Interactions between the atmosphere, oceans and crust: Possible oceanic signals in Earth rotation

Abstract

Observations of changes in the orientation of the Earth (UT1 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 barotropic 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 Niño 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.