The problematic Ψ1 ocean tide

Abstract

SUMMARY Observations of the ψ1 earth tide yield valuable insights into the earth’s free core nutation, especially if the effects of the ψ1 ocean tide can be removed. The ocean tide is extremely small, with amplitudes rarely more than a few millimetres, and developing an accurate model is challenging. Direct observations are inadequate to support a global model. The alternative—numerical simulation—must account for a multitude of possible effects. The ocean tide is forced by the gravitational tidal potential, by pressure loading from atmospheric tides, by seasonal modulation of the nearby K1 constituent, and possibly by non-linear interactions among several other constituents. Here we construct a model of the ψ1 ocean tide which accounts for (or attempts to bound) each of these effects. The radiational component (from atmospheric pressure loading), although relatively small, is complicated by the presence of non-tidal atmospheric variability in the diurnal band. The ocean’s response is dynamic, but there is also high-wavenumber pressure forcing with a near-isostatic response. A general circulation model, forced by both winds and the tidal potential, suggests that annual variability in K1 leads to pronounced ψ1 amplitudes in some marginal seas, especially in the western Pacific.