Chandler Wobble
The Chandler wobble is an excited resonance of the Earth's rotation having a period of about 14 months. Although it has been under investigation for more than a century, its excitation mechanism has remained elusive."
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The 'wobble' has been examined for more than 150 years. It amounts to a change of about 9 metres (30 ft) in the point at which the axis intersects the Earth's surface.
A year 2000 study, from the Jet Propulsion Laboratory, California Institute of Technology, found that ocean pressure variations appear to play a major role :
Numerous investigations have been conducted during the past century in attempts to elucidate the excitation mechanism of the Chandler wobble. Here it has been shown that during 1985.0-1996.0 the single most important mechanism exciting the Chandler wobble has been ocean-bottom pressure fluctuations, which contribute about twice as much excitation power in the Chandler frequency band as do atmospheric pressure fluctuations. Atmospheric winds and oceanic currents have been shown here to play only a minor role in exciting the Chandler wobble during this time"
Source : Geophysical Research Lettersvol. 27, No. 15, Pages 2329-2332
A full picture of the causes still remains elusive.
In addition, the wobble varies in amplitude with a period of around 50 years. This is also unexplained. See : Journal of Physics: Conference Series 955 012033