MAC-OSCILLATIONS OF THE HIDDEN OCEAN OF THE CORE

The dynamics of the stably stratified layer at the top of the core, which we call the H-layer or the hidden ocean of the core (HOC), is considered. It is shown that global axisymmetric eigenoscillations of the H-layer are possible that are similar to MAC-waves. These oscillations have periods of the order of a few decades (similar to 65 yr) if N similar to 2 Omega where N is the Brunt-Vaisala frequency of the H-layer and Omega is the frequency of the Earth's rotation. H-layer oscillations can be excited by an instability mechanism that resembles baroclinic (sloping) instability, and they in turn can excite torsional oscillations (TO) in the bulk of the core. The joint action of these two oscillations provide a mechanism for the generation of the decade geomagnetic secular variations, and the associated variations in the length of day. Rough estimates of the physical parameters of the H-layer are obtained by comparison of the HOC-oscillation theory with observations. The existence of the H-layer has significant consequences for the Earth's dynamo, that are briefly discussed.