Stratospheric circulation changes due to major volcanic eruptions: impact on long-lived species transport and age of air

Large explosive volcanic eruptions are capable of injecting considerable amounts of particles and sulphur gases above the tropopause, causing increases in stratospheric aerosol optical depth even larger than one order of magnitude. Perturbations of stratospheric tracer species transport result from dynamical changes due to both local stratospheric heating and climate changes associated to the increasing scattering of incoming solar radiation by the volcanic aerosols. Summarizing, the dynamical perturbation of the volcanic aerosols is twofold: (a) the stratospheric mean meridional circulation is affected by local aerosol radiative heating and photo-chemically induced ozone changes; (b) the planetary wave propagation in the mid- to high-latitude lower stratosphere is altered as a consequence of perturbed atmospheric stability due to the climate perturbation. The additional heating rates produce a local temperature increase in the lower stratosphere and upset the stratospheric dynamics, since no radiative equilibrium is achieved. For tropical eruptions an additional residual upwelling motion is produced in the tropical stratosphere as a result of a stronger pole-to-equator gradient of net heating rates. The radiatively forced changes of the stratospheric circulation during the first two years after the eruption of Mt. Pinatubo (June 1991) may help explain the observed trend decline of long-lived greenhouse gases (CH4 in particular), as a result of the increased mid- to high-latitude downward flux at the tropopause. Since the stratosphere contains lower methane mixing ratios, a decline in the observed trends could result from a higher degree of exchange between the stratosphere and the troposphere. Results from the ULAQ-CCM model, using an updated version with respect to the one that has participated to the CCMVal-1 and CCMVal-2 inter-comparison campaigns, are shown for long-lived species perturbations due to volcanic eruptions and also for the stratospheric age-of-air. This type of analysis is made by comparing the results of two model simulations of the ULAQ-CCM: a reference case (1960-2010) with no volcanic heating perturbations and a sensitivity experiment where Agung, El Chichon and Pinatubo eruptions have been included.