The problematic Ψ1 ocean tide

Abstract

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 millimeters, 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 nonlinear 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.