrid layers in the vertical, thus representing both barotropic and internal tides. The model system is based on the Hybrid Coordinate Ocean Model (HYCOM)  coupled with the Los Alamos Sea Ice Model (CICE). The results showed good agreement when compared with observations from tide gauges and a data-assimilative global barotropic tidal model. Among other results, the evaluation includes results for tidal amplitude and phase of the most energetic constituents (M2, S2, K1 and Q1).   The model system is currently operational and its development is supported by the Copernicus Marine Environment Monitoring Service (CMEMS) where its forecasts are disseminated.

How to cite: Ali, A., Muller, M., Bertino, L., and Melson, A.: A high resolution three-dimensional model of ocean tides for the pan-arctic region, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-2409, https://doi.org/10.5194/egusphere-egu21-2409, 2021.

How to cite: Innocenti, S., Matte, P., Fortin, V., and Bernier, N.: Residual bootstraps for the uncertainty analysis of tidal models with temporally correlated noise, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-8893, https://doi.org/10.5194/egusphere-egu21-8893, 2021.

Quite a handful of past studies have reported lack of energy near the tidal bands in high-resolution, regional model simulations’ frequency-wave number spectra when compared to observations. A plausible reason for this discrepancy could be the lack of remotely generated internal tides in regional simulations. In this study, we consider the impact of remote internal tides on the energetics in regional simulations of the California Current System (CCS). The CCS is an eddy-rich mid-latitude region, where energetic NIWs and internal tidal waves coexist. We run high-resolution realistic regional simulations using the Regional Ocean Modelling System (ROMS). The ROMS simulations are boundary-forced with high-frequency offline data from the Hybrid Coordinate Ocean Model (HYCOM). We consider a year-long HYCOM expt_06.1 simulation with 8-km horizontal grid resolution and 41 depth layers. The HYCOM simulation is realistically forced with tides and atmospheric forcing.

Time-mean and depth-integrated internal tidal flux computed for the parent HYCOM domain shows radiation of remotely generated internal tide beams into the ROMS domain. These beams comprise mainly of modes 1 & 2. To ensure that we provide satisfactory open boundary conditions (OBCs) for our regional simulation, we conduct sensitivity runs using two main types of OBCs (clamped and adaptive OBCs). For the runs with clamped OBCs, we varied the sponge layer viscosities at the open boundaries from 100 to 800 m²/s. Both nudging parameters and sponge layer viscosities are varied for simulations with the adaptive OBCs.  Although, we observe remotely generated internal tides in all our simulations, we find that the amount of internal tidal energy that is transmitted through the open boundaries and the internal tidal energetics in the interior of the domain depend on the nudging time scales, sponge layer width and/or viscosity values.

In the future, we plan to nest down to increasing high-resolution horizontal and vertical grids and perform simulations with different boundary forcings e.g. with total internal tides, stationary internal tides, and no internal tides. We will also force the ROMS model with unidirectional internal tides. We will evaluate the impacts of these scenarios on the internal tide energetics in the ROMS domain.

How to cite: Siyanbola, O., Buijsman, M., Barkan, R., and Arbic, B.: Finding appropriate boundary conditions for high frequency forcing of Regional Simulations – California Current System as a case study., EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-12204, https://doi.org/10.5194/egusphere-egu21-12204, 2021.

In June of 2018 a project for the establishment of a modern permanent Absolute Gravity Network  on the island of Ireland was initiated by the National Mapping Agency of Ireland, Ordnance Survey Ireland (OSi) with the cooperation of  Institute of Geodesy and Cartography (IGiK), and Land and Property Services (LPS) in Northern Ireland. The project assumes conducting absolute gravity surveys of the network using  the A10 absolute gravimeter on approximately 60 stations homogenously distributed on the island of Ireland.

Data processing includes time variable corrections for body tides, barometric pressure, polar motion as well as ocean tidal loading. For Ireland the ocean tidal loading effect can reach peaks of between 400 nm/s² on the west coast and 200 nm/s² on the east coast. This effect is significant and up to now the authors are unaware of previous historical data or  tidal gravity records being performed in Ireland. Hence it was considered as a valid component of the overall Absolute Gravity Project to evaluate the current situation with ocean tidal loading effect in Ireland using gravimetric tidal records in order to validate available ocean tidal loading models.

In order to assure the most optimal use of ocean tidal model as well as minimize the errors of including ocean tidal correction in absolute gravity processing the LaCoste&Romberg model G spring gravimeter was installed at OSi headquarters in Phoenix Park, Dublin, Ireland. Over a continuous period of 28 months gravity record with more than 99% data completeness at near 2Hz sampling rate was conducted.

The project data was acquired through using a self-programmed Raspberry Pi computer allowing for automatic download and remote access to the data.

A set of CSR, DTU, EOT, FES, GOT, TPXO (ocean tide loading provider – Chalmers, http://holt.oso.chalmers.se/loading/) ocean tidal loading models were used in a joint analysis with the collected tidal record. Analysis included performing tidal adjustment of the gravity data in the ETERNA 3.40 (ET34-X-V73) as well as comparison of IAG (International Association of Geodesy) recommended model combinations with the collected data.

Recommendations by the project team as to which of the ocean tidal models is most suitable to be used in Ireland for the purpose of absolute gravity measurements were made.

How to cite: Dykowski, P., Karkowska, K., Sękowski, M., and Kane, P.: Ocean tidal loading models assessment using 28 months of gravimetric tidal records in Dublin, Ireland, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-2422, https://doi.org/10.5194/egusphere-egu21-2422, 2021.

Tidal de-aliasing of satellite gravimetric data is a critical task in order to correctly extract gravimetric signatures of climate signals like glacier melting or groundwater depletion and poses a high demand on the accuracy of the employed tidal solutions (Flechtner et al., 2016). Modern tidal atlases that are constrained by altimetry data possess a high level of accuracy, especially for partial tides exhibiting large open ocean signals (e.g. M2, K1). Since the achievable precision directly depends on the available density and quality of altimetry data, the accuracy relative to the tidal amplitude drops for minor tidal excitations (worse signal-to-noise ratio) as well as in polar latitudes (sparse satellite-data). In contrast, this drop in relative accuracy can be reduced by employing an unconstrained tidal model acting independently of altimetric data.
We will present recent results from the purely-hydrodynamic, barotropic tidal model TiME (Weis et al., 2008) that benefit from a set of recently implemented upgrades. Among others, these include a revised scheme for dynamic feedbacks of self-attraction and loading; energy-dissipation by parametrized internal wavedrag; partial tide excitations by the tide-generating potential up to degree 3; and a pole-rotation scheme allowing for simulations dedicated to polar areas. Benefiting from the recent updates, the obtained solutions for major tides are on the same level of accuracy as comparable modern unconstrained tidal models. Furthermore, we show that the relative accuracy level only drops moderately for tidal excitations with small excitation strength (e.g. for minor tides), thus narrowing down the accuracy gap to data-constrained tidal atlases. Exemplarily for this, we introduce solutions for minor tidal excitations of degrees 2 and 3 that represent valuable constraints for the expected ocean tide dynamics. While they are currently not considered for GRACE-FO de-aliasing we demonstrate that third-degree tides can lead to relevant aliasing of satellite gravity fields and correspond closely to recently published empirical solutions (Ray, 2020).

How to cite: Sulzbach, R., Dobslaw, H., and Thomas, M.: Unconstrained global simulations of ocean tides up to degree 3 for satellite gravimetry, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-7977, https://doi.org/10.5194/egusphere-egu21-7977, 2021.

Harmonic tidal analysis bases on the presumption that since short records and close frequencies result in an ill-conditioned matrix equation, a record of length T is required to distinguish harmonics with a frequency separation of 1/T (Rayleigh criterion). To achieve stability of the solution, tidal harmonics are grouped. Nevertheless, if any additional information from different harmonics within the assumed groups is present in the data, it cannot be resolved. While the most information in each group is carried by the harmonic with the largest amplitude, time series from other harmonics is properly taken into account in estimated amplitudes and phases. However, if the signal from the next largest harmonic in a group is significantly different from the expectation, the grouping parametrization might lead to an inaccurate estimate of tidal parameters. That might be an issue since harmonics in a group do not have the same admittance factor, or if the assumed relationship between harmonics degree 2 and 3 is false.

The bias caused by grouping tidal harmonics can be investigated with methods used for stabilizing inverse problem solutions. In our study, we abandon the concept of groups. The resulting ill-posedness of the problem is reduced by constraining the model parameters (1) to reference values and (2) to the condition that admittance shall be a smooth function of frequency. The mentioned regularization terms are present in the least-squares objective function, and the trade-off parameter between the model misfit and data residuals is chosen by the L-curve criterion. We demonstrate how this method may be used to reveal system properties hidden by wave grouping in tidal analysis. We also suggest that forcing time series amplitude may be more relevant grouping criterion than solely frequency closeness of harmonics.

How to cite: Ciesielski, A. and Forbriger, T.: Investigation of bias in traditional grouping by means of regularization, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-14747, https://doi.org/10.5194/egusphere-egu21-14747, 2021.

Satellite altimetry recently reached an unprecedented level of global coverage with 7 missions flying simultaneously. While altimeters have been originally designed for open ocean and have improved our understanding of the large-scale ocean dynamic, the exploitation of coastal altimetry data remains a challenge that mobilizes a large effort in the scientific community. The future SWOT mission will solve this issue and certainly revolutionize our view of the coastal waters by mapping SSH with an unprecedented resolution.

One challenging aspect of coastal altimetry is the lack of accuracy in some geophysical corrections, which are critical to derive accurate sea-surface height anomalies (SSHA) near the coast. Especially, uncertainties in ocean tides is still an issue for the exploitation of altimetry in nearshore regions. Global tide models are usually used in most altimetry products. Despite their considerable progress in the last decade, their accuracy tends to decrease near the coast (Lyard, F. et Al., 2020).

Difficulties encountered in modelling the coastal tide are mainly due to its non-linear behaviour caused by changes in depth, shoreline interactions or varying bottom drag as it propagates onto shallower waters. The distortion of tidal propagation can thus be represented as additional tidal waves, which reflect overtides compound tides. These interactions are numerous and a great number of constituents have to be considered in order to reproduce accurately the tidal signal in shallow regions. Consequently, efforts in developing regional modelling of coastal areas are encouraged, as well as the consideration of ocean/shelf/land as a modelling continuum, for the preparation and exploitation of the future SWOT mission (Ayoub, N. et Al., 2015).

Moreover, these shallow-water waves exhibit smaller wavelengths than major astronomical ones, and there is a critical need for observations with short space and time scales to appreciate their spatial variability. While tide models are classically validated against tide-gauges confined to the coast, new opportunities are emerging with the development of kinematic GNSS systems. Chupin et Al. (2020), have demonstrated the ability of the Cyclopée system (a combination of a GNSS antenna and an acoustic altimeter) mounted on an USV to map sea surface height in motion. At a fixed point, the Cyclopée system provides similar accuracy than the best tide-gauge systems (and is therefore a way to propagate tide gauges measurements under satellites tracks).

Through a methodology based on crossover measurements; we demonstrate in this study the potential of the USV PAMELi, developed at the University of La Rochelle, for assessing tide corrections under altimetry tracks, in the scope of future coastal altimetry applications (e.g. storm surge or wave setup). For this purpose, the Pertuis Charentais area (France) is addressed as a modelling case study with a new regional barotropic configuration based on SCHISM model (Zhang, J. et al., 2016). After being compared against coastal tide-gauges, our SCHISM model as well as other available global solutions are assessed though this methodology applied under the pass 216 of Sentinel-3A.

How to cite: Tranchant, Y.-T., Chupin, C., Testut, L., and Ballu, V.: Assessing tide correction under altimetry tracks: an innovative validation methodology using USV (Unmanned Surface Vehicle) in-situ measurements, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-16339, https://doi.org/10.5194/egusphere-egu21-16339, 2021.

Deep time investigations of the Earth have revealed a relationship between plate tectonic motion and the intensity of the tide. Tidal energetics change as continental plates disperse and aggregate in the supercontinent cycle, altering ocean basins around them. The question is, could enhanced tides occur on Earth before plate tectonics started e.g., during the Archean?  

Here we have coupled an established tidal model with an ensemble of potential topographies of the Archean Earth to establish a statistically significant approximation of Archean tidal energetics. Land area is restricted to 5 – 15% with the rest representing primordial ocean – containing no major plate tectonic features i.e., trenches and ridges. Ocean volume is preserved at close to present-day which means oceans are on average 1 km shallower than present-day oceans. Archean day length is set at 13.1 hours with the semi-diurnal tide occurring every 6.8 hours. Equilibrium tide is around 3.4x the present-day value due to the proximity of the Moon.

The aim of this study is to assess the relationship of the Earth Moon system during this primordial stage to better understand the potential role tides had in the origin of life, and to quantify the tidal state of a primordial rocky planet with a young, nearby moon. Understanding the tidal state of Earth at this early time is important for exoplanetary studies as it broadens our scope of planets which may be hospitable to life.

We found coastal and open ocean resonance in many of the ensemble topographies. Total global dissipation in the ensembles varies from 75 – 150% of present-day dissipation rates due to elevated equilibrium tide and greater area where the tide can dissipate. When regional and open ocean resonance does occur, it can raise total global dissipation to >150% of present-day values and can caue regional macrotidal amplitudes (>2m).

The authors would like to acknowledge funding support from FCT – UIDB/50019/2020 – IDL.

How to cite: Davies, H., Green, M., and Duarte, J.: Analysing the tidal state of a pre-plate tectonic Earth during the Archean Eon (3.9 Ga), EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-1582, https://doi.org/10.5194/egusphere-egu21-1582, 2021.

Tide gauge records provide the main source of data behind the study of sea level change over the past 200 years. However, our understanding of changes in mean sea levels, tides and extremes is limited by the length of the records available. A large amount of potential data exists in libraries and archives across the world in the form of historical tidal ledgers and charts that have never been converted into digital data suitable for use in scientific studies. The Intergovernmental Oceanographic Commission’s Global Sea Level Observing System (GLOSS) has been encouraging organisations to locate, catalogue and digitise such material.

Unfortunately, the processes required to extract usable data from charts and ledgers is slow, laborious work. Promising attempts have been made to automate this using optical character recognition, but these are often hindered by changes in document formats, and hard to decipher handwriting, particularly in older records.

A possible solution is to use online citizen science platforms such as Zooniverse that bring together scientists and volunteers in projects as diverse as searching for supernovae, identifying whale sounds, transcribing manuscripts from the archives of natural history museums, and helping train algorithms that analyse images of cancer cells. Last year, 5.25 million rainfall observations from the UK were digitised in a few weeks by about 16,000 volunteers.

Here we present a citizen science project to digitise 16,000 images of ledgers recording 15-minute observations of sea level from North West England that is currently in progress. We describe the process the volunteers undertake, the lessons learnt from early testing, and an overview of the results obtained so far. Finally, we discuss some potential extensions of the project, including the possibility of using the platform to digitise tidal charts.

How to cite: Matthews, A., Bradshaw, E., and Williams, J.: Rescuing historical sea level data using a citizen science platform, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-3211, https://doi.org/10.5194/egusphere-egu21-3211, 2021.

Mean sea levels are changing worldwide, and local tidal changes have been widely reported. Knowledge of regional changes in mean sea level, and local changes in tides are crucial to inform effective climate adaptation. An essential element of this is the availability of accurate observations of sea level. Sea level data in the Republic of Ireland, prior to the establishment of the National Tide Gauge Network in the mid-2000s, is very limited but belies a wealth of historical data available in archival form. In this study, we digitize records located in Cork Harbour, Ireland from 1842 and show how short duration (6 weeks), high quality data, with a large interval (177 years) to the present, can accurately inform tidal and mean sea level changes. We consider error sources in detail and estimate that for M2 the accuracy of these historical measurements is 1% and 2 minutes for amplitude and timing respectively; and our mean sea level estimates are accurate to the centimetre level. Our results show remarkable tidal stability with a 2% change in the amplitude of the M2 component and 4 minute change in the phase over a period of 177 years; and a mean sea level rise of 41 cm in the Cork Harbour area since 1842, approximately in line with global mean sea level trends plus local glacial isostatic adjustment. More broadly, we show that with careful seasonal, nodal, and atmospheric corrections, together with good knowledge of benchmark provenance, these historic, survey-oriented data can accurately inform of sea level changes.

How to cite: McCarthy, G., Pugh, D., Edwards, R., Hogarth, P., and Woodworth, P.: Mean Sea Level and Tidal Change in Ireland since 1842: A case study of Cork, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-14626, https://doi.org/10.5194/egusphere-egu21-14626, 2021.

Many locations in the U.S. have experienced large trends in their tidal range since the 19^th century, often in response to altered coastal and estuarine morphology.  Such tidal changes may enhance the vulnerability of an area towards flooding. In this contribution, >1000 estimates of tidal range from around the contiguous United States are digitized from the published tide tables of 1899 and compared to the tide table of 2020. Our approach enables much greater spatial coverage than previous studies. Tidal range has more than doubled in many regions due to anthropogenic development, including Miami, the Saint Johns River, and the Connecticut River. Important changes are noted in other tidal rivers, including the Sacramento, Savannah, and James Rivers. On average, gauges located inland experienced the largest changes in tidal range, followed by estuary stations; coastal stations showed the least variability. Amplified tidal range increases the prevalence of minor (nuisance) flooding.  As shown by case studies of San Francisco, Wilmington (North Carolina) and Miami (Florida), the prevalence of minor (nuisance) flooding events has greatly increased due to tidal evolution. In locations without historical time-series, we infer the changed flooding using a statistical model that estimates changes to tidal constituents based on the observed change in tidal range and known constituent ratios.  Results show that tidal change may be a previously underappreciated factor in the increasing prevalence of nuisance flooding in cities like Miami and Jacksonville, Florida, where long time series of data back to the 19^th century are not available.  Understanding the reasons for tidal change may provide planners and engineers with new tools to adapt to climate change effects like sea-level rise.

How to cite: De Leo, F. and Talke, S. A.: Trends in tidal range around the U.S. and potential implications for flooding occurrence, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-13975, https://doi.org/10.5194/egusphere-egu21-13975, 2021.

Tide gauges throughout the North Sea basin show significant changes in the local tidal regime since the mid-20th century, especially in the German Bight area. These changes were analyzed within the DFG-funded project TIDEDYN (Analyzing long term changes in the tidal dynamics of the North Sea, project number 290112166) and the final results were recently published in Jänicke et al. (2020, https://doi.org/10.1029/2020JC016456).

In this paper, we document an exceptional large-spatial scale case of changes in tidal range in the North Sea, featuring pronounced trends between -2.3 mm/yr at tide gauges in the UK and up to 7 mm/yr in the German Bight between 1958 and 2014. These changes are spatially heterogeneous and driven by a superposition of local and large-scale processes within the basin. We use principal component analysis to separate large-scale signals appearing coherently over multiple stations from rather localized changes. We identify two leading principal components (PCs) that explain about 69% of tidal range changes in the entire North Sea including the divergent trend pattern along UK and German coastlines that reflects movement of the region’s semidiurnal amphidromic areas. By applying numerical and statistical analyses, we can assign a baroclinic (PC1) and a barotropic large-scale signal (PC2), explaining a large part of the overall variance. A comparison between PC2 and tide gauge records along the European Atlantic coast, Iceland and Canada shows significant correlations on time scales of less than 2 years, which points to an external and basin-wide forcing mechanism. By contrast, PC1 dominates in the southern North Sea and originates, at least in part, from stratification changes in nearby shallow waters. In particular, from an analysis of observed density profiles, we suggest that an increased strength and duration of the summer pycnocline has stabilized the water column against turbulent dissipation and allowed for higher tidal elevations at the coast.

We would like to present these research results and the content of the paper (cf. Jänicke et al., 2020) at vEGU21, hoping to encourage subsequent questions and further discussions.

How to cite: Jänicke, L., Ebener, A., Dangendorf, S., Arns, A., Schindelegger, M., Niehüser, S., Haigh, I. D., Woodworth, P., and Jensen, J.: Assessment of tidal range changes in the North Sea from 1958 to 2014, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-14706, https://doi.org/10.5194/egusphere-egu21-14706, 2021.

The existence of seasonal variations in major tides has been recognized since decades. Where Corkan (1934) was the first to describe the seasonal perturbation of the M2 tide, many others have studied seasonal variations in the main tidal constituents since. However, most of these studies are based on sea level observations from tide gauges and are often restricted to coastal and shelf regions. Hence, observed seasonal variations are typically dominated by local processes and the large-scale patterns cannot be clearly distinguished. Moreover, most tide models still perceive tides as annually constant and seasonal variation in tides is ignored in the correction process of satellite altimetry. This results in reduced accuracy of obtained sea level anomalies.

To gain more insight in the large-scale seasonal variations in tides, we supplemented the clustered and sparsely distributed sea level observations from tide gauges by the wealth of data from satellite altimeters. Although altimeter-derived water levels are being widely used to obtain tidal constants, only few of these implementations consider seasonal variation in tides. For that reason, we have set out to explore the opportunities provided by altimeter data for deriving seasonal modulation of the main tidal constituents. Different methods were implemented and compared for the principal tidal constituents and a range of geographical domains, using data from a selection of satellite altimeters. Specific attention was paid to the Arctic region where seasonal variation in tides was expected to be significant as a result of the seasonal sea ice cycle, yet data availability is particularly limited. Our study demonstrates the potential of satellite altimetry for the quantification of seasonal modulation of tides and suggests the seasonal modulation to be considerable. Already for M2 we observed changes in tidal amplitude of the order of decimeters for the Arctic region, and centimeters for lower latitude regions.

How to cite: Bij de Vaate, I., Guarneri, H., Slobbe, C., and Verlaan, M.: Global mapping of seasonal variations in tides from tide gauge and satellite altimeter data, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-10717, https://doi.org/10.5194/egusphere-egu21-10717, 2021.

To study the effect of changing climate and declining sea ice on tides it is pertinent to include the effects of sea ice in a tidal model. Most of the hydrodynamic global tidal models either ignore the effect of sea ice on tides or approximately model it as an addition to the existing bottom frictional stress. Our focus is to extend an existing global tidal model (Global tide and storm surge model(GTSM), Verlaan et. al 2015) to include the effects of the Arctic sea ice on tides without coupling it to a sea ice model.

We propose to divide the sea ice cover into different regimes: landfast ice, free drift sea ice, and ice drifting under strong internal stresses, and treat each regime based on the physics between the respective regime and the tides.

It is seen that the free drift sea ice (almost) exactly follows the tides and has little to no effect on the tidal amplitudes and phases. In the case of landfast ice, we use the differences in landfast ice cover between the winter (maximum) and summer (zero) to check for the resulting differences in water levels and thus, comment on the performance of the model. Finally, the physics between the sea ice drifting under strong internal stresses and water is studied to model the effect of such ice on tides.

How to cite: Vasulkar, A., Verlaan, M., and Slobbe, C.: Towards the inclusion of sea-ice into a global tidal model, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-15359, https://doi.org/10.5194/egusphere-egu21-15359, 2021.

In this research, the sea level variability in the Russian Arctic seas caused by the Moon and the Sun tidal forces is considered. For a long time, it was thought that the tides can be easily calculated based on a small series of observations made in summer, but as shown in a few recent publications, describing tides in the different parts of the Arctic Ocean, tidal characteristics change significantly during the year. The main attention is paid to their seasonal variability in the seas of the Russian Arctic. The most interesting results have been obtained for the east sector of the Russian Arctic seas, where the tides were poorly known, and the long-term data from the tide gauges have been processed for the first time. We have used the long-term hourly sea-level data from several stations in the White, Kara, Laptev and Chukchi seas. The temporary coverage for the White Sea stations includes rather continuous sea-level records from 2004 to 2014 yrs. The maximum length of records made from 1981 to 2005 at the stations of the east sector of the Arctic was found at the Tiksi station. In this work we also analysed unique data obtained from the bottom pressure loggers installed on the Laptev-sea shelf in the period 2018-2020. The results of this study allow us to conclude that the classical harmonic analysis applied to the precomputation of tides does not provide an accurate estimate of the tidal characteristics in individual water areas in the Arctic. Accounting of the seasonal variability in the tidal characteristics will make it possible to clarify tidal maps important for navigation and coastal construction in the Arctic Region.

How to cite: Kulikov, M. and Medvedev, I.: Seasonal variability of tides in the Russian Arctic seas, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-11880, https://doi.org/10.5194/egusphere-egu21-11880, 2021.

Storm surges pose an increasing risk to coastline communities. These events, combined with high tide, can result in coastal flooding. To reduce the impact of storm surges, an accurate estimate of coastal flood risk is necessary. Specifically, estimates are required for the return level of sea levels (still water), which is the level with annual exceedance probability p. This estimate is used as an input to determine the height for a coastal defence, such as a sea wall. The return level estimation requires statistical analysis based on extreme value theory, as we need to know about the frequency of events that are more extreme than those previously observed.

Large storm surges exhibit seasonality, they are typically at their worst in the winter and least extreme in the summer. This seasonal pattern differs from that of the tide, whose seasonality is driven astronomically, resulting in tidal peaks at the spring and autumn equinoxes. Hence, the worst levels of these two components of still water level are likely to peak at different times in the year, and so statistical methods that treat them as independent variables are likely to over-estimate return levels.

We focus on the skew surge: the difference between the observed and predicted high water within a tidal cycle. Williams et al. (2016) show that tide and skew surge are independent conditional on the time of year. Batstone et al. (2013) used this property to derive estimates used for UK coastal flood defences. They used generalised Pareto distributions for the skew surge tail but did not account for the separate seasonality of tide and skew surge.

This work aims to model how the distribution of skew surges changes over a year and we combine our results with the known seasonality of tides to derive estimates of still water level return levels. We compare our results with the Batstone et al. (2013) approach at a few locations on the UK coastline.

References:

Batstone, C., Lawless, M., Tawn, J., Horsburgh, K., Blackman, D., McMillan, A., Worth, D., Laeger, S. and Hunt, T., 2013. A UK best-practice approach for extreme sea-level analysis along complex topographic coastlines. Ocean Engineering, 71, pp.28-39.

Williams, J., Horsburgh, K.J., Williams, J.A. and Proctor, R.N., 2016. Tide and skew surge independence: New insights for flood risk. Geophysical Research Letters, 43(12), pp.6410-6417.

How to cite: D'Arcy, E., Tawn, J., Joly-Laugel, A., and Sifnioti, D.: Accounting for Seasonality in Extreme Sea Level Estimation, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-12276, https://doi.org/10.5194/egusphere-egu21-12276, 2021.

Over the last few years a number of groups have created maps of the baroclinic tide from satellite altimeter measurements of sea-surface height (SSH). These maps can be used as predictive models for the baroclinic tides, e.g., for removing aliased tidal signals from altimetry, but they can also be used to diagnose aspects of the tidal dynamics. This presentation uses the High Resolution Emprical Tide (HRET) model to compute the phase speed, energy, energy flux, and energy flux divergence of the first few baroclinic modes for the M2, S2, K1, and O1 tides, and compares these with independent estimates from the literature.

The phase speed of the waves in HRET are compared with the theoretically-predicted phase speeds computed from stratification. For the mode-1 M2 waves which are determined most accurately, the theoretical and observed phase speeds agree very well; however, there is a small bias, namely, the theoretical phase speed exceeds the observed phase speed by 1 to 2%. This offset could reflect either a methodological estimation bias, issues with the data used to compute the theoretical phase speed, or a limitation of the theory for the vertical modes.

The phase speed results provide some confidence in the usefulness of linear wave dynamics for interpreting the HRET SSH. Using a simplified form of the momentum equations, the area-integrated kinetic plus potential energy of the mode-1 M2 tide is found to be 43 PJ, larger than in other baroclinic tide models, and with nearly isotropic directional distribution. For mode 1, the divergence of the energy flux diagnosed from HRET agrees well with previous estimates based on the barotropic tides. For the most accurately-determined mode-1 M2 tide, the results provide new information about sources and sinks of baroclinic energy along the continental shelves, and they are used to examine the accuracy of a commonly-used approximation of the baroclinic energy flux.

How to cite: Zaron, E. and Musgrave, R.: The Energy Budget of an Altimeter-Derived Baroclinic Tide Model, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-7887, https://doi.org/10.5194/egusphere-egu21-7887, 2021.

The disintegration of the equatorward-propagating K₁ internal tide in the South China Sea (SCS) by parametric subharmonic instability (PSI) at its critical latitude of 14.52ºN is investigated numerically. The multiple-source generation and long-range propagation of K₁ internal tides are successfully reproduced. Using equilibrium analysis, the internal wave field near the critical latitude is found to experience two quasi-steady states, between which the subharmonic waves develop constantly. The simulated subharmonic waves agree well with classic PSI theoretical prediction. The PSI-induced near-inertial waves are of half the K₁ frequency and dominantly high modes, the vertical scales ranging from 50 to 180 m in the upper ocean. From an energy perspective, PSI mainly occurs in the critical latitudinal zone from 13–15ºN. In this zone, the incident internal tide loses ~14% energy in the mature state of PSI. PSI triggers a mixing elevation of O(10^-5–10^-4 m²/s) in the upper ocean at the critical latitude, which is several times larger than the background value. The contribution of PSI to the internal tide energy loss and associated enhanced mixing may differ regionally and is closely dependent on the intensity and duration of background internal tide. The results elucidate the far-field dissipation mechanism by PSI in connecting interior mixing with remotely generated K₁ internal tides in the Luzon Strait.

How to cite: Liu, K. and Zhao, Z.: Disintegration of the K1 internal tide in the South China Sea due to parametric subharmonic instability, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-2695, https://doi.org/10.5194/egusphere-egu21-2695, 2021.

In this study, we investigate the seasonal and interannual variability of internal tides in the global ocean using a Hybrid Coordinate Ocean Model (HYCOM) and altimetry data. The variability of internal tides is caused by the time varying stratification, mesoscale activity, large-scale shifts in amphidromic points, and changes in ice cover. The variation in the background fields generates the non-phase locked internal tides which are non-stationary. Non-stationary internal tides are less predictable than stationary tides, complicating regional model forcing with remote internal tide signals and the separation of internal tides from mesoscales. We will use 6 years of steric SSH extracted from a global HYCOM simulation with a horizontal resolution of 8 km and 32 layers to study the variability of internal tides. Our objective is to analyze the spatial and temporal variability of the amplitude and phase of the diurnal and semidiurnal internal tides. The SSH time series will be divided into time segments with different durations. The least-squares harmonic analysis will be used to extract SSH amplitude and phase for M2, K1, O1, and S2 constituents for these time segments. It has been found that the stationary amplitude decreases with an increase in the duration of the time series. We will also use empirical orthogonal functions (EOF) analysis to determine the seasonal and interannual variability in the monthly-mean internal tide amplitude and phase. The global maps of the non-stationarity fraction for the internal tidal constituents will be shown for each season. These results will be compared with 25 years of satellite altimetry data to find out whether similar variance decay trends are observed in the altimetry data.

How to cite: Kaur, H. and Buijsman, M.: Seasonal and interannual variability of global internal tides, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-13491, https://doi.org/10.5194/egusphere-egu21-13491, 2021.

How to cite: Williams, J.: Salinity effects on pressure-based tide gauges in a macro-tidal estuary, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-3227, https://doi.org/10.5194/egusphere-egu21-3227, 2021.

Tidal asymmetry in deltas is caused by both the intrinsic asymmetry, resulting from the combination of astronomical tides, and by nonlinear tidal interactions that occur in shallow water. In recent years, nonlinear tidal interactions in deltas have become more complex due to the influence of topographic changes. The relative importance of these sources of tidal asymmetry in delta channel networks, partially due to the limitations of classical harmonic analysis (HA) in hindcasting nonstationary tides, has remained poorly studied. We take the Pearl River Delta (PRD) as an example to examine the spatial-temporal variations of tides and tidal asymmetry in deltas. For hydrological data from 14 stations in the PRD spanning the period1961-2012, the non-stationary harmonic analysis method (NS-TIDE) is used. The spatiotemporal variation of multiple sources of tidal asymmetry is quantified by a skewness metric, revealing the development of alternative sources of tidal asymmetry develop in the delta subject to study. As tides propagate into delta channel networks, analytical results show the development of tides becoming increasingly more asymmetric. In the course of the 1990s and 2000s, tidal skewness has decreased in the parts of the PRD where the water depth varies greatly, indicating that the tidal asymmetry has reduced. Our findings demonstrate that deepening of the channel system is associated with a reduction of the flood-dominant tidal asymmetry. Deeper channels tend to be more often ebb-dominant than shallow areas. Due to extensive sand excavation, the abrupt changes in bathymetry in the delta are likely to be responsible for the observed spatial variations in tidal response that reduce the flood-dominant tidal asymmetry in this region.

How to cite: Zhang, W. and Bao, S.: The evolution of the tidal asymmetry in a river networks system, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-11127, https://doi.org/10.5194/egusphere-egu21-11127, 2021.

In this contribution, we show that channel deepening can amplifiy tide and storm surge--while simultaneously decreasing the river slope during both normal conditions and during floods.  We investigate the Saint Johns River Estuary, Florida, an example of a hyposynchronous, strongly frictional estuary with a landward decay in tidal amplitudes. Records since the 1890s and numerical modeling show that tidal range doubled in Jacksonville, Florida (40 km from coast), while tidal discharge approximately doubled everywhere. Overall, an increase in channel depth from 5 to 10m drove the observed changes, with width and length changes comparatively minor factors. Tidal amplitude evolved in a spatially variable way--negligible at the coast and inland, maximal 20-30km from the ocean.  The change in the M2 constituent is approximated by the equation x * exp(mu*x), where x is the distance from the ocean and mu is a damping coefficient that depends on depth, drag coefficient, and other system properties.  The observed tidal evolution is similar to storm surge:  Numerical modeling of hurricane Irma (Sept. 2017) under 1898 and 2017 bathymetric conditions confirms that both tidal and storm surge amplitudes have increased over time, with a maximum change about 20-25km from the inlet. Nonetheless, hurricane Irma produced overall high water levels in the historical bathymetric configuration. The reason is that the mean water level slope required to move water out of the modern estuary has decreased. An analytical model confirms that reduced slope is caused primarily by channel deepening.  However, greater tides and storm surge imply an increased vulnerability to a worst-case scenario hurricane. 

How to cite: Talke, S., Jay, D., and Familkhalili, R.: Alteration in tides and flood dynamics caused by channel deepening:  case study of the Saint Johns River, Florida, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-14012, https://doi.org/10.5194/egusphere-egu21-14012, 2021.