GCM evaluation of a mechanism for El Niño triggering by the El Chichón ash cloud

Abstract

The El Chichón volcanic eruption in 1982 was immediately followed by the strongest El Niño of the century in 1982–83. We investigate a possible mechanism proposed by Hirono whereby volcanic eruptions might trigger or enhance El Niños. Three distributions of volcanic ash aerosol in the troposphere over the eastern Pacific Ocean were used to force simulations with a modified version of the NCAR CCM1 atmospheric general circulation model for the month of April 1982, using observed sea surface temperatures. Only in the case of mid‐tropospheric heating is there a strong dynamical reaction in the atmosphere resulting in a weakening of the trade winds in the eastern Pacific Ocean north of the equator, consistent with surface wind observations and with the first part of Hirono's theory. However, the 1982 ENSO event had started before this wind anomaly, and only trade wind collapses in the western equatorial Pacific can initiate El Niños. The results suggest that, while tropospheric aerosols from volcanic eruptions can influence atmospheric circulation, the timing and location of the El Chichón eruption and the large ENSO event that followed was a coincidence, and that Hirono's mechanism was not responsible for the strength or timing of the El Niño.