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 49

According to the thermal wind balance at the equator, which basically says that
the vertical gradient in the zonal wind is compensated by temperature anomalies
(e.g., Andrews et al. 1987 ; Randel et al. 1999 ), strong equatorial westerly (easterly)
wind shears are accompanied by positive (negative) temperature anomalies (on the
order of

4 K, see Baldwin et al. 2001 ). The positive (negative) temperature
anomalies induce adiabatic warming (cooling) of sinking (rising) air (e.g., Baldwin
et al. 2001 ), which preserves these temperature anomalies. Thus, vertical motions
relative to the upwelling of the mean meridional circulation (MMC) occur. In
addition, the equatorial temperature anomalies are linked to anomalies in the MMC
in the tropics. A negative anomaly in the Northern Hemisphere MMC and a positive
anomaly in the Southern Hemisphere MMC at the same time induce a secondary
meridional motion towards the equator and vice versa (Baldwin et al. 2001 ; Ribera
et al. 2004 ). Due to mass continuity, the equatorial sinking (rising) motion during
westerly (easterly) wind shear periods and the related meridional motion lead to
subtropical rising (sinking) air (Baldwin et al. 2001 ). This

±

finally completes the

SMC in the tropical and subtropical stratosphere.
The SMC modulates the large-scale tracer transport in the stratosphere that is
associated with the MMC. Based on a microphysical model of the life-cycle of
stratospheric aerosols, Hommel et al. ( 2014 ) showed in detail how the stratospheric
aerosol layer is lofted by the SMC during the QBO easterly shear. This lofting is
followed by a slow descent over about a quarter QBO period, with a subsequent
stronger descent under the in

uence of the emerging QBO westerly wind shear.
This motion of aerosols in the equatorial stratosphere is pretty well re

fl

ected in the
vertically-resolved time series of SCIAMACHY observed aerosol extinction
coef

fl

cients at 750 nm (Fig. 2 ). Furthermore, the SCIAMACHY aerosol record
nicely shows the hemispheric asymmetry in the QBO modulated meridional aerosol
transport (Fig. 1 ), which is stronger in the Northern Hemisphere, where the wave
driving in the stratosphere is stronger (e.g., Andrews et al. 1987 ).
The zonal-mean zonal winds in Fig. 3 a indicate that an easterly shear between

10 10 hPa. Thus, we

can use the wind at 10 hPa as a proxy for the underlying wind shear period and the
resulting direction of the aerosol transport by the SMC in order to examine the
correlation between the aerosol load at 30 km and the wind shear period. Figure 4
shows the relationship between the zonal wind at 10 hPa (

30 hPa often coincides with strong easterly winds at about 5

- -

31 km) and the aerosol
load at 30 km altitude in two different ways. Figure 4 a is a latitude-time cross
section and shows that easterly winds stronger than

≈

30 m/s occur at the same time
as maximum aerosol loads. Figure 4 b illustrates the time series of the aerosol
extinction at 30 km (monthly mean averaged between 5

−

° N and 5 ° S) and the zonal

wind speed at 10 hPa (averaged between 3 gures, a corre-

lation between easterly wind (which has negative sign) and the aerosol extinction is
clearly visible.
Finally, the lower aerosol load at 30 km during the last few years (mid-2009

° N and 3 ° S). In both

2012)
of the SCIAMACHY measurements should be discussed. We explain it as follows.
Figure 3 a shows that from about April 2008 to February 2010, a relatively long
westerly phase in the lower stratosphere occurs. The next westerly phase starts in

-