Stratospheric Aerosols from Major Volcanic Eruptions: A Composition-Climate Model Study of the Aerosol Cloud Dispersal and e-folding Time

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.

Quantification of sulfur deposition changes under sulfate geoengineering conditions

“…They found that the aerosol burden non-linearly correlate with the QBO phase because of a wide range of reasons, amongst those the rather wide differences in the size range of the aerosols. The QBO impact on the e-folding time of stratospheric sulfate aerosols injected in past major volcanic eruptions was studied in Pitari et al (2016b), where a clear correlation is found between a larger e-folding time and a QBO E shear of 10 the mean zonal equatorial winds, as a consequence of a higher aerosol confinement in the tropical pipe (consistently with the findings of Trepte and Hitchman (1992)). It should be noted that the stratospheric aerosol distribution in case of SG, or after a major tropical explosive volcanic eruption, is so different with respect to the atmospheric background, both spatially and in size (see Fig.…”

“…As it has been shown in Fig. 3, this is a result of both the larger cross-tropopause tropical downward flux and the larger mid-upper tropospheric mixing toward tropical latitudes in GEOS-Chem compared to ULAQ-CCM (see also 20 the discussion relative to Fig. 9-11).…”

“…Important model updates regarding horizontal and vertical resolution (now T21 with 126 log pressure levels), species cross sections and Schumann-Runge bands treatment, and upgrades of the radiative transfer code were described and tested in Pitari et al (2014). This radiative module, crucial for a good prediction of the sulfate aerosol interaction 10 with shortwave solar and longwave planetary radiation has been tested for tropospheric aerosols in SPARC-AEROCOM (Randles et al (2013)) and also for stratospheric aerosols after major volcanic eruptions (Pitari et al (2016b)). The shortwave radiative module uses a two-stream delta-Eddington approximation and operates on-line in the ULAQ-CCM.…”

“…Geoengineering sulfate aerosols (or those produced after major volcanic eruptions) may significantly perturb wavelength-dependent aerosol extinction, absorption and asymmetry parameter at all model grid-points, thus allowing on-line calculation of radiative flux perturbations, with consequent changes of O 2 and O 3 photolysis, O 3 heating rates and aerosol heating rates in the solar and planetary infrared ranges (Pitari et al (2014);Pitari et al (2016b)). …”

Evaluating the link between the sulfur-rich Laacher See volcanic eruption and the Younger Dryas climate anomaly

“…2). Measurable quantities of aerosols remained in the atmosphere for approximately 3 years even after the 1980 Mount St Helen's eruption (46 • N, 122 • W) (Pitari et al, 2016), which produced only 2.1 Mt SO 2 (Baales et al, 2002;Pitari et al, 2016) and erupted laterally (Eychenne et al, 2015). In short, the LSE eruption probably occurred during the late spring or early summer, but even if the eruption were a winter eruption, the LSE's aerosols would have certainly persisted over at least the following summer, with the potential to catalyse the positive feedback we invoke.…”