Geoscience Reference
In-Depth Information
rate is called
Madden-Julian Oscillation
(Madden and Julian
1971
). While mass
redistributions and changing winds are equally important for the equatorial part of
intraseasonal Earth rotation variations, the axial component is almost solely affected
by zonal winds, especially those below an isobaric level of 10hPa (Gross et al.
2004
).
On short time scales such as daily and subdaily periods, the contribution of ocean
tides to Earth rotation is about 30 times larger than that of the atmosphere (Brzezi nski
et al.
2002
), both for variations in Earth's rotation rate (Ray et al.
1994
) and for
polar motion (Chao et al.
1996
). Nonetheless, by using data from numerical weather
models (NWM), the high-frequency contribution of the atmosphere to EOP can be
estimated. The discernable effects are predominantly tidal waves of thermal origin
that cause sharp peaks at
T
24h in the excitation spectra. Bizouard et al.
(
1998
) or Brzezi nski et al. (
2002
) highlighted the considerable impact of atmospheric
tides on the nutational motion of the Earth, demonstrating that geophysical excitation
in this frequency range is largely amplified by the presence of the FCN resonance.
Diurnal and semidiurnal atmospheric effects on polar motion and changes in LOD are
below 10
=±
12
,
±
s, respectively, and thus by one order of magnitude smaller than
the atmosphere-forced nutation amplitudes. Yet, the atmosphere certainly exerts an
indirect influence on Earth rotation via the oceanic motion that is triggered by surface
pressure and wind variations (de Viron and Dehant,
1999
). The oceanic response to
such an atmospheric forcing is, however, still poorly understood.
Two different but fundamentally equivalent methods can be applied in order to
evaluate the effect of the atmosphere or, more generally, any mobile fluid on Earth
rotation. The first method, the angular momentum approach, considers the system
Earth + atmosphere + oceans as isolated, thus conserving angular momentum. In
this case, any change of angular momentum in the atmosphere (AAM) is compen-
sated by an opposite change of equal magnitude in the angular momentum of the
system's remaining parts, which in turn can be observed as variation in the rotation
of our planet. The second method, referred to as torque approach, requires the direct
computation of the interaction torque between the atmosphere and the solid Earth +
oceans ('solid Earth' is understood to comprise both mantle and core). The resulting
time-dependent quantity is then introduced as forcing function in the equations of
motion (Brzezi nski et al.
2002
), as shown, e.g, by Iskenderian and Salstein (
1998
)for
the axial component of the Earth rotation vector and by de Viron and Dehant (
1999
)
for the equatorial component. In the present review, emphasis is placed on the AAM
approach, with the torque approach moderately further expounded in Sect.
2.6
.
µ
as or 10
µ
2 Modeling Geophysical Excitation of Earth Rotation
2.1 Liouville Equations
The dynamical equation of motion of a rotating body such as the Earth in a space-fixed
reference system is (e.g. Moritz and Müller
1987
)
