Abstract
Large explosive volcanic eruptions are capable of injecting considerable amounts of particles and sulfur gases above the tropopause, causing large increases in stratospheric aerosols. Five major volcanic eruptions after 1960 (i.e., Agung, St. Helens, El Chichón, Nevado del Ruiz and Pinatubo) have been considered in a numerical study conducted with a composition-climate coupled model including an aerosol microphysics code for aerosol formation and growth. Model results are compared between an ensemble of numerical simulations including volcanic aerosols and their radiative effects (VE) and a reference simulations ensemble (REF) with no radiative impact of the volcanic aerosols. Differences of VE-REF show enhanced diabatic heating rates; increased stratospheric temperatures and mean zonal westerly winds; increased planetary wave amplitude; and tropical upwelling. The impact on stratospheric upwelling is found to be larger when the volcanically perturbed stratospheric aerosol is confined to the tropics, as tends to be the case for eruptions which were followed by several months with easterly shear of the quasi-biennial oscillation (QBO), e.g., the Pinatubo case. Compared to an eruption followed by a period of westerly QBO, such easterly QBO eruptions are quite different, with meridional transport to mid- and high-latitudes occurring later, and at higher altitude, with a consequent decrease in cross-tropopause removal from the stratosphere, and therefore longer decay timescale. Comparing the model-calculated e-folding time of the volcanic aerosol mass during the first year after the eruptions, an increase is found from 8.1 and 10.3 months for El Chichón and Agung (QBO westerly shear), to 14.6 and 30.7 months for Pinatubo and Ruiz (QBO easterly shear). The corresponding e-folding time of the global-mean radiative flux changes goes from 9.1 and 8.0 months for El Chichón and Agung, to 28.7 and 24.5 months for Pinatubo and Ruiz.
Highlights
Surface temperature records [1] have demonstrated that major volcanic eruptions reaching the stratosphere globally cool the surface by a few tenths of a degree
The e-folding time during the second year is rather similar for all the tropical eruptions (10–12 months), since the coarser particles have already been lost by sedimentation and a significant amount of the aerosol has been transported outside the tropics
Major volcanic eruptions after 1960 have been considered for the first time in a numerical study conducted with a composition-climate coupled model including an aerosol microphysical code for aerosol formation and growth
Summary
Surface temperature records [1] have demonstrated that major volcanic eruptions reaching the stratosphere globally cool the surface by a few tenths of a degree. The stratospheric aerosol layer may be perturbed by more moderate eruptions (~0.5–3 Tg-SO2 injected in the upper troposphere and lower stratosphere [2]), with small but non-negligible impact on global surface temperatures [3]. These eruptions have contributed to the generally increasing levels of stratospheric aerosols between 2002 and 2010, as observed from measurements by satellite [4]; ground-based lidars [5,6]; and ground-based sun-photometers, lidar and balloons [7].
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