Adv. Space Res. Vol. 13, No. 11, pp. (11)291—(1 1)300, 1993 0273—1177/93 $6.00 + 0.00 Printed in Great Britain. All rights reserved. Copyright (6 1993 COSPAR INTERACTIONS BETWEEN THE ATMOSPHERE, OCEANS AND CRUST: POSSIBLE OCEANIC SIGNALS IN EARTh ROTATION T. M. Eubanks Code TSEO, U.S. Naval Observatory, Washington DC, 20392, U.S.A. ABSTRACT Observations of changes in the orientation of the Earth (UT! and polar motion) provide a novel means of studying the dynamics of the atmosphere and oceans as variations in the angular momentum of the oceans and atmosphere must be balanced by changes in the rotation of the “solid” Earth (the crust and mantle). Estimates of the total angular momentum of the atmosphere are routinely available as a by-product of medium range weather forecasting, and these data have greatly facilitated the study of the terrestrial angular momentum balance. Although the role of the oceans in non-tidal rotational fluctuations remains obscure due to a lack of suitable oceanic data, oceanic angular momentum exchanges may well be visible in current Earth rotation data. This paper presents a simple dynamical model of baroiropic and baroclinic ocean waves to illustrate possible oceanic excitations of rotational variations. Sea floor bottom pressure measurements strongly suggest that bottom pressure changes are the cause of currently unexplained polar motions at short periods (weeks to months), and it is shown that these are probably dominated by barotropic oscillations forced by surface wind stresses. At longer periods, baroclinic ocean waves are part of the El Nub Southern Oscillation phenomena, and it is shown that these may cause observable interannual variations in rotation rate. By contrast, the close correlation found between rotation rate and atmospheric zonal winds indicates that the high frequency variations in the axial wind stress torque on the oceans must be rapidly transmitted to the solid Earth. Recent work indicates that this is done by the rapid zonal propagation of barotropic ocean waves to the continental margins of the ocean basins, which may provide a detectable geodetic signal. INTRODUCTION The rotation of the Earth is not uniform, butexhibits both regular and irregular variations on time scales from less than a day to millennia. The irregular variations are largely due to exchanges of angular momenta between the solid Earth and the atmosphere, oceans, and liquid outer core, which can cause both changes in the direction of the pole of rotation (or polar motions) as well as variations in rotation rate (generally described in terms of the changes in the Length of the Day, or LOD). Geodetic observations of the LOD and the polar motion are thus a novel means of observing the global fluid dynamics of the atmosphere, ocean and core, with recent reviews of research in this area being provided byNandi2/~ The modem space geodetic techniques of Very Long Baseline Interferometry (VLBI), Satellite Laser Ranging (SLR) and Lunar Laser Ranging (LLR) have improved both the precision and temporal resolution of rotational measurements by almost two orders of magnitude over the older technique of optical asirometry, and make it possible to observe relative variations as small as a few parts in iO~ over durations as short as a few hours. These improvements in geodetic accuracy have been matched by improvements in the ability to estimate the Atmospheric Angular Momentum (AAM), as meteorological AAM estimates are currently available from a number of weather centers as a by-product of numerical modeling of the atmosphere for medium-range weather forecasting,2’ci3’. The atmosphere has been found to clearly dominate the excitation of sub-seasonal LOD changes, down to periods as small as 8 days/4’c and is an important source of polar motions at those frequencies as well/S’ç,’b< At longer periods, inter-annual LOD variations have been correlated with the Southern Oscillation, or Soiii,s~ç and with the Quasi-Biennial Oscillation, or QBO,W.