Tropical explosive volcanic eruptions can trigger El Niño 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 Niño within 2 years following the eruption. Modelling studies have, however, so far reached no consensus on either the sign or physical mechanism of El Niño response to volcanism. Here we show that an El Niño 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 Niñas, lengthen El Niños 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 Niño-like response.

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  1. A series of model experiments in this study show that tropical explosive volcanic eruptions, such as the 1964 eruption of Mount Agung in Indonesia, the 1991 eruption of Mount Pinatubo in Philippines, and the 1982 eruption of El Chicon in Mexico, quickly cool the tropical land and ocean around the globe due the reduced shortwave radiation. The land mass cools much faster than the ocean due to their lower heat capacity. Due to the proximity and the large area of land mass, the tropical Africa land mass cools much faster than in other tropical regions. A Gill-type response later kicks in producing westerly wind anomalies over the tropical Indian and west Pacific Oceans, and easterly wind anomalies in the tropical Atlantic. These wind anomalies in turn could initiate El Nino event in the Pacific and Atlantic Nina (i.e., cold SST anomalies in the eastern and central equatorial Atlantic) in the Atlantic.

    This is very interesting given that Bali’s Mount Agung volcano eruption is imminent. Let’s see if this scenario actually plays out this time.

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