Volcanic Impacts on El Niño / Southern Oscillation

Abstract

El Niño / Southern Oscillation (ENSO) is arguably the most influential climate driver on Earth, so there is immense value for accurately forecasting it. The strong explosive volcanic eruptions provide a valuable opportunity to study the ENSO response to external forcing. Such volcanic eruptions can inject millions of tons of SO2 into the stratosphere, where they can convert into sulfate aerosols. For equatorial volcanoes, these aerosols can spread globally, scattering and absorbing incoming sunlight, and induce surface cooling worldwide. Despite this global cooling effect, the tropical Pacific Ocean surface often shows El Niño-like warming after strong volcanic eruptions. However, limited instrumental data cannot uncover the actual ENSO response to a robust external perturbation. Even modeling studies provide a limited understanding of the Volcano-and-ENSO interaction mechanism. This dissertation develops a firm understanding of the dynamical mechanisms of the ENSO response to tropical and high-latitude eruptions. It does so by developing a unified modeling framework that combines the roles of the seasonal cycle, stochastic forcing, eruption magnitude, and various tropical Pacific climate feedbacks. This study analyzes specifically designed climate model simulations spanning over 20000 years. This framework illuminates the nature of ENSO’s responses to past eruptions and explains why the El Niño-like response is more likely to occur during particular seasons and ENSO phases. It clarifies why prior studies obtained different and seemingly conflicting results. The ENSO response to strong volcanic eruptions is analyzed here in terms of stochastic and deterministic components. The partial contribution of these components determines the predictability and strength of the ENSO response to volcanic perturbation, and the ratio between them varies depending on the perturbation season and the ocean preconditioning. For boreal winter eruptions, stochastic dispersion largely obscures the deterministic response, being the largest for the strong El Niño preconditioning. Deterministic El Niño-like responses to summer eruptions are well seen on neutral ENSO and weak to moderate El Niño onsets and grow with the eruption magnitude. This improved understanding is expected to advance climate model simulations, predictions, and projections of ENSO, and its response to both tropical and high-latitude volcanic eruptions.