1. 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

2. 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)

3. 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)

4. Enhanced stratospheric circulation? Adapted from Schmidt et al. (2001) Residual circulation Mixing barriers LowerLower- most Large-scale mixing processes

5. Increasing upper branch of the Brewer-Dobson circulation? Mean age of air in the midlatitudes above 30 hPa Engel et al., Nature Geosciences, 2008 Slope = +0.24 ±0.22 years

6. Enhanced stratospheric circulation? Adapted from Schmidt et al. (2001) Residual circulation Mixing barriers LowerLower- most Large-scale mixing processes

7. Observations: Higher N 2 O in the LMS In-situ aircraft measurements ▬ Before 2001 ▬ After 2001 Winter (February) STREAM (1997) vs. SPURT (2003) Summer (July) STREAM (1998) vs. SPURT (2003)

8. Diagnostic: N 2 O-O 3 Correlation Concept of Plumb and Ko, JGR, 1992 tropical reference correlation upper envelope extratropical reference correlation lower envelope Isentropes

9. Diagnostic: N 2 O-O 3 Correlation Enhanced isentropic transport relative to the downwelling leads to steeper (more tropical) correlation ▬Tropics ▬ Extratropics (No Vortex) ---Isentropes In-mixing of tropospheric air In-mixing of tropical stratospheric air

10. Seasonality of the residual circulation Winter (DJF) strongest downwelling Summer (JJA) weakest downwelling

11. Seasonality of the N 2 O-O 3 correlation 270 290 310 Winter (February) STREAM (1997) Spring (May) STREAM (1996) Summer (July) STREAM (1998) Seasonality strongest downwelling weakest downwelling strongest influence of downwelling phase shifted by ~3-4 month

12. Changing N 2 O-O 3 correlation before and after 2001 270 290 310 SPURT (2003)SPURT (2002)SPURT (2003) 270 290 310 Winter (February) STREAM (1997) Spring (May) STREAM (1996) Summer (July) STREAM (1998) steeper gradients Before 2001 After 2001

13. Changing N 2 O-O 3 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

14. Other reasons? Changing O 3 photochemistry in the stratosphere  Can not explain higher N 2 O mixing ratios in the LMS.  Unlikely in the LS because of long effective lifetime for O 3 there. Changing influence of arctic vortex O 3 -depletion on trace gas composition in the LS, especially in spring

15. Other reasons? O 3 -depletion [DU] Year Arctic vortex ozone depletion between 380 K and 550 K calculated from HALOE methane und ozone profiles, numbers from Tilmes et al. (2004) SPURT (May) SPURT (April) STREAM (May)

16. Summary and Outlook Enhanced tropical upwelling after 2001 (Randel et al., 2006) is coupled with an intensification of the lower BD circulation branch.  higher N 2 O mixing ratios in the LMS after 2001  steeper N 2 O-O 3 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

17. 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

18. Thank you for your attention