Stefano	Pierini	Universit&agrave; di Napoli <i>Parthenope</i>	
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Coherence resonance and stochastic tipping points in climate 
dynamics
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The concepts of relaxation oscillation and
excitable system play a fundamental role in
explaining abrupt climate changes such as the
glacial terminations and the Dansgaard-Oeschger
events on the millennial time scale, and the
Kuroshio Extension (KE) bimodal variability on a
much smaller (decadal) time scale. A crucial
aspect currently under debate is whether these
abrupt transitions are associated with a global
bifurcation (or "tipping point") being exceeded,
in which case precursors could be identified, or
if they are rather excited by fast "noise" 
dynamics through the 
coherence resonance (CR)
mechanism, with an external periodic forcing
(e.g., a Milankovitch cycle) possibly acting as a pacemaker.
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In this communication these aspects are
considered in the context of the KE bimodality,
chosen as a significant oceanic case study.
Results of numerical simulations based on both a
geophysical fluid dynamics PDE model and a
low-order (4D) ODE model are used to investigate
general phenomena such as: the intrinsically
induced variability, Hopf, period-doubling and
homoclinic bifurcations and transition to chaos,
homoclinic orbits as relaxation oscillations and
related stable and unstable manifolds, CR and
phase selection. Finally, the new concept of
stochastic tipping points is proposed with the
aim of reconciling, at least formally, the bifurcation and CR 
views.