Polar motion

[4][5]: 1 The slow drift, about 20 m since 1900, is partly due to motions in the Earth's core and mantle, and partly to the redistribution of water mass as the Greenland ice sheet melts, and to isostatic rebound, i.e. the slow rise of land that was formerly burdened with ice sheets or glaciers.

This less dramatically westward drift of motion is attributed to the global scale mass transport between the oceans and the continents.

[5]: 2 Major earthquakes cause abrupt polar motion by altering the volume distribution of the Earth's solid mass.

These shifts are quite small in magnitude relative to the long-term core/mantle and isostatic rebound components of polar motion.

[6] In the absence of external torques, the vector of the angular momentum M of a rotating system remains constant and is directed toward a fixed point in space.

In the case of the Earth, it is almost identical with its axis of rotation, with the discrepancy due to shifts of mass on the planet's surface.

[2][7] The observed angle between the figure axis of the Earth F and its angular momentum M is a few hundred milliarcseconds (mas).

[8] Observations show that the figure axis exhibits an annual wobble forced by surface mass displacement via atmospheric and/or ocean dynamics, while the free nutation is much larger than the Euler period and of the order of 435 to 445 sidereal days.

This polar motion should not be confused with the changing direction of the Earth's rotation axis relative to the stars with different periods, caused mostly by the torques on the Geoid due to the gravitational attraction of the Moon and Sun.

The Chandler wobble is usually considered a resonance phenomenon, a free nutation that is excited by a source and then dies away with a time constant τD of the order of 100 years.

[21] There exist two external forces to excite polar motion: atmospheric winds, and pressure loading.

Such standing wave represents the seasonally varying spatial difference of the Earth's surface pressure.

It is difficult to estimate the effect of the ocean, which may slightly increase the value of maximum ground pressure necessary to generate the annual wobble.

One possible explanation for the observed frequency-amplitude behavior would be a forced, but slowly changing quasi-periodic excitation by interannually varying atmospheric dynamics.

Polar motion in arc-seconds as function of time in days (0.1 arcsec ≈ 3 meters). [ 1 ]
Figure 2. Rotation vector m of the annual component of polar motion as function of year. Numbers and tick marks indicate the beginning of each calendar month. The dash-dotted line is in the direction of the major axis. The line in the direction of the minor axis is the location of the excitation function vs. time of year. ( 100 mas (milliarcseconds) = 3.082 m on the Earth's surface at the poles)