The sensitivity of the El Ninõ-Southern Oscillation to volcanic aerosol spatial distribution in the MPI Grand Ensemble

Using the Max Planck Institute Grand Ensemble (MPI-GE) with 200 members for the historical simulation (1850-2005), we investigate the impact of the spatial distribution of volcanic aerosols on the El Ninõ-Southern Oscillation (ENSO) response. In particular, we select three eruptions (El Chichón, Agung and Pinatubo) in which the aerosol is respectively confined to the Northern Hemisphere, the Southern Hemisphere or equally distributed across the Equator. Our results show that relative ENSO anomalies start at the end of the year of the eruption and peak in the following one. We especially found that when the aerosol is located in the Northern Hemisphere or is symmetrically distributed, relative El Ninõ-like anomalies develop, while aerosol distribution confined to the Southern Hemisphere leads to a relative La Ninã-like anomaly. Our results point to the volcanically induced displacement of the Intertropical Convergence Zone (ITCZ) as a key mechanism that drives the ENSO response, while suggesting that the other mechanisms (the ocean dynamical thermostat and the cooling of tropical northern Africa or the Maritime Continent) commonly invoked to explain the post-eruption ENSO response may be less important in our model.