Ten-Year SCIAMACHY Stratospheric Aerosol Data Record: Signature of the Secondary Meridional Circulation Associated with the Quasi-Biennial Oscillation - Towards an Interdisciplinary Approach in Earth System Science - page 45

shorter periods (e.g., Trepte and Hitchman 1992 ). One dominant dynamic feature of
the atmosphere (below around 50 km) is the residual mean meridional circulation
(reviewed in Butchart 2014 ), also known as Brewer-Dobson circulation, which is
primarily driven by planetary wave breaking in the stratosphere. The main prop-
erties of this atmospheric circulation are tropical upwelling combined with air mass
transport from the tropics to the extratropics and to the poles (shallow branches) in
the lower and middle stratosphere and from the tropics to the winter pole (deep
branch) in the middle and upper stratosphere.
Another relevant dynamical process in the stratosphere is the quasi-biennial
oscillation (QBO) of the equatorial zonal wind. The QBO is caused by the inter-
action of the zonal wind with tropical gravity, inertia-gravity, Kelvin, and Rossby-
gravity waves and appears as easterly and westerly wind regimes propagating
downward from the stratopause (around 50 km) to the tropopause (around 16 km)
while at a

fixed altitude the wind changes from east to west and back to east within
28 months ( 2 years) on average. The period and the amplitude of the zonal wind

QBO are, for instance, in

≈

uenced by the residual mean meridional circulation
(Kawatani and Hamilton 2013 ; Butchart 2014 ).
The QBO has a signi

fl

cant impact on the global chemistry and climate, as it
affects the distribution of trace species and aerosols in the stratosphere (reviewed in
Baldwin et al. 2001 ). Furthermore, it causes the secondary meridional circulation in
the tropical and subtropical stratosphere (Baldwin et al. 2001 ; Ribera et al. 2004 ).
Aerosol effects associated with the secondary meridional circulation of the QBO
have been first described by Trepte and Hitchman ( 1992 ) for the stratospheric

background aerosol, based on SAGE II aerosol extinction ratios. Bingen et al.
( 2004 ) showed that the QBO has a clear in uence on the aerosol size distribution

derived from SAGE II data over the period 1984

fl

2000.
One satellite observation method for SAs, which began with the SAM II mission
in the late 1970s followed by SAGE I

-

III, exploits the solar occultation geometry
(e.g., McCormick et al. 1979 ). The use of limb scattered solar radiation measure-
ments, e.g., by OSIRIS/Odin (Llewellyn et al. 2004 ) and SCIAMACHY/ENVISAT
(see Sect. 2 ) is a new approach to quantify stratospheric aerosols during the day. We
employ the SCIAMACHY limb stratospheric aerosol record from August 2002 to
April 2012 (Ernst et al. 2012 ; Ernst 2013 ) in order to focus on the tropical aerosol
distribution in the upper part of the aerosol layer at an altitude of about 30 km.

-

2 Instrument and Data

This study makes use of aerosol extinction coef cients derived from limb scattered

sunlight measurements of the SCIAMACHY (SCanning Imaging Absorption
spectroMeter for Atmospheric CartograHY) satellite instrument (Burrows et al.
1995 ; Bovensmann et al. 1999 ) aboard the ENVISAT (ENVIronmental SATellite)
of the European Space Agency (ESA). The satellite was launched on 1st March
2002 into a polar, sun-synchronous orbit of about 800 km altitude. It ceased