A Likely Role for Stratification in Secular 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 2019 caused open-ocean trends of order 0.1 mm yr -1 in the barotropic M 2 tide, similar in structure and magnitude to secular M 2 changes deduced from satellite altimetry. Amplitude trends are predominantly negative, implying enhanced energy transfer to internal tides since the 1990s. Effects of stratification on the M 2 amplitude increase to -0.4 mm yr -1 in several coastal regions and are the dominant driving factor for contemporary decreases in tidal amplitude along the coasts of Northwest Australia, the United Kingdom, and western North America.