Numerical modeling of the global semidiurnal tide in the present day and in the last glacial maximum

TLDR

The background is that internal tide drag depends on ocean stratification, which is poorly known for the last glacial maximum, leaving significant uncertainties about paleotides even when bathymetry is well constrained. The study uses a hydrodynamic model that self‑consistently accounts for ocean self‑attraction and loading and parameterizes internal tide drag to assess the accuracy of barotropic tide modeling without data and to explore tidal energetics during the last glacial maximum. The authors employ a global hydrodynamic model that self‑consistently incorporates ocean self‑attraction and loading and a physically based internal tide drag parameterization. High‑resolution M2 solutions with present‑day bathymetry match satellite altimetry within 5 cm RMS, and this agreement, along with accurate energy‑dissipation estimates, requires inclusion of SAL and IT drag; model sensitivity to bathymetry errors explains remaining elevation errors, while the ~100 m sea‑level drop at the LGM increases North Atlantic M2 amplitudes by factors of two or more and raises global dissipation by ~50 % (tripling in the deep ocean), a robust result that remains sensitive to stratification assumptions.

Abstract

A hydrodynamic model incorporating a self‐consistent treatment of ocean self‐attraction and loading (SAL), and a physically based parameterization of internal tide (IT) drag, is used to assess how accurately barotropic tides can be modeled without benefit of data, and to explore tidal energetics in the last glacial maximum (LGM). M 2 solutions computed at high resolution with present day bathymetry agree with estimates of elevations from satellite altimetry within 5 cm RMS in the open ocean. This accuracy, and agreement with atlimetric estimates of energy dissipation, are achieved only when SAL and IT drag are included in the model. Solutions are sensitive to perturbations to bathymetry, and inaccuracies in available global databases probably account for much of the remaining error in modeled elevations. The ≈100 m drop in sea level during the LGM results in significant changes in modeled M 2 tides, with some amplitudes in the North Atlantic increasing by factors of 2 or more. Dissipation is also significantly changed by the drop in sea level. If IT drag estimated for the modern ocean is assumed, dissipation increases by about 50% globally, and almost triples in the deep ocean. However, IT drag depends on ocean stratification, which is poorly known for the LGM. Tests with modified IT drag suggest that the tendency to a global increase in dissipation is a robust result, but details are sensitive to stratification. Significant uncertainties about paleotides thus remain even in this comparatively simple case where bathymetry is well constrained.