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| Titel |
The dynamics of a low-order model for the Atlantic Multidecadal Oscillation |
| 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 |
250034543
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| Zusammenfassung |
| Observations and model studies provide ample evidence for the presence of multidecadal
variability in the North Atlantic sea-surface temperature known as the Atlantic Multidecadal
Oscillation (AMO). This variability is characterised by a multidecadal time scale, the
westward propagation of temperature anomalies, and a phase difference between the
anomalous meridional and zonal overturning circulations.
We derive a low-order model which captures the characteristics of the AMO. The starting
point is a minimal model consisting of a thermal wind balance and an equation for the
advection of temperature in a 3-dimensional box-shaped ocean basin. The low-order model is
obtained by an orthogonal projection onto a finite-dimensional function space. Flows are
forced by restoring the sea surface temperature to an idealised atmospheric temperature
profile with an equator-to-pole gradient ÎT as a control parameter. A second control
parameter, γ, interpolates between restoring (γ = 0) and prescribed heat flux (γ = 1)
conditions.
For the standard values ÎT = 20-C and γ = 0 the low-order model has a stable
equilibrium which corresponds to a steady ocean flow. By increasing the parameter γ from 0
to 1 this equilibrium becomes unstable through a supercritical Hopf bifurcation and we find a
periodic attractor with the physical signature of the AMO. In turn, this attractor can bifurcate
through (cascades of) period doublings when ÎT is increased. Next, we impose a
time-periodic forcing, modelling annual variations in the ocean-atmosphere heat flux. In this
setting the AMO appears through a Hopf-NeÄmark-Sacker bifurcation as an invariant 2-torus
attractor. Hence, we have to study bifurcations of invariant tori. For ÎT -¥ 22-C the 2-torus
associated with the AMO bifurcates through a sequence of quasi-periodic period
doublings, which can give birth to strange attractors of quasi-periodic Hénon-like type. |
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