![Hier klicken, um den Treffer aus der Auswahl zu entfernen](images/unchecked.gif) |
Titel |
Influence of the North Atlantic Oscillation and the Quasi-Biannual Oscillation on Earth Orientation Parameters |
VerfasserIn |
Nana Schoen, Uwe Ulbrich, Gregor Leckebusch, Peter Névir, Maik Thomas |
Konferenz |
EGU General Assembly 2011
|
Medientyp |
Artikel
|
Sprache |
Englisch
|
Digitales Dokument |
PDF |
Erschienen |
In: GRA - Volume 13 (2011) |
Datensatznummer |
250052169
|
|
|
|
Zusammenfassung |
Variations in the earth rotation parameters are strongly influenced by atmospheric and
oceanic variability patterns. In order to develop a climate index from Earth rotation
parameters, the influence of known large-scale climate variability features on Earth
rotation must be assessed. This can be done using Atmosphere-Ocean General
Circulation Models (CGCMs), simulating the climate system in a physically consistent
way.
The analysis performed is based on the computation of effective angular momentum
functions derived from: a) an ocean model (OMCT) driven with ECMWF (ERA
Interim/ERA40) atmospheric reanalysis data, and with a 500 year run of the ECHAM5/OM1
model, developing its climate without an observational forcing. Results obtained from
re-analysis and the simulated ocean can be directly a compared with the observational IERS
geodetic earth orientation data (C04 excitation functions). Data from the free model run shall
demonstrate in how far the fully coupled model is able to reproduce the same features for the
geodetic variations.
One of the variability features investigated is the North Atlantic Oscillation (NAO), the
dominant atmospheric winter teleconnection pattern for the Northern Hemisphere. Its
influence on polar motion(e.g. Chao and Zhou, 1998) was thought to be caused largely by
mass redistribution. This assumption is, however, inconsistent with the inverted barometer
assumption, telling that atmospheric pressure anomalies over the ocean (where the larger part
of the NAO anomalies lies) should be outweighed by an elastic response of the ocean
surface.
Our results suggest that, instead of atmospheric mass redistribution, the influence of the
NAO on polar motion is exerted through changes in wind speed and resulting oceanic
transport, mainly via the Ï1 motion components of the atmospheric (AAM) and oceanic
(OAM) effective angular momentum (EAM) functions.
As a second variability feature, the possible influence of the Quasi-Biannual Oscillation
(QBO) on polar motion is examined. Because of the link between NAO and QBO
(significantly correlated with r=0.22 in the ERA Interim dataset), an indirect influence of the
QBO on earth orientation would be expected. However, no significant correlation between the
EAM functions and the QBO index is found.
To investigate this connection further, Granger causality is used, a statistical tool to
determine whether the knowledge of past values of one timeseries (y) is useful in predicting
future values of a second timeseries (x) over and above the knowledge of past values of x
alone. It is shown that, for the ERA Interim period, the QBO “granger-causes” the winter
AAM Ï-1 mass component as well as the OAM Ï-1 motion component at 99%
significance level, meaning that previous QBO index values may influence the earth
orientation.
The comparison with data from the ECHAM5/OM1 model - which does not include a
well resolved stratosphere and fails to reproduce correctly the QBO - is used to determine
whether the influence of the NAO on earth rotation is modified by the existence of the QBO. |
|
|
|
|
|