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Another hint for a changing stratospheric circulation after 2001

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

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Another hint for a changing stratospheric circulation after 2001

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

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

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

  7. Winter (February) STREAM (1997) vs. SPURT (2003) Summer (July) STREAM (1998) vs. SPURT (2003) Observations: Higher N2O in the LMS In-situ aircraft measurements ▬ Before 2001 ▬ After 2001

  8. Diagnostic: N2O-O3 Correlation tropical reference correlation upper envelope Isentropes Concept of Plumb and Ko, JGR, 1992 extratropical reference correlation lower envelope

  9. Diagnostic: N2O-O3 Correlation In-mixing of tropical stratospheric air ▬ Tropics ▬ Extratropics (No Vortex) --- Isentropes Enhanced isentropic transport relative to the downwelling leads to steeper (more tropical) correlation In-mixing of tropospheric air

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

  11. 270 290 310 270 290 310 270 290 310 Seasonality of the N2O-O3 correlation Winter (February) STREAM (1997) Spring (May) STREAM (1996) Summer (July) STREAM (1998) strongest downwelling weakest downwelling Seasonality strongest influence of downwelling phase shifted by ~3-4 month

  12. 270 290 310 270 290 310 270 290 310 270 290 310 270 290 310 270 290 310 Changing N2O-O3 correlationbefore and after 2001 Winter (February) STREAM (1997) Spring (May) STREAM (1996) Summer (July) STREAM (1998) Before 2001 steeper gradients SPURT (2003) SPURT (2002) SPURT (2003) After 2001

  13. Changing N2O-O3 correlationbefore 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 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

  15. O3-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) Other reasons? STREAM (May) SPURT (April) SPURT (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 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

  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

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