The observed variations, displayed on Fig. 1, reflect the difference of the actual celestial motion of the pole with the one predicted by the conventional IAU 1980 precession and nutation models. Most of these variations are explained by errors in a few terms of the IAU models: secular term, periodic terms (mainly 18.6, 1.0, 0.5 years and 14 days). These terms are well modelled in the present precession-nutation model IAU 2000.
The transition from (dψ,dε) / UAI 1980 to (dψ,dε) / UAI 2000 or (dX, dY) / UAI 2000 or inversely can be achieved numerically by the UAI 2000 package (see readme here) proposed by the EOP center.Presently all the available EOP time series provide still (dψ,dε) with respect to IAU 1980, except one version of the combined EOP series C04 for the years 2002 et 2003 where we give (dX,dY) / UAI 2000. In the future (2005-2007) the series (dψ,dε) / UAI 1980 will disappear for the benefit of (dX,dY) / UAI 2000.
Figure 2: Celestial pole offsets dX/dY referred to IAU 2000 precession-nutation model. Unit : mas (0.001"). The IAU 2000 precession-nutation model matches the observations within +/- 0.001".