1. Observed Recent Changes in the Tropopause Dian Seidel NOAA Air Resources Laboratory ~ Silver Spring, Maryland USA Bill Randel NCAR Atmospheric Chemistry Division ~ Boulder, Colorado USA AGU Chapman Conference on the Role of the Stratosphere in Climate & Climate Change 24-28 September 2007 ~ Santorini, Greece

2. Motivation Tropical tropopause changes may relate to –Stratospheric water vapor changes –Changes in stratospheric and/or tropospheric dynamics Global tropopause changes may relate to –Anthropogenic (radiatively forced) climate change –Vertical temperature profile changes Evolving understanding of –Thermal tropopause –Multiple tropopauses –Tropopause layers

3. Observations Daily radiosoundings from 100 stations NCEP/NCAR reanalysis (and ERA 40) GPS Met

4. Radiosonde Stations

5. Climatological Tropopauses

6. Inhomogeneity in Tropopause Times Series

7. Zonal Tropopause Trends 1980-2004

8. Global Tropopause Trends 1980-2004 +64 ± 21 m/decade -1.7 ± 0.6 hPa/decade -0.41 ± 0.09 K/decade 160 m rise 4.2 hPa drop 1.0 K cooling

9. Short-term Co-Variability of Tropopause Height and Atmospheric Temperature

10. Tropopause Height and Temperature Trends

11. Bimodal Tropopause Climatology in Subtropics

12. Bimodal Tropopause in Radiosonde Data

13. Bimodal Tropopause In Reanalysis Data

14. Double Tropopauses in GPS Data

15. How often is the tropopause high?

16. Tropopause in Subtropics More Frequently Tropical

19. Increases in High Tropopause Frequency

20. Measuring the Width of the Tropics

21. Expansion of the Tropical Belt

22. Corroboration in Other Observations

23. Issues and Implications of a Wider Tropical Belt Observed rate exceeds model projections. Why? Seasonal, regional structure of changes have not been studied. Related changes in –Hydrologic cycle, esp. subtropical dry zones –Jet streams and storm tracks –Tropical weather systems –Brewer-Dobson circulation

24. Summary The global tropopause has risen and cooled since 1980. –Data issues pose problems estimating trends further back. –Short time scale changes assoc. with stratospheric and tropospheric T changes. Trends only assoc. with strat. T trends. Tropopause heights are bimodally distributed in subtropics –Region is also where double tropopauses occur –Indicates ‘edge’ of tropical belt –Robust metric for trend detection Tropical belt has expanded 2-8 deg lat in past 25 yrs –Evidence from tropopause and other, independent observations –Many open questions about processes, implications

25. References  Hu, Y., and Q. Fu: Observed poleward expansion of the Hadley circulation since 1979. Atmos. Chem. Phys. Disc., submitted.  Hudson R. D., M. F. Andrade, M. B. Follette, and A. D. Frolov, 2006: The total ozone field separated into meteorological regimes – Part II: Northern Hemisphere mid-latitude total ozone trends. Atmos. Chem. Phys. 6, 5183- 5191.  Randel, W. J., D. J. Seidel, and L. L. Pan, 2007: Observational characteristics of double tropopauses. J. Geophys. Res., 112, D07309, doi:10.1029/2006JD007904.  Reichler, T., and I. Held: Widening trend of the Hadley cell over the past 40 years. American Meteorological Society, Conference on Climate Variability and Change (Cambridge, MA, 2005).  Seidel, D.J., and W.J. Randel, 2007: Recent widening of the tropical belt: Evidence from tropopause observations. J. Geophys. Res., in press.  Seidel, D.J., and W.J. Randel, 2006: Variability and trends in the global tropopause estimated from radiosonde data, J. Geophys. Res., 111, D21101, doi:10.1029/2006JD007363.

26. Thank you Ευχρισώ

27. Example of Tropopauses in Sounding Data

28. WMO (1957) Lapse Rate Tropopause Definition The first tropopause is defined as the lowest level at which the lapse rate decreases to 2°C/km or less, provided also the average lapse rate between this level and all higher levels within 2 km does not exceed 2°C/km. If above the first tropopause the average lapse rate between any level and all higher levels within 1 km exceeds 3°C/km, then a second tropopause is defined by the same criterion as under (a). This tropopause may be either within or above the 1 km layer.