Oceanic tidal angular momentum and Earth's rotation variations

Luni-solar tides affect Earth's rotation in a variety of ways. We give an overview of the physics and focus on the excitation of Earth rotational variations by ocean tides under the conservation of angular momentum. Various models for diurnal and semidiurnal tidal height and tidal current fields have been derived, following a legacy of a number of theoretical tide models, from the Topex/Poseidon (T/P) ocean altimetry data. We review the oceanic tidal angular momenta (OTAM) predicted by these T/P models for the eight major tides (Q~, O~, P~, K~, N2, M2, $2, K2), and their excitations on both Earth's rotational speed variation (in terms of length-of-day or UTI ) and polar motion (prograde diurnal/semidiumal components and retrograde semidiurnal components). These small, high-frequency effects have been unambiguously observed in recent years by precise Earth rotation measurements via space geodetic techniques. Here we review the comparison of the very-long-baseline-interferometry (VLBI) data with the T/P OTAM predictions. The agreement is good with discrepancies typically within 1-2 microseconds for UTI and 10-30 microarcseconds for polar motion. The eight tides collectively explain the majority of subdaily Earth rotation variance during the intensive VLBI campaign Cont94. This establishes the dominant role of OTAM in exciting the diurnal/semidiurnal polar motion and paves the way for detailed studies of short-period non-OTAM excitations, such as atmospheric and oceanic angular momentum variations, earthquakes, the atmospheric thermal tides, Earth librations, and the response of the mantle lateral inhomogeneities to tidal forcing. These studies await further improvements in tide models and Earth rotation measurements. © 1998 Elsevier Science Ltd. All rights reserved 1. KINEMATICS AND MEASUREMENTS OF EARTH'S ROTATION Earth's spin has long been humankind 's best timekeeper, at least until recently. The time it keeps is called Universal Time (UT), or UT1 after an empirical correction to allow for polar motion (see below). The first evidence indicating the non-uniformity of UT, i.e. that the Earth actually spins not at a constant rate, emerged when UT was compared against the Ephemeris Time kept by motions of solar system bodies according to Newton's gravitational law, or against the time kept by the best mechanical clocks. The advent of the atomic clocks in the 1950s provided the truly accurate and uniform time-keeping standard known as the atomic time (AT); and the difference between UT1 and AT became the standard quantity that reflects the Earth's spin variation as a function of time. The spin variation is often conveniently expressed in terms of the length-of-day (LOD), which is proportional to the time derivative of UT1, if the time scale of interest is longer than one day. The Earth's spin, of course, is only one component, namely the axial component, of the 3-