Towards Improved Shallow-Water Tide Modelling for Gravimetric Observations

Ocean tide signatures are omnipresent in geodetic observations. This applies to direct observations of sea surface height variations or tidal transports, as well as to geodetic observables that are implicitly affected by them, including horizontal and vertical deformation of the solid Earth, tidal variations of the terrestrial magnetic field, and most importantly, the terrestrial gravity field. Since ocean tide-induced oscillations often have a significant magnitude, they can introduce artefacts into observation time series and thereby degrade the overall observation quality if they are not corrected by a skilful model. A prominent example is tidal aliasing in satellite gravimetric observations, caused by imperfections in ocean tide models, which still contribute significantly to the GRACE(-FO) uncertainty budget.

While modern ocean tide atlases have achieved high accuracy through the incorporation of satellite altimetric observations, several parts of the ocean tide spectrum remain insufficiently known. This particularly includes shallow-water tides excited by hydrodynamical nonlinearity, which can reach high amplitudes in extended shelf seas. Consequently, they can strongly impact, for example, altimetric and gravimetric observations. Due to their small amplitudes and short wavelengths, shallow-water tides are more difficult to observe from space; therefore, modern ocean tide atlases rely heavily on purely numerical modelling. However, the underlying hydrodynamical processes are still not well understood and are only approximately parameterised. Although purely numerical models succeed in reproducing general patterns of shallow-water tides, they are not yet accurate enough to yield a meaningful variance reduction in geodetic observations. For instance, the M₄ tide from the TiME22 ocean tide atlas achieved a variance reduction of only 15–25 %.

In this contribution, we combine several approaches to improve the modelling accuracy of TiME over the European shelf – a well-suited test region due to high tidal amplitudes and the availability of long gravimetric time series at near-coastal locations. Modelling strategies include (i) increasing the model resolution to obtain a more realistic representation of bathymetry, (ii) accounting for wetting and drying processes—particularly important in the Wadden Sea—and (iii) representing the divergence between the alignment of mean and bottom-layer flow. While each of these effects improves the overall model performance, their combination substantially enhances the large-scale (temporal and spatial) prediction accuracy of shallow-water tidal dynamics and enables the generation of a geodetic correction atlas for a wide range of nonlinear tides.