Three drivers of 21st‐century changes in ocean tides

Numerical model simulations are conducted to study the response of barotropic ocean tides to 21st-century climate change, as manifested by sea level rise, increasing ocean stratification, and expanding Antarctic ice shelf cavities. Emphasis is placed on surface elevations, with projections of (Formula presented.), (Formula presented.), (Formula presented.), and (Formula presented.) made in decadal time slices (2050–2100) under the Representative Concentration Pathways (RCP) 4.5 and 8.5. The simulations suggest that enhanced energy transfer to baroclinic motion in an increasingly stratified ocean weakens barotropic tides on a global scale and dominates secular and decadal amplitude variability in the open ocean for most of the 21st century. Under RCP8.5, significantly reduced ((Formula presented.) 50%) boundary layer dissipation beneath thinning ice shelves readjusts the excitation of semidiurnal normal modes, leading to amplitude changes by (Formula presented.) 2090–2100 of order a few cm on (sub-) basin scales. Sea level rise, on the other hand, causes localized anomalies—primarily (Formula presented.) and (Formula presented.) amplitude increases—in numerous shallow coastal regions. These local signals are modulated in places (e.g., Gulf of Maine, Patagonian, Northwest European, and Northwest Australian shelf areas) by spatially extended tidal changes associated with the other forcing factors, with a tendency for counteraction between sea level and stratification effects. The combined impact of all three drivers on coastal high tide water levels, as would be of interest in flood risk assessments, varies with location and climate scenario. Under RCP4.5 and at 2100, changes (Formula presented.) 5 cm are mostly restricted to river deltas, but wider sections of coastline could be affected under the upper-bound RCP8.5 scenario.