Changes in ocean tides by the end of the 21st century in response to stronger stratification

Recent model results in combination with observations have provided a first coherent picture of secular changes in ocean tides since 1993. Strengthening of ocean stratification has been identified as an important driver of the observed secular trends, where the barotropic tide is primarily affected through enhanced tidal conversion at topography. These changes are responsible for open-ocean trends in the order of 0.1 mm yr^-1 for the barotropic M₂ tide, increasing to magnitudes comparable to the tidal response to sea level change (0.2—0.4 mm yr^-1) in several coastal regions. This has ramifications for global projections of future extreme sea levels, which either neglect changes in tides or consider them solely as a function of sea level rise. In this study, we employ a global high-resolution (1/12°) internal-tide permitting numerical ocean model to quantify future changes in ocean tides until 2100 as a result of upper-ocean warming and the concomitant increase in stratification. We simulate the evolution of leading tidal constituents in 5-year average time slices and use EC-Earth3P HighResMIP density data to constrain the model’s background stratification. As the Representative Concentration Pathway (RCP) in the EC-Earth3P simulation is a high greenhouse gas emission scenario (RCP8.5), we also consider data from a CM2.6 coupled global climate model, which is more closely aligned with a medium stabilisation scenario (RCP6.0).