The changing statistics of storm surges across the Northwest of Ireland in a warming world.

Abstract

Unequivocal change in the climate system has put coastal regions around the world at increasing risk from climate-related hazards like storm surge flooding. With studies demonstrating an increase in the frequency of storm surges along the Northwest coast of Ireland, within the EU H2020 project SCORE (Smart Control of the Climate Resilience in European Coastal Cities) a numerical modelling approach has been undertaken using SHYFEM (System of HYdrodynamic Finite Element Model) to simulate the storm surges into 2100 for the NW of Ireland under different Climate scenarios.   Using the significant grid flexibility offered by the finite elements within SHYFEM, unstructured grids are implemented for two computational domains. One confined to the region of interest, specifically the Donegal Bay, namely the Limited Area Model (LAM) and the other extended to span the entirety of the North Atlantic Basin namely the Basin Scale Model (BSM). The two model applications are designed to capture the development of storm surges due to wind shear stresses and barometric pressure fluctuations associated with the passage of storm systems across the North Atlantic and to quantify the influence of the tides on the tide surge interaction. Given the large tidal excursion in the region, baroclinicity is neglected and the models are implemented in the barotropic mode. The LAM is forced with surface atmospheric forcing (pressure and winds) from ECMWF and sea surface height (SSH, including eleven tidal components) from CMEMS at its open boundary, whereas the BSM is solely forced with the atmospheric forcing. Extensive calibration of wind drag and bottom friction coefficients for the LAM and BSM using tidal gauge observations for 2020, shows high P. Corr and acceptable RMSE. Only negligible differences, around 3 cm, are seen in the surge magnitudes between the LAM and the BSM implying the negligible non-linear interaction of the tides on the storm surge dynamics in the Donegal Bay.  Thus, the BSM has been forced with atmospheric forcing obtained from CMIP6 climate models to predict the impact of climate changes on storm surge intensity and frequency under different climate scenarios up to 2100. Finally, by accounting for the tidal contribution, total water levels projections have been determined thus facilitating the generation of useful statistics (for instance Annual exceedance probabilities) and flood maps to inform relevant policies for coastal resilience.