A likely role for stratification in long-term changes of the global ocean tides

Stratification-that is, the vertical change in seawater density-exerts a subtle control on the energetics and thus the surface elevation of barotropic (depth independent) flows in the ocean. Changes in stratification therefore provide a plausible pathway to explain some of the puzzling trends in ocean tides evident in tide gauge and, more recently, satellite altimetry data. Using a three-dimensional global ocean model, we estimate that strengthening of stratification between 1993 and 2020 caused open-ocean trends of order 0.1 mm yr^-1 in the barotropic M₂ tide, similar in structure and magnitude to long-term M₂ changes deduced from satellite altimetry. Amplitude trends are predominantly negative, implying enhanced energy transfer to internal tides since the 1990s. Effects of stratification are also a relevant forcing of contemporary M₂ trends at the coast, where they may modulate or even overprint the tidal response to sea level rise. Salient examples for such significant near-shore influence of stratification (≥ 95% confidence) include the Northwest Australian Shelf ( - 0.5 mm yr^-1) and the coasts of western North America ( - 0.1 mm yr^-1), commensurate with observed M₂ amplitude trends at tide gauges.