Tropical explosive volcanic eruptions can trigger El Ninõ by cooling tropical Africa

Stratospheric aerosols from large tropical explosive volcanic eruptions backscatter shortwave radiation and reduce the global mean surface temperature. Observations suggest that they also favour an El Ninõ within 2 years following the eruption. Modelling studies have, however, so far reached no consensus on either the sign or physical mechanism of El Ninõ response to volcanism. Here we show that an El Ninõ tends to peak during the year following large eruptions in simulations of the Fifth Coupled Model Intercomparison Project (CMIP5). Targeted climate model simulations further emphasize that Pinatubo-like eruptions tend to shorten La Ninãs, lengthen El Ninõs and induce anomalous warming when occurring during neutral states. Volcanically induced cooling in tropical Africa weakens the West African monsoon, and the resulting atmospheric Kelvin wave drives equatorial westerly wind anomalies over the western Pacific. This wind anomaly is further amplified by air-sea interactions in the Pacific, favouring an El Ninõ-like response.