Why the Earth ‘Wobbles’

Redistribution of surface mass, determined from direct measurements of the Earth’s shape, matches measurements of polar motion (aka Chandler wobble):
http://www.sciencedaily.com/releases/2004/05/040520065656.htm

The principal cause, it turns out, is pressure fluctuations on the ocean floor.  These in turn are caused by temperature, salinity, and wind-driven changes in the circulation of ocean currents.
http://www.sciencedaily.com/releases/2000/07/000717123812.htm

Not just the polar motion but also the Earth’s rotation period changes due to mass motions on the surface.  In fact, various wind and atmospheric pressure measurements have been matched with variations in the length of the day:
http://www.sciencedaily.com/releases/2003/03/030306075514.htm