El Niño–Southern Oscillation Dynamical Response to Volcanic Forcing During the Last Millennium

Abstract. The El Niño–Southern Oscillation (ENSO) is the dominant mode of interannual climate variability in the global climate system, with widespread impacts on temperature and precipitation. Volcanic eruptions represent one of the most significant external perturbations to the climate system, yet how volcanic forcing influences ENSO remains divided; some studies suggest El Niño-like responses, others La Niña-like responses, or no consistent signal at all. This disagreement may stem from a shared assumption that volcanic forcing acts primarily on ENSO amplitude. This study reframes the question around ENSO’s dynamical evolution, combining Palaeoclimate Modelling Intercomparison Project (PMIP) 3/4 past1000 climate model simulations and palaeoclimate ENSO reconstructions over the Last Millennium (850 to 1850 CE). Three dynamical diagnostics are assessed: growth and decay rates of Niño3.4 anomalies following eruptions, year-to-year phase transition probabilities, and active-phase duration, conditioned on ENSO phase, eruption seasonality and location. Post-eruption growth and decay rates are predominantly negative across both datasets, with the clearest signal being a suppression of the strong positive growth that normally characterises Neutral phases, suggesting volcanic forcing dampens the development of new active phases. However, this pattern does not reach statistical significance in either dataset. Transition probabilities show a modest, consistent increase in phase persistence across all starting phases, while phase duration remains largely unchanged relative to non-volcanic conditions. Together, these results indicate a state-dependent volcanic influence on ENSO, consistent with transient suppression of the Bjerknes feedback through an atmospheric pathway. This dynamical framing motivates further investigation into whether sustained, clustered volcanism produces longer term shifts in ENSO behaviour.