Stratospheric aerosols from major volcanic eruptions: QBO impact on the aerosol cloud dispersal and optical depth

Abstract:

Explosive volcanic eruptions, by injecting considerable amount of particles and sulfur gases above the tropopause, can cause large increases in stratospheric aerosol mass concentration and optical depth. This is particularly true for tropical eruptions, because the efficient confinement of atmospheric tracers in the tropical pipe tends to prolong the aerosol residence time into the stratosphere. The amount of time it takes for the aerosol cloud to disperse in the extra-tropics varies mainly because of two factors: (a) the modulation of stratospheric dynamics produced by the quasi-biennial oscillation (QBO) of the equatorial winds and (b) the aerosol radiative feedback on the stratospheric circulation. The latter takes place both directly in shortwave and longwave radiation wavelengths and also indirectly through induced photochemical ozone changes and ozone absorption of aerosol diffused UV radiation. The QBO may strongly influence how effectively the aerosols can be transported out of the tropics, as well as the magnitude of tropical upwelling. All major volcanic eruptions from 1960 to 2015 have been considered in a numerical study conducted with the composition-climate coupled model ULAQ-CCM. This is a chemistry-climate model with on-line a microphysics code for aerosol formation and growth (including SO_x chemistry and explicit gas-particle interactions). Model results are compared between two different simulations: (a) the first including the aerosol radiative effects and the observed phase of the QBO (VE); (b) the second where the nudged QBO has been shifted ahead of one year, thus allowing for all the volcanic eruptions to take place under a different QBO phase (VESH). VESH-VE differences highlight the QBO phase importance in regulating the aerosol confinement in the tropical pipe; this effect is also visible in comparing the aerosol cloud dispersal in the VE case, for eruptions taking place in different QBO regimes. The impact on the aerosol cloud e-folding time and optical depth is discussed for major tropical eruptions (Agung, El Chichón and Pinatubo) and also for eruptions of smaller magnitude, as those recorded after Pinatubo.