The motion of the rotation axis of the earth relative to the crust has three major components. A free oscillation with period about 435 days (Chandler wobble) and an annual oscillation forced by the seasonal displacement of air and water masses, beating which each other, give the characteristic pulsating shape of the motion, represented in Figure 1 (dots at 5-day intervals). The mean pole has an irregular drift in the direction to 80deg. West, shown as a solid line.
Figure 2 and Figure 3 show a decomposition of the x and y coordinates of the pole since 1890 into a trend, the seasonal and Chandler terms. The series shown is EOP(IERS) C 01. The residual motion in the lower part of the figures includes irregularities with recurrence times ranging from days to years that are forced by the atmosphere.
Table 1 gives yearly coordinates of the mean rotation axis in the IERS Terrestrial Reference Frame, obtained by filtering the Chandler and seasonal terms. Their uncertainty is 0.010".
Polar motion has diurnal and semi-diurnal variations with amplitudes of a fraction of millisesond of arc (mas) that are due to the oceanic tides. A correction model is available in the IERS Conventions (1996).