Perturbations to the Global Atmosphere Associated With the El Chichon Volcanic Eruption of 1982 (Paper 7R0120)

In terms of effects on the stratosphere, the 1982 eruption of the Mexican volcano El Chichón (17°N) is believed to have been one of the two or three greatest in the past century. In terms of providing opportunities for testing physical and chemical models in the stratosphere, it is unprecedented. Earlier recent eruptions (Fuego in 1974, St. Helens in 1980, and Alaid in 1981) were not of the magnitude required to investigate the more subtle chemical and concomitant radiative and dynamical consequences, and while Agung in 1963 may have been, the necessary measurements are absent. Thus for the first time, trace chemical species such as HCℓ and OH were observed to increase, and other species, possibly related to these, were observed to decrease (O₃, NO, and NO₂) following the eruption. Volcanic SO₂ was observed from space for the first time, and its conversion into H₂SO₄, vapor, condensation nuclei, and finally, sulfuric acid aerosol were monitored closely by both in situ and remote measurements. In this manner the very uncertain reaction sequence, which converts SO₂ to H₂SO₄ could be ascertained. The optical depth of about 0.3, produced by the sulfuric acid aerosol which formed after the eruption, ranks it with the largest in recent history (for example, Krakatau in 1883). At least three factors contributed to the severity of the eruption aftermath. These include the apparently large fraction of sulfur involved for an eruption of only moderate proportions, the latitude of the volcano, and the time of year of the eruption. As in the case of the eruption of Agung (8°S) in 1963, the stratosphere was observed to warm, especially in equatorial regions. However, the expected surface cooling did not apparently occur. A highly enhanced El Niño, which is followed by tropospheric warming, may have acted to counterbalance the surface cooling in the case of El Chichón. Evidence that enhanced El Niño events usually accompany major low-latitude eruptions suggests a volcanic effect on global atmospheric transport, thus providing a climatic self-healing effect.