Volcanic aerosol perturbations and strong El Niño events: No general correlation

We test the hypothesis that El Niño/Southern Oscillation (ENSO) events are caused or enhanced by volcanic aerosol perturbations using two methods: 1) A case-by-case comparison of the 16 strongest El Niño events of the last 150 years with the characteristics of the largest concurrent volcanic events, and 2) Comparison of the timing of strong El Niño events and the independently determined record of stratospheric optical depth (τ) perturbations. Many eruptions that occurred near times of strong El Niño years produced small amounts of stratospheric aerosols, in many cases the relative timing of the two events argues against triggering. The correlation of 4 of 11 peaks in global stratospheric optical depth (τ ≥ 0.025) within 3 years of strong El Niño events over the last 150 years is what would be expected by chance. Moreover, five strong El Niños occurred between 1915 and 1960, when the stratosphere was largely free of volcanic aerosols. Of the three strongest tropical volcanic aerosol perturbations that coincided with or were followed by strong El Niños in the period studied (Krakatau in 1883, El Chichón in 1982, and Pinatubo in 1991), the two modern ones occurred after the earliest SST wanning of the El Niño events. The coincidence of these two phenomena indicates no causative relationship.