arXiv subjects

Raphael Römer

Publications and source records attributed to Raphael Römer.

4 recordsLinked to original sources

arXiv2026

The effect of timescale separation on the tipping window for chaotically forced systems

Tipping behavior can occur when an equilibrium of a dynamical system loses stability in response to a slowly varying parameter crossing a bifurcation threshold, or where noise drives a system from one attractor to another, or some combination of these effects. Similar behavior can be expected when a multistable system is forced by a chaotic deterministic system rather than by noise. In this context, the chaotic tipping window was recently introduced and investigated for discrete-time dynamics. In this paper, we find tipping windows for continuous-time nonlinear systems forced by chaos. We characterize the tipping window in terms of forcing by unstable periodic orbits of the chaos, and we show how the location and structure of this window depend on the relative timescales between the forcing and the responding system. We illustrate this by finding tipping windows for two examples of coupled bistable ODEs forced with chaos. Additionally, we describe the dynamic tipping window in the setting of a changing system parameter.

nlin.CD↗

arXiv2026

Chaotic Kramers' Law: Hasselmann's Program and AMOC Tipping

In bistable dynamical systems driven by Wiener processes, the widely used Kramers' law relates the strength of the noise forcing to the average time it takes to see a noise-induced transition from one attractor to the other. We extend this law to bistable systems forced by fast chaotic dynamics, which we argue is in some cases a more realistic modeling approach than unbounded noise forcing. Transitions similar to the noise-driven case can only occur if the amplitude of the chaotic forcing is large enough. If this is the case, in our numerical example - a reduced-order model of the Atlantic Meridional Overturning Circulation (AMOC) - we observe the chaotic Kramers' law to hold even when the chaotic forcing is far from the stochastic limit. We discuss the limitations of the chaotic Kramers' law, how to address the numerical issues associated with the timescale separation, and give a possible explanation for the dynamics of recently found AMOC collapses and recoveries in complex climate models.

nlin.CD↗

arXiv2024

Saddle avoidance of noise-induced transitions in multiscale systems

In multistable dynamical systems driven by weak Gaussian noise, transitions between competing states are often assumed to pass via a saddle on the separating basin boundary. By contrast, we show that timescale separation can cause saddle avoidance in non-gradient systems. Using toy models from neuroscience and ecology, we study cases where sample transitions deviate strongly from the instanton predicted by Freidlin-Wentzell theory, even for weak finite noise. We attribute this to a flat quasipotential and present an approach based on the Onsager-Machlup action to aptly predict transition paths.

math.DS↗

arXiv2024

Contrasting chaotic and stochastic forcing: tipping windows and attractor crises

Nonlinear dynamical systems subjected to a combination of noise and time-varying forcing can exhibit sudden changes, critical transitions or tipping points where large or rapid dynamic effects arise from changes in a parameter that are small or slow. Noise-induced tipping can occur where extremes of the forcing causes the system to leave one attractor and transition to another. If this noise corresponds to unresolved chaotic forcing, there is a limit such that this can be approximated by a stochastic differential equation (SDE) and the statistics of large deviations determine the transitions. Away from this limit it makes sense to consider tipping in the presence of chaotic rather than stochastic forcing. In general we argue that close to a parameter value where there is a bifurcation of the unforced system, there will be a chaotic tipping window outside of which tipping cannot happen, in the limit of asymptotically slow change of that parameter. This window is trivial for a stochastically forced system. Entry into the chaotic tipping window can be seen as a boundary crisis/non-autonomous saddle-node bifurcation and corresponds to an exceptional case of the forcing, typically by an unstable periodic orbit. We discuss an illustrative example of a chaotically forced bistable map that highlight the richness of the geometry and bifurcation structure of the dynamics in this case. If a parameter is changing slowly we note there is a dynamic tipping window that can also be determined in terms of unstable periodic orbits.

nlin.CD↗