Dispersion of the Nabro volcanic plume and its relation to the Asian summer monsoon

Abstract. We use nighttime measurements from the Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO) satellite, together with a Lagrangian trajectory model, to study the initial dispersion of volcanic aerosol from the eruption of Mt. Nabro (Ethiopia/Eritrea) in June 2011. The Nabro eruption reached the lower stratosphere directly, and dispersal of the plume in the upper troposphere and lower stratosphere (UTLS) was constrained initially by the Asian anticyclone, which prevails in the UTLS from the Mediterranean Sea to East Asia during boreal summer. CALIPSO detected aerosol layers, with optical properties consistent with sulfate, in the lower stratosphere above the monsoon convective region in South and South-East Asia within 10 days of the eruption. We show that quasi-isentropic differential advection in the vertically sheared flow surrounding the Asian anticyclone explains many of these stratospheric aerosol layers. We use Meteosat-7 data to examine the possible role of deep convection in the Asian monsoon in transporting volcanic material to the lower stratosphere during this time, but find no evidence that convection played a direct role, in contrast with claims made in earlier studies. On longer time scales, we use CALIPSO data to illustrate diabatic ascent of the Nabro aerosol in the lower stratosphere, at rates of ~10 K per month for the first two months after the eruption, falling to ~3 K per month after the Asian anticyclone dissipates. We attribute ~15% of this to self-lofting. We show that aerosol optical depth associated with the Nabro plume peaks in July; we find the associated radiative forcing is relatively small; top of atmosphere (TOA) radiative forcing peaks at ~1.6 W m−2 locally in July, ~10% of that reported for the impact of Pinatubo in 1991 and within observed interannual variability.