Phase relations for high frequency core–mantle coupling and the Earth’s axial angular momentum budget

Abstract

Although it has long been thought that dynamical core–mantle coupling is necessary for understanding decadal variations in the length of day (LOD), the physics of the coupling mechanism are very poorly constrained. There several hypotheses — electromagnetic, topographic, gravitational, viscous — but it is notoriously difficult to distinguish between them on observational grounds. There is both an expectation [Geophys. J. Int. 138 (1999) 679] and some evidence [J. Geophys. Res. 100 (1995) 8233; Geophys. Res. Lett. 24 (1997a) 1799] that there is significant core–mantle coupling on much faster timescales, down to subannual periods. At such frequencies, the core would no longer be the dominant driving force for LOD, but would instead interact with both the mantle and external reservoirs of angular momentum such as the atmosphere and ocean. On these much faster timescales, it is possible that some information that is obscured on decadal timescales becomes available: in particular, any characteristic timescales at very short periods may become observable. Here, I examine the angular momentum budget of the Earth on subannual to annual timescales for any additional implied constraints on the physics of angular momentum exchange between the core and the mantle. I find that the discrepancy between atmospheric angular momentum and mantle angular momentum on timescales where the ocean is thought to be unimportant has a distinctive signature which is difficult to reproduce using simple models of core–mantle coupling, although an ad hoc “frictional” model does moderately well. Rather than indicating simplicity, this may instead be symptomatic of great complexity in the physics of the core–mantle boundary region.