Geoscience Reference
In-Depth Information
of the displayed series. ENSO events, on the contrary, are significant anomalies in
the usual annual seesawing of oceanic and atmospheric circulation over the entire
eastern equatorial Pacific ocean region (Philander
1990
). As apparent from Fig.
16
c,
such irregularities have been particularly strong during 1982-1983 and 1997-1998,
normally lasting about 14-18 months. Since ENSO events originate from interac-
tions in the atmosphere-ocean system, their impact on changes in LOD can only
be studied properly if reverting to a coupled geophysical model that yields both
atmospheric and oceanic angular momentum. However, Fig.
16
c encompasses pure
AAM, so that the correlation coefficient between the residual geodetic observations
and AAM fluctuations is
ρ
≈
0
.
86, only.
3.4 The Angular Momentum Budget
In accordance with the central theme of this review, the given comparisons of Earth
rotation (orientation) parameters and physical model results have been solely based
on atmospheric data. As suggested by Eqs. (
5
)-(
7
), a thorough application of the
angular momentum approach would require to additionally consider the angular
momentum portion of Earth's other subsystems in terms of variations of the tensor
of inertia and relative angular momentum
I
(
a
)
+
Δ
I
(
o
)
+
Δ
I
(
c
)
+
Δ
I
(
h
)
Δ
=
Δ
I
(98)
h
(
a
)
+
h
(
o
)
+
h
(
c
)
+
h
(
h
)
.
h
=
(99)
Herein, the contributions of the atmosphere (
a
), the oceans (
o
) as well as hydro-
logical excitation (
h
), associated with land water, soil moisture and snow, and the
effect of the core (
c
) have been linearly superposed. The resulting cumulative angular
momentum of Earth's fluids has to be balanced by that of the solid Earth by means of
small rotational fluctuations, which in turn can be monitored by high-accuracy space
geodetic techniques. The underlying mathematical scheme capable of relating those
observations to geophysical excitation in the form of angular momentum functions
has been established and illustrated in the previous sections. Figures
11
and
16
reveal
the large impact of atmospheric processes to polar motion and changes in length of
day in different frequency bands, but also imply that full closure of the angular
momentum budget has to likewise involve the contributions of other fluid layers. It
is widely recognized that mass redistributions in the core are substantial for Earth
rotation variations at decadal periods, see e.g. Dickey et al. (
2010
). The comprehen-
sive studies of Ponte (
1997
) and Gross et al. (
2003
) point out the instrumental role
of oceanic excitation mechanisms in balancing the equatorial angular momentum at
seasonal, intraseasonal and interannual time scales. In detail, combined time series
of atmospheric and oceanic excitation may account for up to 67% of the variance in
observed polar motion values at intraseasonal frequencies, while at longer periods
the agreement between observed and modeled excitation function decreases (Gross
