Percolation anticipates abrupt changes in coupled FitzHugh-Nagumo oscillators and the El Ni\~no-Southern Oscillation: an explanatory analysis

Functional networks are powerful tools to study statistical interdependency structures in spatially extended or multi-variable systems. They have been used to get insights into the dynamics of complex systems in various areas of science. In particular, percolation properties of correlation networks have been employed to identify precursors for critical transitions. In this work, we further investigate the corresponding potential of percolation measures for the anticipation of different types of sudden shifts in the state of coupled irregularly-oscillating systems. As a paradigmatic model system, we study the dynamics of a ring of diffusively coupled noisy FitzHugh-Nagumo oscillators and show that, when the oscillators are nearly completely synchronized, the percolation-based precursors successfully anticipate the rapid switches between the two states of the system. By isolating the stochastic component of the individual dynamics from the deterministic mean-field behavior, we unveil the mechanisms behind the percolation transitions in the correlation network: namely common trends and the synchronization of stochastic fluctuations. We then apply the same methodology to real-world data of sea surface temperature anomalies during different phases of the El Ni\~no-Southern Oscillation. This leads to a better understanding of the factors that make percolation precursors effective as early warning indicators of incipient El Ni\~no and La Ni\~na events.