What does the “mean pole” mean

Abstract:

Rotational deformation (also called the pole tide) is the response of the solid earth and ocean to the Earth’s wobble, which includes gravitational and surface displacement components. An important part of the model is the Conventional Mean Pole, which is required to remove the slow variation in the polar motion. This ensures that the rotational deformation model removes only the principal periodic motions (annual, Chandler and any other high frequency variations). Previously, no dependable mean pole product had been available. Consequently, for the 2010 Conventions, a cubic was fit to a filtered time series of polar motion and this model was adopted, along with a simple linear extrapolation into the future that was expected to be adequate until the next Conventions update. In light of the large changes in the mean pole due to recent ice mass losses, such extrapolations may not be reliable. To avoid this, the next Conventions update will include the use of a regularly updated mean pole table. Some subtleties in that choice will be noted.

However, the original pole tide model was conceived when the mean pole was moving (more or less) linearly, largely in response to glacial isostatic adjustment (GIA). With the recent ice mass losses, particularly in Greenland, this motion is decidedly non-linear, and it is unclear whether the pole tide model correctly reflects this new reality. There are questions as to whether the mean pole should be the slow variation as conceived currently or should it reflect only the long-term linear motion (either based on a GIA model or a linear fit over an appropriate time span).