NATIONAL REPORT OF THE FEDERAL REPUBLIC OF ... - IAG Office

M. Thomas, M. Soffel, H. Drewes: Earth Rotation – Theory and Analysis 91
and KUTTERER (2005) performed a sensitivity study revealing
that the pole tide Love number k 2 is the most critical
parameter, while the dependence on other parameters is
marginal.
2.2 Excitation of earth rotation by geophysical fluids
ENDLER (2007) investigated the relationship between
interannual variations in Length of Day (LOD) and selected
El Nino/Southern Oscillation (ENSO) events. The study
confirms that changes in the atmospheric angular momentum
due to zonal winds are well correlated with LOD
variability on timescales varying between several days and
years. Strong correlations (at the 99% significance level)
between the interannual amplitudes of LOD and the atmospheric
wind term with sea-surface temperatures and
selected ENSO indices clearly demonstrate a significant
relation between interannual LOD variability, zonal atmospheric
wind anomalies and the ENSO phenomenon.
Although the overall correlation between LOD and ENSO
is significantly varying in time depending on specific
characteristics of the individual ENSO event, there is
evidence that observed variations in the amplitude of LOD
can be used as an indication for changes in the low and high
frequency spectrum of hemispheric circulation systems led
off by warm ENSO events. (LEHMANN et al., 2007).
SEITZ et al. (2005) and STUCK et al. (2005) investigated the
role of atmospheric and oceanic dynamics in exciting polar
motion in the annual and Chandler wobble frequency band
by means of simulations with the gyroscopic model
DyMEG consistently forced with output from the atmospheric
climate model ECHAM and the ocean model
OMCT. According to STUCK et al. (2005), the annual
oscillation of polar motion is predominantly due to atmospheric
pressure forcing, while the motion component is
less important. A regional statistical analysis of AAM
turned out that strong annual pressure variations over Asia,
in particular at the Himalayas, is the primary component
responsible for accelerating forced polar motion. Both
STUCK et al. (2005) as well as SEITZ et al. (2005) came to
the conclusion that stochastic processes in atmosphere and
ocean are sufficient to excite the Chandler wobble. Neither
a significant nor at least an increased signal in the frequency
domain of 14 to 16 months was found and regional statistical
analysis of angular momentum gave no hint for an
oscillation with a typical timescale of 14 to 16 months. This
is in agreement with the findings of THOMAS et al. (2005)
who calculated power spectral densities from effective
angular momentum functions deduced from various
consistent model combinations (NCEP/MIT, NCEP/ECCO,
ECHAM/OMCT). The investigated model combinations
led to similar excitation power in the Chandler frequency
band always exceeding the observed power.
The impact of oceanic mass redistributions due to pressure
loading of atmospheric tides and gravitational tides at
frequencies S1 and S2 was estimated by THOMAS et al.
(2007) by means of simulations with OMCT driven by
operational atmospheric data provided by ECMWF. The
study demonstrates that ECMWF's 3-hourly forecasts can
be used to represent atmospheric mass redistributions and
corresponding oceanic responses down to semidiurnal
timescales and, consequently, to determine short-term
effects of the atmosphere-ocean system on earth's rotation.
In contrast to, e.g., altimetry observations, the applied
method principally allows a separation of effects due to
gravitational and pressure tides.
From simulations with the Hydrological Discharge Model
(HDM) WALTER (2005) deduced hydrologically induced
excitations of earth rotation on seasonal to decadal timescales.
Although the model simulations were higly sensitive
to applied atmospheric forcing conditions, the results
generally agreed with respect to the annual excitation of
LOD, suggesting that about 25 :s of the annual amplitude
have to be attributed to hydrological mass redistributions.
Applying the high-resolving unconstrained ocean model
TiME forced by the complete lunisolar tidal potential
derived from ephemerides, WEIS (2006) estimated the effect
of several partial tides and shallow-water tides on earth
rotation. Although the unconstrained model generally
overestimates tidal amplitudes, the high-resolving real-time
model agreed better with data assimilation models than
partial tide model approaches. The total energy dissipated
by the complete tidal oscillation system was estimated by
WEIS (2006) to be 4.8 TW; the contribution of ocean tides
to tidal friction was calculated to be 4.1 TW, while other
recent studies agree on a lower value of 3.0 TW. However,
some less significant partial tides, which had not been
included in any modelling study, so far, were in excellent
correspondence with results from both VLBI and GPS
measurements with correlations of 90-96%. The effects of
shallow-water tides on UT1 and polar motion turned out
to be about three orders of magnitude lower than major
astronomical partial tides, but should be above the detection
limit of modern observation techniques within the near
future.
2.3 Internal processes
One part of the earth rotation theory concerns the modelling
of the influence of core processes (e.g. fluid motions,
electromagnetic forces) on the earth’s rotation. To this
regard, GREINER-MAI et al. (2003) gave an outline about
appropriate methods, results and unsolved problems.
To determine core motions and coupling torques from the
geomagnetic field, it is necessary to extend the geomagnetic
field from the earth’s surface to the core-mantle boundary
through an electrically conducting mantle. To solve this
problem, a new inversion method for the induction equation
of the mantle was developed until 2002. GREINER-MAI et
al. (2004) have extended this method for determining the
geomagnetic field in a differentially rotating upper core
layer.
GREINER-MAI et al. (2003) discussed a kinematical model
of forced inner-core wobble (ICW) by which the decadal
variations of polar motion may be explained. Complementary,
GUO et al. (2005a) checked the detectability of a free
ICW with a period of about 6 years in the measured variations
of the gravity field and polar motion. They found no
firm evidence of the ICW in polar motion data used, which
have an accuracy of few milliarc seconds.