arXiv subjects

Jérôme Vialard

Publications and source records attributed to Jérôme Vialard.

3 recordsLinked to original sources

arXiv2026

A Reduced-Order Coupled Oscillator Model for the Emergence of Episodic Convection

Extreme precipitation is a societal concern, and understanding tropical moist convection is increasingly important in a warming climate. Climate simulations show that, under hothouse Earth, convection can transition from quasi-steady precipitation to an episodic regime characterized by intense rainfall bursts separated by dry intervals, although the mechanisms governing this transition remain debated. Here we develop a reduced-order framework that represents tropical convection through column-integrated moisture (q) and thermal stratification (Delta), rather than bulk measures of convective instability. Analysis of cloud-resolving model (CRM) simulations shows that these variables evolve together: during dry periods, surface fluxes recharge moisture, while radiative and dynamical processes erode stratification generated by previous convection. We couple this evolution to the nonlinear moisture dependence of tropical precipitation (P), whereby rainfall increases sharply above a critical moisture threshold. A third prognostic variable, C, represents convective persistence, allowing convection to continue below its onset threshold and thereby producing hysteresis. With parameters constrained by CRM output, the model reproduces the evolution of q, Delta, and P in both regimes, representing quasi-steady convection as stable, noise-driven oscillations and episodic convection as an unstable, self-sustained limit cycle. Linear stability analysis shows that the transition depends on moisture-recharge and stratification-adjustment timescales, thermal-moisture sensitivity, and convective persistence. Because radiative and dynamical processes enter through their effects on thermal stratification, the framework is not tied to a specific mechanism, providing a unified dynamical link between moisture-threshold precipitation behavior and the emergence of episodic convection under warming.

physics.ao-ph↗

arXiv2026

Detectability of Forced ENSO Changes under Global Warming: Insights from the Recharge Oscillator

We use perfect-model, large-ensemble nonlinear Recharge Oscillator (RO) simulations to quantify ENSO internal variability and the detectability of forced changes in ENSO characteristics. Fixed-parameter simulations show that linear trends in ENSO standard deviation, period, and skewness as large as those observed since 1955 can arise from internal variability in 10% of simulations. RO parameters estimated from single realizations exhibit spurious drifts even without forcing, underscoring the need for ensembles. Using 100-member ensembles, comparable to the largest climate-model ensembles, we identify detectable parameter trends. For ENSO amplitude, the detectability thresholds for forced changes in underlying processes are 15% per century for stochastic forcing, 25% per century for basin adjustment, and 50% per century for the Bjerknes feedback. For ENSO period, the detectability threshold for forced changes in the underlying recharge--discharge processes and delayed oceanic feedback is 15% per century. These results provide a testbed for interpreting RO-diagnosed ENSO changes in climate-model ensembles.

physics.ao-ph↗

arXiv2025

Realistic ENSO Dynamics Requires a Damped Nonlinear Recharge Oscillator

The dynamics of the El Niño-Southern Oscillation (ENSO) are succinctly captured by the Recharge Oscillator (RO) framework. However, to simulate ENSO realistically, careful choices must be made regarding the RO's key parameters. In particular, nonlinear parameters govern how well the model reproduces ENSO asymmetries-El Niño events tend to be stronger but relatively short, often transitioning into La Niña, whereas La Niña events are typically weaker but may last longer. While amplitude asymmetry has been studied within the RO framework, duration and transition asymmetries remain less explored and their causes are debated. In this study, by systematically exploring the RO parameter space-rather than relying on commonly used fitting methods-we identify optimal parameter values that successfully capture key linear and nonlinear ENSO characteristics. In doing so, we revisit several foundational elements of the RO framework. First, we analytically derive the phase relationship between temperature and heat content anomalies, showing that it depends on the signs of the Bjerknes feedback and the ocean damping timescale. We show that self-sustained oscillations fail to reproduce the observed kurtosis of Niño indices. We further derive an analytical expression for the power spectrum and argue that incorporating red noise forcing, rather than white noise, introduces unnecessary complexity. The most realistic yet simplest RO configuration is a strongly damped oscillator, with a decay timescale shorter than the dominant period, forced by multiplicative white noise and influenced by weak deterministic nonlinearities. Identifying these minimal components preserves the RO framework's clarity and isolates the core physical processes underlying ENSO behavior.

physics.ao-ph↗