Another hint for a changing
stratospheric circulation
after 2001
Harald Bönisch (1), Andreas Engel (1), Thomas Birner (2), Peter Hoor (3)
(1) Institute for Atmospheric and Environmental Sciences, J. W. Goethe
University, Frankfurt am Main, Germany
(2) Department of Atmospheric Science, Colorado State University, Fort
Collins, CO, USA
(3) Max Planck Institute for Chemistry, Air Chemistry department, Mainz,
Germany
The Extratropical UTLS
Community workshop at the National Center for Atmospheric Research, Boulder, Colorado
19-22 October 2009
Another hint?
Decreases in stratospheric water vapor after 2001:
Links to changes in the tropical tropopause and the
Brewer-Dobson circulation
(Randel et al., JGR, 2006)
Another hint?
Decreases in stratospheric water vapor after 2001:
Links to changes in the tropical tropopause and the
Brewer-Dobson circulation
(Randel et al., JGR, 2006)
Enhanced stratospheric circulation?
Residual
circulation
Large-scale
mixing
processes
Mixing
barriers
Lower
Lowermost
Adapted from
Schmidt et al.
(2001)
Increasing upper branch of the
Brewer-Dobson circulation?
Mean age of air in the midlatitudes above 30 hPa
Slope = +0.24 ±0.22 years
Engel et al., Nature Geosciences, 2008
Enhanced stratospheric circulation?
Residual
circulation
Large-scale
mixing
processes
Mixing
barriers
Lower
Lowermost
Adapted from
Schmidt et al.
(2001)
Observations: Higher N2O in the LMS
Winter (February)
Summer (July)
STREAM (1997) vs. SPURT (2003)
STREAM (1998) vs. SPURT (2003)
In-situ aircraft
measurements
▬ Before 2001
▬ After 2001
Diagnostic: N2O-O3 Correlation
tropical
reference correlation
upper envelope
Isentropes
extratropical
reference correlation
lower envelope
Concept of Plumb and Ko, JGR, 1992
Diagnostic: N2O-O3 Correlation
In-mixing of
tropical
stratospheric air
▬ Tropics
▬ Extratropics
(No Vortex)
--- Isentropes
In-mixing of
tropospheric air
Enhanced isentropic transport relative to the downwelling
leads to steeper (more tropical) correlation
Seasonality of the residual circulation
Winter (DJF)
strongest downwelling
Summer (JJA)
weakest downwelling
Seasonality of the N2O-O3 correlation
Winter (February)
Spring (May)
Summer (July)
STREAM (1997)
STREAM (1996)
STREAM (1998)
270
290
310
270
290
310
strongest
downwelling
270
290
310
weakest
downwelling
Seasonality
strongest influence
of downwelling
phase shifted by
~3-4 month
Changing N2O-O3 correlation
before and after 2001
Winter (February)
Spring (May)
Summer (July)
STREAM (1997)
STREAM (1996)
STREAM (1998)
Before 2001
270
290
310
SPURT (2003)
270
290
310
SPURT (2002)
270
290
310
SPURT (2003)
steeper
gradients
After 2001
270
290
310
270
290
310
270
290
310
Changing N2O-O3 correlation
before and after 2001
Hypothesis:
More tropical air masses in the LMS after 2001 caused by
the intensification of the lower BD circulation branch
Other reasons?

Changing O3 photochemistry in the stratosphere

Can not explain higher N2O mixing ratios in the LMS.
Unlikely in the LS because of long effective lifetime
for O3 there.

Changing influence of arctic vortex O3-depletion on
trace gas composition in the LS, especially in spring

Other reasons?
Arctic vortex ozone depletion between 380 K and 550 K
calculated from HALOE methane und ozone profiles, numbers
from Tilmes et al. (2004)
O3-depletion [DU]
STREAM
(May)
SPURT
(April)
SPURT
(May)
Year
Summary and Outlook
Enhanced tropical upwelling after 2001 (Randel et al.,
2006) is coupled with an intensification of the lower
BD circulation branch.
higher N2O mixing ratios in the LMS after 2001
 steeper N2O-O3 gradients in the LMS after 2001
 non-decreasing mean age of air above 30 hPa

Including more aircraft campaigns
 Looking for changes of other tracers and
correlations
 Looking for changes of dynamical parameters, e.g.
EP-flux

Summary and Outlook
=> Changes in the BD might be coupled with changes
of the TIL and tropopause structure
e.g. “Residual circulation and tropopause structure”
Thomas Birner
=> Chemical composition in the LMS above the ExTL
is dominated by the lower branch of the BD circulation.
e.g. “Transport timescales and surface source regions of UTLS
air from START-08 tracer correlations”
Eric Ray, F. Moore, K. Rosenlof
Thank you for your
attention