Atmospheric Excitation of the Earth's Wobble

Hassan (1960) has computed mean monthly values of the atmosphere's moments and products of inertia for the period 1873 to 1950 using all available station-level pressures. On the basis of these time series the excitation of the seasonal and 14-month (Chandler) wobble is discussed. With respect to the seasonal wobble, our calculations confirm the conclusion by Jeffreys and others that it is due largely to atmospheric excitation. By working directly with station-level pressures we have avoided the procedure followed previously of removing the sea-level correction from sea-level pressure charts (thus “uncorrecting” for a large and undesirable correction). Our amplitudes are 25 per cent smaller than those given by Jeffreys (1959). With respect to the Chandler wobble, it has been suggested by Jeffreys (1940), Rudnick (1956), and by Munk & MacDonald (1960) that this represents a resonance amplification of the irregular (non-seasonal) variation in atmospheric inertia. The computed spectral density of the atmospheric variation at the Chandler frequency falls short by one to two orders of magnitude to meet the requirement of this hypothesis. Excitation by irregular motion in the core is briefly considered. Here the electromagnetic coupling (limited by conductivity in the lower mantle) appears to be far too weak to account for the observed wobble. The excitation of the Chandler wobble must be considered an unsolved problem.