Contributors: Kun Yan (Deltares), Sanne Muis (Deltares), Natalia Aleksandrova (Deltares), Robyn Gwee (Deltares), Jelmer Veenstra (Deltares), Job Dullaart (VU Amsterdam), Jeroen Aerts (VU Amsterdam), Trang Duong (IHE), Roshanka Ranasinghe (IHE)
Issued by: Kun Yan, Sanne Muis, Natalia Aleksandrova (Deltares)
Issued Date: 16/12/2024
Official refence number service contract: 2020/C3S_435_Lot8_Deltares, 2024/C3S2_422
Acronyms
Acronym | Description |
C3S | Copernicus Climate Change Service |
CDR | Climate Data Record |
CDS | Climate Data Store |
CMS | Content Management System |
EQC | Evaluation and Quality Control |
RCP | Representative Concentration Pathway |
RCM | Regional Climate Model |
SIS | Sectoral Information System |
GTSM | Global Tide and Surge Model |
CMIP6 | Coupled Model Intercomparison Project Phase 6 |
HighResMIP | High Resolution Model Intercomparison Project |
1. Introduction
1.1. Executive Summary
The Global Sea Level Change Time Series for 1950 to 2050 Derived from Reanalysis and High Resolution CMIP6 Climate Projections forms one of two catalogue entries consisting of tides, storm surges and sea level rise data and can be used to characterize water levels in present day and future conditions. The second catalogue entry provides similar indicators computed for three different 30-years periods corresponding to historical, present and future climate conditions (1951-1980, 1985-2014, and 2021-2050). The two datasets may be used to evaluate sea level variability, coastal flooding, coastal erosion, and accessibility of ports in both present day conditions and to assess changes under climate change.
The time series are computed based on hydrodynamic simulations with the Deltares Global Tide and Surge Model (GTSM) version 3.0. To assess the impacts of climate change on extreme sea levels, we use 5 climate models of the HighResMIP (High Resolution Model Intercomparison Project) multi-model ensemble. HighResMIP is part of the CMIP6 (Coupled Model Intercomparison Project Phase 6), and the ensemble is specifically tailored to be used for assessing climate extremes such as tropical cyclones. This is mainly because of the high-resolution of the HighResMIP climate models, which is at least 50 km in the atmosphere and 0.25° in the ocean. By using a multi-model ensemble, the dataset explicitly considers the uncertainties associated with the climate forcing. The simulations span from 1950 to 2050, with 1950-2014 being the historical period that is largely constrained by observations, and 2015-2050 being the future period that largely corresponds with the high-emission scenario SSP5-8.5.
In addition to the HighResMIP ensemble, a reanalysis dataset is computed by forcing GTSM with ERA5 reanalysis from 1950-2024. This provides recent historical water levels that can be used to look at specific (extreme) events in the past.
This dataset was produced by a Deltares led consortium including Vrije Universiteit Amsterdam, IHE Delft on behalf of the Copernicus Climate Change Service.
1.2. Scope of Documentation
This document describes one of the two global water level change datasets ingested into the CDS as two catalogue entries. It provides an overview of metadata and variable description, as well as how the product is produced. This includes a description of the model, product validation, and indicator calculation.
Specifically, section 1 provides an executive summary of the product. Section 2.1 summarize the data gap and product added value. Section 2.2 provides product overview with metadata information and variable description. Section 2.3 describes input data, climate scenarios, modelling and validation. Section 3 concludes the user guidance and the product limitations.
1.3. Version History
The global dataset of tidal elevation, mean sea level, total water levels and surges based on CMIP6 multi-model ensemble is version 1, added in 2021.
The dataset of water levels and surges for the reanalysis experiment forced with ERA5 data is available in version 2. This version of the dataset includes the extension of the dataset to period 1950-2024 (compared to the original 1979-2018), and a new version of the 1979 data with improved spin-up and sea level rise values consistent with the extended dataset.
2. Product Description
2.1. Product Target Requirements
Extreme sea levels, consisting of tides, storm surges, and mean sea levels, can cause a range of coastal hazards. The world's coastal areas are increasingly at risk due to rising mean and extreme sea levels, which can lead to the permanent submergence of land; increased coastal flooding; enhanced coastal erosion; loss of coastal ecosystem; and salinization (Oppenheimer, et al., 2019). Global projections of extreme sea levels can be used to assess the impacts of these coastal hazards and provide information on the projected changes for the coming decades.
This User Guide provides a description of a new global dataset of extreme sea levels for a multi-model ensemble of high-resolution climate models and for a reanalysis model. The dataset provides indicators that can be used to characterize water level in present-day conditions, but also assess changes under climate change. This dataset provides an update of a previous dataset which was developed in the contract C3S_422 Lot2. There are two main improvements compared to the previous dataset. First, the geospatial coverage is expanded from Pan-European to global. Second, uncertainty from the climate forcing and the extreme value analysis has been considered. More details are provided in Section 2.3.
2.2. Product Overview
2.2.1. Data Description
In this section an overview of the dataset is provided (Table 2-1).
Table 2-1: Overview of key characteristics of the water level change time series.
Data Description | |
Dataset title | Global sea level change indicators from 1950 to 2050 derived from reanalysis and high resolution CMIP6 climate projections |
Data type | Reanalysis / Climate projections |
Topic category | Sea and coastal regions, Natural hazard |
Sector | Coastal flood risk, integrated coastal zone management, harbor and port |
Keyword | Extreme sea level, CMIP6, time series |
Domain | Global |
Horizontal resolution | Coastal grid points: 0.1° |
Temporal coverage | ERA5 reanalysis: from 1950 to 2024 |
Temporal resolution | 10min (all), hourly and daily maximum (reanalysis only) |
Vertical coverage | Surface |
Update frequency | No updates expected |
Version | Historical and Future datasets based on CMIP6 models - version 1. Reanalysis dataset of water levels and surges based on ERA5 - version 2. |
Model | Global Tide and Surge Model (GTSM) version 3.0 |
Provider | Deltares (Kun Yan) |
Terms of Use | Copernicus Product License |
2.2.2. Variable Description
In this section more details are given about the variables listed in the time series datasets (Table 2-2).
Table 2-2. Overview and description of variables for water level change time series.
Variables | |||
Long Name | Short Name | Unit | Description |
Mean sea level | msl | m | Mean sea level height relative to the reference period (1986-2005). |
Storm surge residual | Surge | m | The storm surge residual is calculated as the difference between the total water level and the tide-only water level simulation. The effect of changes in annual mean sea level is included in both simulations and for both the historical and future period. |
Tidal elevation | Tide | m | The tidal elevation is derived from the tide-only simulation. The tide-only simulation is a hydrodynamic simulation without meteorological forcing (i.e. wind and pressure at mean sea level), the outputted water level includes only the pure tide without the storm surge residual. Sea level rise is included in both the historical and future period. |
Total water level | Water level | m | Total water level is derived from simulation including tidal and atmospheric forcing. Sea level rise is included for both the historical and future period. |
2.3. Method
2.3.1. Background
Extreme sea levels, consisting of tides, storm surges, and mean sea levels, can cause a range of coastal hazards. The world's coastal areas are increasingly at risk due to rising mean and extreme sea levels, which can lead to the permanent submergence of land; increased coastal flooding; enhanced coastal erosion; loss of coastal ecosystem; and salinization (Oppenheimer, et al., 2019). Global projections of extreme sea levels can be used to assess the impacts of these coastal hazards and provide information on the projected changes for the coming decades. In a previous contract (C3S_422_Lot2), a Pan-European dataset with consistent projections of mean sea level, tides, surges and wave condition has been developed (Muis et al., 2020). This dataset has been used for coastal applications such as offshore wind maintenance, port operations and planning, and coastal flood risk assessment.
The dataset described in this User Guide (C3S_435 Lot8 and C3S2_422) implements several improvements compared to the previous dataset. First, the spatial coverage is expanded from pan-European to global. The global coverage is highly valuable for global studies of the climate change impact on extreme sea levels and coastal flooding, including the effect of climate mitigation and adaptation. Moreover, the global coverage is very beneficial for countries where no regional information is available. Second, uncertainty from the climate forcing and the extreme value analysis has been considered. We provide extreme sea level projections based on a multi-model ensemble of CMIP6 climate models opposed to the previous pan-European dataset which was based on a single CMIP5 climate model. Climate models have large uncertainties and the use of multi-models can be used to increase the confidence in the extreme sea level projections. Moreover, the CMIP6 models have a higher model resolution and improved physics, and as such they are expected to better capture storm surges compared to the previous generation of climate models. In addition, compared to the previous one, this dataset applies more advanced extreme values analysis (peaks-over-threshold instead of annual maxima), and provides an estimate of the uncertainty of the fitted distribution (low and high bound in addition to the best fit).
2.3.2. Dataset design
This User Guide describes a global dataset of (changes in) extreme sea level under future climate change based on the HighResMIP multi-model ensemble. The dataset is produced by simulations with the Global Tide and Surge Model version 3.0 (GTSMv3.0), a 2D hydrodynamic model with global coverage which incorporates tides, surges and mean sea-levels dynamically. The HighResMIP ensemble, part of the CMIP6 experiments, is used as atmospheric forcing (Haarsma et al., 2016). The HighResMIP ensemble consists of high-resolution climate models with resolutions of at least 50 km in the atmosphere and 0.25° in the ocean. The enhanced resolution of HighResMIP has added value for resolving climate extremes such as tropical cyclones (Roberts et al., 2020), which is important for the modelling of storm surges. This new dataset is developed with the aim to assess how extreme sea levels change between 1950 to 2050 under influence of sea-level rise and climate change based on SSP5-8.5.
For the reanalysis experiment, ERA5 reanalysis is used as atmospheric forcing. This allows us to compute historical water levels and surges that closely correspond to observed events.
Two main products are ingested in the CDS:
- Time series of total water levels, tides, storm surges and mean sea level for the HighResMIP ensemble (1950-2050) and for the ERA5 reanalysis (1950-2024).
- Indicators of total water level, tides and storm surge statistics for historical, present and future time periods (1951-1980, 1985-2014, and 2021-2050), including changes and uncertainties;
2.3.3. Input Data
The global dataset of extreme sea levels described in this User Guide is based on the ERA5 climate reanalysis and the HighResMIP climate simulations. For the simulations of total water levels we force GTSM with wind and atmospheric pressure. More specially, the following variables are required: the mean sea level pressure (psl), zonal surface wind speed (uas), and meridional surface wind speed (vas). For each of these variables, we use a the highest temporal resolution available. For ERA5 this is hourly. For the HighResMIP ensemble, this is 3- or 6-hourly depending on the climate model.
ERA5 is the global climate reanalysis of the Copernicus Climate Change Service, which is available at the CDS (Hersbach et al., 2020). It is the successor of the ERA-Interim dataset with a spatial resolution of 0.25° × 0.25° (∼31 km), and it is available from 1940 to present. Note that it is reported that the 10m winds in ERA5 can become unrealistically large in a particular location (see https://confluence.ecmwf.int/display/CKB/ERA5%3A+large+10m+winds). Also, it is reported that some of the ERA5 data on the CDS was corrupted, resulting in spurious values (see https://confluence.ecmwf.int/display/CKB/ERA5+CDS%3A+Data+corruption). These caveats were issued after the C3S-435 Lot8 simulations for 1979-2018 were completed, and may affect the products described in this User Guide. Nevertheless, the GTSM results have been extensively validated as reported in Section 2.3.5 below and show no major issues of concern. The impact of unrealistic wind values and corrupted files would result in the output dataset with erroneous value to be unavailable. For the extreme value statistical indicators, we have attempted to remove erroneous values but for the time series this was not desirable. We do not expect the ERA5 caveats to have large-scale effects, but the unrealistic wind values can result in localized extreme water levels that are too high. When using the time series for a specific event in a local setting, we would recommend users to verify the results and validate.
HighResMIP is a set of model experiments carried out as part of the Climate Model Intercomparison Phase 6 (CMIP6). The experiments cover the period 1950-2050 for a set of climate models with a resolution higher than 50 km. The historical periods (1950-2014) is constrained by observations, while the future period (2015-2050) is based on a high-emission scenario (i.e. SSP5-8.5). In general, the difference between emission scenarios are rather small until the mid-century. The members that are included are largely based on the availability at the time the GTSM simulations were carried out. An ensemble of 5 climate models is used. This ensemble consist of a mix of are both coupled and atmosphere-only (i.e. SST-forced) simulations (Table 2-3). The experimental design of HighResMIP is explained in more detail in Haarsma et al., (2018).
Both the historical and future simulations with GTSM include sea level rise. A spatially-varying SLR fields at a 1°x1° resolution is used as input, and updated for each year of the simulation (i.e. for each year a static spatial SLR field is prescribed). In the model we use the mean SLR in the period 1986-2005 as the reference level for sea level rise. Processes are computed and combined using the probabilistic model described in Le Bars (2018), here we use the median of these probabilistic SLR projections. The period 1950-2016 is informed by observations-based products; Antarctic and Greenland ice sheets (Mouginot et al., 2019; Rignot et al., 2019), the glaciers (Marzeion et al., 2015), thermal expansion between 0 and 2000 m depth (Levitus et al., 2012), and climate-driven water storage (Humphrey and Gudmundsson, 2019). The ice sheets are assumed to be in equilibrium before 1979 for Antarctica and 1972 for Greenland because no data are available before these dates. The period 2016-2050 is based on the Fifth Assessment Report (AR5) of the Intergovernmental Panel on Climate Change (IPCC) for the RCP8.5 scenario (Church et al., 2013). For the dynamics of the Antarctic contribution we use the re-evaluation presented in the Special Report on the Ocean and Cryosphere in a Changing Climate (SCROCC) by the IPCC (Oppenheimer et al., 2019). Additionally, glacial isostatic adjustment is taken from the ICE-6G model (Peltier et al., 2015) but do not take into account other changes in land elevation (subsidence or tectonics).
Table 2-3: Overview scenarios and epochs in the water level change time series simulation
Scenario | Type | Period | Meteorological forcing |
ERA5 Reanalysis | Climate reanalysis | 1950-2024 | ERA5 |
Historical | Baseline climate scenario | 1950-2014 | HighResMIp ensemble, consisting of a mix of SST-forced (HadGEM3GC31-HM, and GFDL-CMC192) and coupled (EC-Earth3P-HR, CMCC-CM2-VHR4, and HadGEM3-GC31-HM) climate simulations |
Future | Future climate scenario based on SSP5-8.5 | 2015-2050 | HighResMIP ensemble, consisting of a mix of SST-forced (HadGEM3-GC31-HM and GFDL-CMC192) and coupled (ECEarth3P-HR, CMCC-CM2-VHR4, and HadGEM3-GC31-HM) climate simulations |
Tide only | Tide-only simulation | 1950-2050 | N/A |
2.3.4. Model / Algorithm
2.3.4.1. Global Tide and Surge Modelling (GTSM)
The Global Tide Surge Model (GTSM v3.0) is used as modelling tool to produce the C3S-435 dataset. The GTSMv3.0 is a depth-averaged hydrodynamic model with global coverage that dynamically simulates water levels resulting from tides and storm surges. GTSM has no open boundaries. Tides are modelled by forcing with the tide-generating forces using a set of 60 frequencies. Self-attraction and loading and dissipation of energy through generation of internal tides are considered (Irazoqui Apecechea et al., 2017). Storm surges are modelled by forcing with wind speed and surface pressure. Because tides and storm surge are modelled simultaneously, non-linear interaction effects are included.
GTSMv3.0 uses the unstructured Delft3D Flexible Mesh software (Kernkamp et al., 2011). The spatially-varying resolution leads to high accuracy at relatively low computational costs. It has an unprecedented high coastal resolution globally (2.5 km, 1.25km in Europe, Figure 2-1). The resolution decreases from the coast to the deep ocean to a maximum of 25km. Grid resolution is refined in areas in the deep ocean with steep topography areas to enable the dissipation of barotropic energy through generation of internal tides. GTSMv3.0 also has high temporal resolution producing output at 10-minute intervals. The 10-minutes time series are physically realistic since two types of forcing are used; that is, tidal and meteorological forcing. The tidal forcing is internally generated based on position of the earth, moon and sun. The meteorological forcing is available at hourly (or coarser) resolution, but is internally interpolated to the model timestep. Because tides vary at high-frequency and can non-linearly interact with storm surge (so the sum of the two is different from the individual components) we use an temporal resolution of 10 minutes. Especially for output stations with a wide and shallow continental shelf (such as the North Sea) lower temporal resolution, i.e. hourly resolution, can be too coarse and may miss the peak water levels.
Figure 2-1: GTSMv3.0 grid in SouthEast Asia (left) and Europe (right).
2.3.4.2. Time series of total water levels, tides, storm surges, and mean sea levels
GTSM output is processed using a post-processing toolbox written in Python. Time series for total water levels, tides and storm surge are available at a 10-minute resolution. Time series of mean sea levels have an annual resolution. For the user's convenience, we also derive hourly mean and daily maxima time series for the ERA5-based simulations. From 1950 to 2050, we ran simulations of total water levels (combination of tides and storm surges) including annually updated sea-level rise. In addition, we ran a tide-only simulation including annually updated sea-level rise. Time series of storm surges are computed by subtracting the tide-only simulation from the total water level simulation. This approach implies that any surge-tide interaction effect is part of the surge time series. Moreover, the time series for total water levels and tides include the relative sea-level rise.
2.3.5. Validation
GTSM is the backbone of the global water level change time series and indicator products, and the validation of the model is thus key to the product validation. The GTSM model has been thoroughly calibrated and validated based on tide gauge observations, satellite products, and comparison with other hydrodynamic models. Table 2-4 provides a summary of the key references and description of the results.
Table 2-4: Key references of GTSM validation
Reference | Description |
Muis, S., Verlaan, M., Winsemius, H. C., Aerts, J. C. H., & Ward, P. J. (2016). A global reanalysis of storm surges and extreme sea levels. Nature Communications, 7(1), 1-12, doi:10.1038/ncomms11969 | Validation of GTSM2.0 for the modelling of storm surges and estimation of return periods. Results show good agreement with observations. Storm surges, especially those induced by tropical cyclones, are slightly underestimated; this is mainly due to the resolution of the meteorological forcing. |
Dullaart, J.C.M., Muis, S., Bloemendaal, N. & Aerts, J. C. H. (2020). Advancing global storm surge modelling using the new ERA5 climate reanalysis. Climate Dynamics 54, 1007–1021, doi:10.1007/s00382-019-05044-0 | Evaluation of the performance of GTSM3.0 for the global modelling of storm surges for historical extreme events, and the advances due to ERA5 climate reanalysis |
Muis, S., Apecechea, M. I., Dullaart, J., de Lima Rego, J., Madsen, K. S., Su, J., Kun, Y. & Verlaan, M. (2020). A High-resolution global dataset of extreme sea levels, tides, and storm surges, including future projections. Frontiers in Marine Science, 7, 263, doi:10.3389/fmars.2020.00263 | Validation of GTSM3.0 for application to climate change projections. Comparison against observations shows a good performance with observed sea levels demonstrated a good performance with the annual maxima having mean bias of -0.04 m. |
Wang, X., Verlaan, M., Apecechea, M. I., & Lin, H. X. (2021). Computation‐Efficient Parameter Estimation for a High‐Resolution Global Tide and Surge Model. Journal of Geophysical Research: Oceans, 126(3), e2020JC016917. | Description of the calibration of the GTSM. Result show that the accuracy of the tidal representation can be improved significantly at affordable cost. |
Irazoqui Apecechea, M., Rego, J., Verlaan, M (2018) GTSM setup and validation. Project report C3S_422_Lot2_Deltares - European Services | Description of the calibration of the GTSM. |
The validation of total water levels and storm surges in Muis et. al. (2016; 2020) indicates a good performance for the modelling of extreme sea levels. In general, when comparing modelled and observed time series the root-mean-squared-errors are low (<10cm) and correlation coefficients are high (>0.7). Also return periods show a good performance with a mean bias of 10cm for a 10-year return period. The high accuracy of GTSM is attributed to the increased model resolution at the coast, which is where the highest storm surges are generated. Moreover, the good quality of the ERA5 climate reanalysis also contributes to the model performance. The model performance is generally lower in regions with little variability and storm surges dominantly induced by tropical cyclones. This is linked to the resolution of the meteorological forcing which is too low to fully resolve tropical cyclones. In topographically complex areas, such as estuaries and semi-enclosed bays, the model resolution of GTSM may be insufficient to accurately capture the storm surge.
The validation of tides is not published, and it is described in more detail here. We validated GTSM against observed tides from the University of Hawaii Sea Level Center (UHSLC) dataset, which contains 251 tide gauge stations. In addition, we validated GTSM against modelled tides derived with the FES2012 model, which is an assimilative global tide model. GTSM shows, in general, a good agreement with the observed and models tides (Table 2-5). In general, GTSM seems to overpredict tidal amplitudes (Table 2-5). Errors near the coast are larger than in the open ocean. However, with an average M2 vector difference error of 10.5cm at the coast, the model has a comparable accuracy to state-of-the-art assimilative global tide models. Moreover, when compared to non-assimilated global tide models, the GTSMv3.0 performs significantly better. For the semi-enclosed seas, a good agreement with observations proves more difficult. The semi-enclosed sea of the Baltic is sensitive to the narrow connection with the adjacent North Sea and the geometry of such connection, which seems to negatively affect the model performance. In some areas we cannot properly assess the model performance because of the lack of a full spatial coverage and long records of time-series.
Table 2-5: Model performance of GTSM against the UHSL dataset and the FES2012 model. The metrics used are standard deviation of errors (STDE), relative range, and correlation coefficient (R).
Geographical Area | UHSCL tide gauge stations | FES2012 assimilative tide model | ||||||
No. of stations | STDE | Relative | R | No. of stations | STDE | Relative | R | |
Antarctic | 1 | 0.07 | 101 | 0.98 | 3 | 0.14 | 107 | 0.96 |
Arctic | 3 | 0.12 | 115 | 0.94 | 40 | 0.05 | 125 | 0.85 |
South East Asia | 27 | 0.28 | 113 | 0.90 | 0 | - | - | - |
Indian Ocean | 39 | 0.20 | 114 | 0.94 | 72 | 0.07 | 112 | 0.98 |
North Atlantic | 48 | 0.18 | 106 | 0.86 | 30 | 0.07 | 102 | 0.97 |
North Pacific | 75 | 0.15 | 102 | 0.95 | 65 | 0.07 | 104 | 0.98 |
South Atlantic | 13 | 0.16 | 114 | 0.94 | 43 | 0.05 | 111 | 0.99 |
South Pacific | 45 | 0.14 | 109 | 0.93 | 94 | 0.07 | 111 | 0.97 |
Total | 251 | 0.18 | 108 | 0.92 | 347 | 0.06 | 111 | 0.96 |
The model performance is also assessed in terms of energy budget. In general, the global and regional estimates of M2 energy dissipation through bottom friction and internal wave drag are in good agreement with satellite altimetry derived estimates by Egbert and Ray (2001). Sensitivity tests show that these energies are slightly sensitive to bottom friction coefficient changes within a range of typical values. The dissipation estimated seems quite sensitive to changes of similar order to the internal wave drag coefficient, showing a positive response in terms of STDE to increasing values of the parameter. However, it is concluded from these tests that spatially non-uniform calibration of both dissipation parameters is needed to optimize the model solution and the agreement with the observed regional dissipation estimates.
UHSCL tide gauge stations | FES2012 assimilative tide model | |
STDE | ||
R |
Figure 2-2: GTSM model validation against the UHSL dataset and the FES2012 model showing the standard deviation of and correlation coefficient (R).
In summary, the validation demonstrates a good model performance of GTSMv3.0 on representing total water-levels, tides and storm surges. The high accuracy is the result of the continuous model developments, which has been focused at improving the model physics, grid resolution, and input datasets. GTSM is capable of simulation of historical events as well as multi-decadal simulations for historical and future climate scenarios.
3. Concluding Remarks
Use of the dataset
This dataset presents water level time series resulting from tides, surges and sea level rise computed for the whole globe. The reanalysis dataset is based on the ERA5 reanalysis, while the climate projections dataset is based on the multi-model HighResMIP ensemble (one of the CMIP6 model experiments). The projections cover the period 1950-2050 and are based on SSP5-8.5. The dataset constitutes a major improvement compared to dataset provided in the C3S 422 Lot2 contract, which was limited to the pan-European domain and included a single CMIP5 model.
The water level change time series allow for evaluation of the impacts of climate change and sea level rise including an assessment of uncertainty. Different coastal sectors and industries, such as flood risk authorities, harbors and ports, coastal zone management institutes, can use the dataset for a first-cut assessment of changes in extreme sea level for the mid-century. In addition, the time series can be used for dynamic downscaling for regional studies of climate change impacts.
Limited applicability at the local level
The dataset is designed specifically for global to continental scale assessment of climate change impacts on coastal water levels, and users should take caution when applying the dataset to smaller scale studies. Although the coastal resolution of GTSM is high for a global model (e.g. 1.25km in Europe, 2.5km rest of the world), this resolution is rather coarse for areas with a complex bathymetry such a estuaries or semi-enclosed bays. Moreover, at the local scale it may be important to consider additional physical processes which are not simulated by GTSM. This includes, for example, the effect of waves during extremes and the seasonal variability in mean sea level driven by baroclinic currents. It is therefore recommended to use this dataset only in large-scale studies. For regional studies, we would recommend using the time series as boundary conditions for hydrodynamic models with a more refined resolution, where also the local data and knowledge can be included.
Uncertainties in projected changes
For the projections dataset, a source of uncertainty is the performance of the individual HighresMIP climate models. Climate models can have large spatial bias and may under- or overestimate the storm surges related statistics. In many places the projected changes in extreme are small compared to the uncertainties. The intermodel agreement may indicate how large the uncertainties are. Users could consider carrying out of more detailed evaluation of the spatial bias of the individual climate models for their specific region of interest, and subsequently, they could decide to give more weight to a good-performing model.
4. Appendix I
FAQ
Q: What is the vertical reference of the dataset? How is it derived?
A: The vertical reference level of the time series is the mean sea level (MSL) calculated over the 1986-2005 reference period as used by the Intergovernmental Panel on Climate Change in the Fifth Assessment Report (IPCC AR5). For each year of simulation, the model is initialized with sea level rise fields for that year, relative to the 1986-2005 mean; this is done by imposing a spatially-varying static pressure field that corresponds to the relative sea level rise (inverse barometer relation). Note the reference mean sea level values may be different from the IPCC AR5 values due to minor differences in the calculation methods. The MSL 1986-2005 datum is applicable for all time series and indicators provided in this dataset and does not change over time (e.g. from historical to future period). Please note that the time series for total water levels and tides include the relative sea-level rise, while for the surge levels the sea-level rise component is removed.
Q: How is sea level rise considered in the model simulation?
A: All simulations include spatially-varying sea level rise (SLR) contributions. The SLR fields are computed using a probabilistic model (Le Bars, 2018) based on observations (1950-2015) and CMIP5 climate models according to RCP8.5 for 2016-2050 and hence is independent of the model selection in this catalogue entry. This includes changes in sea level from various processes including thermal expansion of the ocean, changes in ocean circulation, ice sheet contributions, and glacio-isostatic adjustment (but not subsidence or tectonics). The annual SLR fields are referenced to the mean levels over the period 1986–2005, with a spatial resolution of 1° × 1° and interpolated to the model grid using nearest neighbor. The SLR field is used to initialize the GTSM model at annual timesteps. The same approach is applied both for the projections dataset (based on HighResMIP) and the reanalysis dataset, which means that for the recent years (after 2015) the reanalysis dataset contains sea level rise fields that are based on climate projections, and not on observed sea level rise data. However, for the period of the reanalysis model simulations this is a plausible assumption given the relatively small SLR (and hence small discrepancy between the projections and the observed SLR).
Q: Is the model output at 10-minute temporal resolution physically realistic?
The 10-minutes time series are physically realistic because two types of forcing are used; tidal forcing and meteorological forcing. The tidal forcing is internally generated based on position of the earth, moon and sun. The meteorological forcing is available at hourly (or coarser) resolution and is internally interpolated to the model timestep. Since tides vary at high-frequency and produce non-linear interactions with storm surges (the sum of the two is different from the individual components), we use a temporal resolution of 10-minutes to capture the high frequency signal. This is especially relevant for stations on wide and shallow continental shelves, such as the North Sea, where an hourly resolution may be too coarse and potentially miss the peaks in water level. For further details on the model and validation, see Kernkamp et al. (2011) and Muis et al. (2016, 2022).
5. References
Dullaart, J.C.M., Muis, S., Bloemendaal, N. & Aerts, J. C. H. (2020). Advancing global storm surge modelling using the new ERA5 climate reanalysis. Climate Dynamics 54, 1007–1021, doi:10.1007/s00382-019-05044-0
Egbert, G. D., and Ray, R. D. (2001). Estimates of M2 tidal energy dissipation from TOPEX/Poseidon altimeter data, J. Geophys. Res., 106(C10), 22475–22502, doi:10.1029/2000JC000699.
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