Abstract
Observations and model simulations of the climate responses to strong explosive low-latitude volcanic eruptions suggest a significant increase in the likelihood of El Nino during the eruption and posteruption years, though model results have been inconclusive and have varied in magnitude and even sign. In this study, we test how this spread of responses depends on the initial phase of El Nino-Southern Oscillation (ENSO) in the eruption year and on the eruption's seasonal timing. We employ the Geophysical Fluid Dynamics Laboratory CM2.1 global coupled general circulation model to investigate the impact of the Pinatubo 1991 eruption, assuming that in 1991 ENSO would otherwise be in central or eastern Pacific El Nino, La Nina, or neutral phases. We obtain statistically significant El Nino responses in a year after the eruption for all cases except La Nina, which shows no response in the eastern equatorial Pacific. The eruption has a weaker impact on eastern Pacific El Ninos than on central Pacific El Ninos. We find that the ocean dynamical thermostat and (to a lesser extent) wind changes due to land-ocean temperature gradients are the main feedbacks affecting El Nino development after the eruption. The El Nino responses to eruptions occurring in summer are more pronounced than for winter and spring eruptions. That the climate response depends on eruption season and initial ENSO phase may help to reconcile apparent inconsistencies among previous studies.