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| Titel |
New nonlinear mechanisms of midlatitude atmospheric low-frequency variability |
| VerfasserIn |
Alef Sterk, Renato Vitolo, Henk Broer, Carles Simo, Henk Dijkstra |
| Konferenz |
EGU General Assembly 2010
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 12 (2010) |
| Datensatznummer |
250032975
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| Zusammenfassung |
| We investigate the dynamical mechanisms potentially involved in the so-called atmospheric
low-frequency variability, occurring at midlatitudes in the Northern Hemisphere. This
phenomenon is characterised by recurrent non-propagating and temporally persistent flow
patterns, with typical spatial and temporal scales of 6000-10000 km and 10-50 days,
respectively.
We study a low-order model derived from the 2-layer shallow-water equations on a
β-plane channel. The main ingredients of the low-order model are a zonal flow, a planetary
scale wave, orography, and a baroclinic-like forcing.
A systematic analysis of the dynamics of the low-order model is performed using
techniques and concepts from dynamical systems theory. Orography height (h0)
and magnitude of zonal wind forcing (U0) are used as control parameters to study
the bifurcations of equilibria and periodic orbits. An equilibrium loses stability
(U0 -¥ 12.5 m/s) along two curves of Hopf bifurcations and gives birth to two distinct
families of periodic orbits. These periodic orbits bifurcate into strange attractors along
three routes to chaos: period doubling cascades, breakdown of 2-tori by homo-
and heteroclinic bifurcations, and intermittency (U0 -¥ 14.5 m/s and h0 -¥ 800
m).
The observed attractors exhibit spatial and temporal low-frequency patterns comparing
well with those observed in the atmosphere. For h0 -¤ 800 m the periodic orbits have a
period of about 10 days and patterns in the vorticity field propagate eastward. For
h0 -¥ 800 m, the period is longer (30-60 days) and patterns in the vorticity field are
non-propagating. The dynamics on the strange attractors are associated with low-frequency
variability: the vorticity fields show weakening and strengthening of non-propagating
planetary waves on time scales of 10-200 days. The spatio-temporal characteristics
are “inherited” (by intermittency) from the two families of periodic orbits and are
detected in a relatively large region of the parameter plane. This scenario provides a
characterisation of low-frequency variability in terms of intermittency due to bifurcations of
waves. |
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