On the validity of the geostrophic approximation for calculating the changes in the angular momentum of the core

The changes in the length of day at decade periods are attributable to exch anges of angular momentum between the core and the mantle. It is commonly a ssumed that the changes in angular momentum in the fluid are carried by geo strophic flows which are characterized by the rotation of rigid cylindrical shells aligned with the Earth's rotation axis. In this work, we explore th e validity of this assumption using the simple geometry of a plane fluid la yer between two solid boundaries. Coupling at the boundaries is done by ele ctromagnetic interactions. We quantify the conditions under which the motio n of the fluid that results from this coupling is accurately described by g eostrophic flows. We find that for the region outside the so-called tangent cylinder, the geostrophic approximation is valid for all periods of oscill ation longer than a few days. For the region inside the tangent cylinder, t he geostrophic nature of the flow is broken at annual and shorter periods b y the propagation of MC waves. (C) 1999 Elsevier Science B.V. All rights re served.