Atmospheric Chemistry and Physics
www.atmos-chem-phys.net
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2007
10.5194/acp-7-5611-2007
http://www.atmos-chem-phys.net/7/5611/2007/
http://www.atmos-chem-phys.net/7/5611/2007/acp-7-5611-2007.html
http://www.atmos-chem-phys.net/7/5611/2007/acp-7-5611-2007.pdf
5611
5624
2007-11-14
Trends and variability in stratospheric mixing: 1979–2005
H. Garny
hella.garny@dlr.de
G. E. Bodeker
M. Dameris
Meteorological Institute, University of Munich, Munich, Germany
National Institute of Water and Atmospheric Research, Lauder, New Zealand
DLR-Institut für Physik der Atmosphäre, Oberpfaffenhofen, Germany
Changes in climate are likely to drive changes in stratospheric mixing with
associated implications for changes in transport of ozone from tropical
source regions to higher latitudes, transport of water vapour and source gas
degradation products from the tropical tropopause layer into the mid-latitude
lower stratosphere, and changes in the meridional distribution of long-lived
trace gases. To diagnose long-term changes in stratospheric mixing, global
monthly fields of Lyapunov exponents were calculated on the 450 K, 550 K,
and 650 K isentropic surfaces by applying a trajectory model to wind fields
from NCEP/NCAR reanalyses over the period 1979 to 2005. Potential underlying
geophysical drivers of trends and variability in these mixing fields were
investigated by applying a least squares regression model, which included
basis functions for a mean annual cycle, seasonally dependent linear trends,
the quasi-biennial oscillation (QBO), the solar cycle, and the El Niño
Southern Oscillation (ENSO), to zonal mean time series of the Lyapunov
exponents.
<br><br>
Long-term positive trends in mixing are apparent over southern middle to high
latitudes at 450 K through most of the year, while negative trends over
southern high latitudes are apparent at 650 K from May to August. Wintertime
negative trends in mixing over northern mid-latitudes are apparent at 550 K
and 650 K. Over low latitudes, within 40° of the equator, the QBO
exerts a strong influence on mixing at all three analysis levels. This QBO
influence is strongly modulated by the annual cycle and shows a phase shift
across the subtropical mixing barrier. Solar cycle and ENSO influences on
mixing are generally not significant. The diagnosed long-term changes in
mixing should aid the interpretation of trends in stratospheric trace gases.
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