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

 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 upper troposphere and
lower stratosphere (UTLS) directly, and the plume was initially
entrained by the flow surrounding 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 Southeast 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 timescales, we
use CALIPSO data to illustrate diabatic ascent of the Nabro
aerosol in the lower stratosphere at rates of 10K per month
for the first two months after the eruption, falling to 3K
per month after the Asian anticyclone dissipates. Maps of
stratospheric aerosol optical depth (AOD) show local peaks
of 0.04–0.06 in July in the region of the Asian anticyclone;
we find associated estimates of radiative forcing small, 5–
10% of those reported for the eruption of Mt. Pinatubo in
1991. Additionally, we find no clear response in outgoing
shortwave (SW) flux due to the presence of Nabro aerosol
viewed in the context of SW flux variability as measured by
CERES (Clouds and Earth Radiant Energy System).