Geoscience Reference
In-Depth Information
the frequency definition of Sect.
1.1
, see Mathews et al. (
2002
). Remaining nuta-
tional motions of the axis that are not accounted for by the model can be measured
by means of VLBI (Very Long Baseline Interferometry) and are published by the
IERS as
Celestial Pole Offsets
. These residuals arise from existing model deficien-
cies and from unpredictable effects such as the
Free Core Nutation
(FCN). The FCN
is a free rotational mode of the Earth, caused by a misalignment of the rotation axes
of mantle and core and probably excited by diurnal retrograde atmosphere and ocean
loading of the Earth's surface (Sasao and Wahr
1981
). Studies from VLBI observa-
tions show that the FCN period is likely to be around 430d (Herring et al.
1986
),
while its amplitude is at the level of 0.1-0.3mas (McCarthy and Luzum
2010
).
1.3 Polar Motion
The terrestrial part of CIP variations has an order of magnitude of several meters and
is routinely reported by the IERS as pole coordinates
x
p
,
y
p
in a two-dimensional
coordinate system, of which the
x
-axis is oriented in the direction of the Greenwich
meridian and the
y
-axis points positively towards 90
◦
E longitude (Fig.
1
). If the
Earth were a rigid oblate spheroid, the resulting misalignment of the rotation axis
with the axis of figure would engender a circular motion of the pole with a period
of around 304d, recognized already in 1765 by Euler. The real Earth departs from
this simplified model. Astronomical observations at the end of the nineteenth century
revealed that polar motion is mainly composed of an annual variation of about 90mas
(or 2.7m on the Earth's surface) and the 14-month
Chandler oscillation
or
Chandler
wobble
(CW) with a mean amplitude of about 160mas (or 4.8m). The CW is another
pivotal eigenmode of the Earth, a nearly circular, damped oscillation, which would
have subsided due to friction in Earth's interior, if it were not for a constantly renewed
excitation (Schuh and Böhm
2011
). The exact excitation mechanism is still under
investigation, even though nowadays there is broad consensus that the necessary
energy to maintain the CW emerges from irregular processes in the atmosphere-
ocean-system, such as ocean-bottom pressure fluctuations and air pressure variability
over the continents (Gross
2000
; Brzezi nski et al.
2012
). The interference of the
Chandler mode with the annual motion leads to a distinct beat-like pole behavior
with a maximum amplitude up to 9m every 6.3yr.
The polar motion spectrum below the annual cycle is characterized by semi-
and terannual wobbles as well as retrograde oscillations associated with atmospheric
normal modes at periods of about 1.2 and 10d (Brzezi nski et al.
2002
, and references
therein). Dominant short-periodic variations are due to ocean tides with essentially
diurnal and semidiurnal periods, albeit the total effect is only about 1/100 of the CW.
Processes in Earth's interior, such as dynamic coupling between core and mantle, are
associated with long-term and decadal polar motion signals. The linear trend in the
position of the pole towards 76-78
◦
W is labeled
secular polar motion
and amounts
to about 3.3mas yr
−
1
according to Schuh et al. (
2001
). This effect is supposed to be
predominantly caused by postglacial rebound and melting of polar ice masses.
