European altimetry-derived tide model for improved tide and water level forecasting along the Dutch Continental Shelf

With the continued rise in global mean sea level, accurate operational predictions of tidal height and total water levels have become crucial for early warning of potential extreme events in the coastal region. Ocean tides play an important role in extreme sea level events, with high oceanic tides increasing the likelihood of coastal flooding. The Dutch Continental Shelf Model in Flexible Mesh (DCSM-FM) is developed at Deltares to operationally estimate the total water levels to help trigger early warning systems to combat these extreme events along the Dutch coastline. At the boundaries of this model, a tidal forcing is applied from global ocean tide models to better incorporate the ocean tidal height estimations within the model.

In this study, a regional Empirical Ocean Tide model for the Northwest European Continental Sea (EOT-NECS) is developed with the aim to apply better tidal forcing along the boundary of the regional DCSM-FM. EOT-NECS is developed at DGFI-TUM by using thirty years of multi-mission along-track satellite altimetry to derive tidal constituents which are estimated both empirically and semi-empirically. Compared to the previous global iteration, EOT20, EOT-NECS showed a reduction in the root-square-sum error for the eight major tidal constituents of 0.525 cm compared to in-situ tide gauges.

Water levels of DCSM-FM are forced from a number of sources. At the open model boundaries, a combination of water levels from multiple global tide models, an estimation of the surge levels through an Inverse Barometer Correction based on the local atmospheric pressure, and the forcing of the density driven mean sea surface height from a global ocean recirculation model is used. A part of the water level signal is generated within the model domain. This is based on tidal potential within the model domain, meteorological forcing and baroclinic processes. In the 2D depth-averaged version of the model, the contribution of the latter is forced through a static water level field from the 3D version of the model, representing the Mean Dynamic Topography.

When applying constituents from EOT-NECS at the boundaries of DCSM-FM, an overall improvement of 0.42 cm was seen in the root-mean-square error of tidal height estimations made by DCSM-FM, with some regions exceeding a 1 cm improvement. The results demonstrate that there is a large importance in using the appropriate tide model(s) as boundary forcings and in this manuscript, the use of EOT-NECS has a clear positive impact on the total water level estimations made in the northwest European continental seas.