Global modelling of ocean tides

A longstanding challenge in oceanography is the production of accurate global maps of the amplitude and phase of a given tidal component, simply from Laplace's tidal equations supplemented by the corresponding astronomical forcing and global bathymetry. Over the last forty years, such prognostic models have increased in accuracy, partly due to increases in computing power, but also as the role of additional physical processes have been recognised (such as the effects of oceanic self-attraction and Earth loading, and more recently the role of a significant internal tide drag). By comparison with data-constrained hydrodynamic tide models, which take advantage of near-global satellite altimetry, prognostic tide models can now achieve impressive accuracy, particularly in the open ocean.

We have developed a prognostic tide model with complete global coverage. It uses simple finite-difference numerics with variable spatial resolution, having approximately twice the resolution at high latitudes than at the equator. Thus it allows for detailed study of the polar oceans, which are relatively poorly constrained by satellite altimetry. We place special emphasis on implementing a realistic parameterisation of the internal tide drag, which is often tuned in other models to fit observations.

Using this model, we calculate the amplitude and phase of the largest tidal constituents, and consider their form since the Last Glacial Maximum (LGM). Of particular interest is an examination of the significant enhancement of the lunar M2 tidal amplitude in the Labrador Sea at LGM, reported in previous studies, which has possible implications for polar dynamics. To examine this and other changes with more care, an accurate bathymetry is essential; we use the ICE-5G reconstruction, which has key improvements over datasets used in previous studies. Also important, for the internal tide drag parameterisation, is knowledge of the ocean stratification. In contrast to previous studies, this is diagnosed in a consistent way, by using results from coupled atmosphere-ocean climate simulations for LGM and present-day conditions.