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| Titel |
Forced versus coupled dynamics in Earth system modelling and prediction |
| VerfasserIn |
B. Knopf, H. Held, H. J. Schellnhuber |
| Medientyp |
Artikel
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| Sprache |
Englisch
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| ISSN |
1023-5809
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| Digitales Dokument |
URL |
| Erschienen |
In: Nonlinear Processes in Geophysics ; 12, no. 2 ; Nr. 12, no. 2 (2005-02-17), S.311-320 |
| Datensatznummer |
250010492
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| Publikation (Nr.) |
 copernicus.org/npg-12-311-2005.pdf |
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| Zusammenfassung |
| We compare coupled nonlinear climate models and their simplified forced
counterparts with respect to predictability and phase space topology. Various
types of uncertainty plague climate change simulation, which is, in turn, a
crucial element of Earth System modelling. Since the currently preferred
strategy for simulating the climate system, or the Earth System at large, is
the coupling of sub-system modules (representing, e.g. atmosphere, oceans,
global vegetation), this paper explicitly addresses the errors and
indeterminacies generated by the coupling procedure. The focus is on a
comparison of forced dynamics as opposed to fully, i.e. intrinsically,
coupled dynamics. The former represents a particular type of simulation,
where the time behaviour of one complex systems component is prescribed by
data or some other external information source. Such a simplifying technique
is often employed in Earth System models in order to save computing
resources, in particular when massive model inter-comparisons need to be
carried out. Our contribution to the debate is based on the investigation of
two representative model examples, namely (i) a low-dimensional coupled
atmosphere-ocean simulator, and (ii) a replica-like simulator embracing
corresponding components.Whereas in general the forced version (ii) is able to mimic its fully coupled
counterpart (i), we show in this paper that for a considerable fraction of
parameter- and state-space, the two approaches qualitatively differ. Here we
take up a phenomenon concerning the predictability of coupled versus forced
models that was reported earlier in this journal: the observation that the
time series of the forced version display artificial predictive skill. We
present an explanation in terms of nonlinear dynamical theory. In particular
we observe an intermittent version of artificial predictive skill, which we
call on-off synchronization, and trace it back to the appearance of unstable
periodic orbits. We also find it to be governed by a scaling law that allows
us to estimate the probability of artificial predictive skill. In addition to
artificial predictability we observe artificial bistability for the forced
version, which has not been reported so far. The results suggest that
bistability and intermittent predictability, when found in a forced model
set-up, should always be cross-validated with alternative coupling designs
before being taken for granted. |
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