Geoscience Reference
In-Depth Information
1.4 Length of Day
The parameter which is usually quoted to represent changes in Earth's rotation rate
is LOD, the so-called excess length of day or the deviation of the effective length of
day from the nominal value of 86400 s (Schuh and Böhm
2011
), cf. also Sect.
3.3
.By
integration, LOD can be alternatively related to dUT1, which is directly connected
to the Earth rotation angle.
Variations of Earth's angular velocity can be assigned to different frequency bands
and different physical processes. For daily and subdaily periods, the strongest influ-
ence emerges from the ocean tides as a response to the gravitational attraction of
the Sun and the Moon. On time scales of a few weeks to months, solid Earth tides
provide distinct peaks near 14 and 28d. Additional fluctuations of the axial rotational
component can be attributed to the atmosphere, which are highlighted in the next
section. A pronounced annual spectral component in LOD, predominantly associated
with the annual seesawing of wind patterns, is amplified every 4-6yr by large-scale
climate anomalies (El Ni
no-Southern Oscillation phenomenon). Angular momen-
tum changes in the fluid core generate decadal LOD fluctuations and are transfered
to the mantle via mechanic or electromagnetic coupling. Tidal dissipation (angular
momentum losing to the Earth-moon system) and long-termmass variations steadily
decrease the Earth's angular velocity. This secular prolongation of the day amounts
to 1.8ms in 100yr (Morrison and Stephenson
2001
).
˜
1.5 Influence of the Atmosphere on Earth Rotation
The substantial influence of the atmosphere on all three components of the Earth's
rotation vector has been recognized for a long time, see e.g. the pioneering study
of Jeffreys (
1916
). While the sum of all air masses amounts only to about 1/300
of the oceanic mass, the atmosphere's mobility, chiefly driven by the diurnal and
seasonal thermodynamic cycles, is unprecedented compared to that of the oceans
or the core. By interaction with the underlying mantle (attached to the crust), the
resulting atmospheric angular momentum (AAM) is transfered to the solid parts of
the Earth and can be recognized as substantial geophysical variation of Earth rotation.
Large-scale atmospheric mass redistributions as well as changes in the pattern
of winds are the dominating driving agents for observed changes in LOD and polar
motion at periods from a few days up to several years, see the works of Lambeck
(
1980
) or Eubanks (
1993
). The distinct seasonal excitation signals in ERP, which are
generated by atmospheric pressure and wind variations at annual, semiannual and
terannual frequencies, have been treated thoroughly by Gross et al. (
2003
) and Gross
et al. (
2004
). Earlier studies on this subject are Wahr (
1983
) or Barnes et al. (
1983
).
Intraseasonal fluctuations, i.e. non-seasonal excitation signals at periods ranging from
4d up to 1yr, are present in both polar motion and LOD. On those time scales, the
most prominent feature in the atmosphere coupling with variations in Earth's rotation
