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Man-made Widening of the "Tropics": Causes and Trends

This study explores the expansion of the "Tropics" using different measures and investigates the role of SST changes and radiative effects. The results indicate that only under radiative forcing, including GHG and O3, can the expansion of the tropics be reproduced, suggesting anthropogenic sources for this phenomenon. Further research is being conducted to understand the specific contributions of GHG and O3 forcing.

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Man-made Widening of the "Tropics": Causes and Trends

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  1. A Man-made Widening of the “Tropics” Jian Lu Collaborators: Clara Deser Tomas Reichler CCSM Workshop, Breckenridge, June 18, 2008

  2. How wide is the Tropics?

  3. Expansion from different measures Seidel et al. 2008, Nature Geoscience chemical radiative dynamical dynamical thermal

  4. Expansion from different measures Seidel et al. 2008, Nature Geoscience Tropopause-based measure

  5. Objective • What causes the tropopause-based expansion of the “Tropics”? • SST changes or radiative effects(GHG, ozone, volcanic and sulphate aerosols, solar output)? • Tools: GFDL AM2.1 (NCAR CAM3.0) • 1958-2000

  6. Data • Radiosondes: HadRT V2.1 (1957-2001) (Parker et al., 1997) • Reanalysis:ERA40 (1957/10—2001/09) (Uppala et al. 2005) NCAR/NCEP • GFDL AM2.1 simulations • “SST”: Obs SST/SIC 1950-2000 • “SST+RAD”: Obs SST/SIC + radiative forcings including: GHG, O3, solar and sulphate and volcanic aerosols • “SST+RAD” – “SST”: Effects of radiative forcings

  7. Method Tropopause-based metric • WMO criterion: the lowest level at which the lapse rate decreases to 2C/km, provided also the average lapse rate between this level and higher levels within 2km does not exceed 2C/km. • Reichler (2003) algorithm -dT/dz=2Cº/km

  8. Annual mean tropopause hPa

  9. A snap shot of tropopause • Shape of the tropopause

  10. PDFs of tropopause---metric of Seidel and Randel • Compute tropopause for each day using Reichler algorithm  H(x,y); for a year: H(days,x,y) • For each year, compute the probability density function of H(d,x,y) in to [75:5:350] pressure(hPa) bins  P(bins,x,y) • Zonal average of P(bin,x,y)  P(bin,y) Seidel and Randel, 2007

  11. PDFs of daily tropopause P(bin,y) 120 hPa ------------following Seidel and Randel latitude Pressure bins

  12. ∑P(bin≤120hPa, y) 1999 1958 200 days days

  13. Expansion from different measures Seidel et al. 2008, Nature Geoscience Tropopause-based measure

  14. Broadening of tropics ANN Agung El Chichon Pinatubo NH SH

  15. Trend in PDFs of daily tropopause P(bin,y), DJF SST+RAD Δ(=RAD) SST

  16. Significance of the Widening Trend 1958-99 Trend 1978-99

  17. Fingerprints of radiative forcings volcano solar natural O3 GHG anthropogenic sulphate Sum

  18. Conclusion and outlook • The agreement between ERA40 and the GFDL AM2.1 simulations corroborates the reality of an expansion of the tropics since the 1950s. • SSTs alone drive no trend (or even a slightly shrinking trend) in tropical width. Only under the radiative forcing, especially GHG and O3, can AM2.1 reproduce the expansion of the tropics, a result that points to anthropogenic sources for the expansion. • Further attribution is underway to investigate the respective roles of GHG and O3 forcing using AM2.1. • The tropical expansion in SH summer (DJF) varies hand-in-hand with the expansion of the Hadley cell and the associated change in subtropical rain (P-E) pattern. Puzzles remain for NH and other seasons.

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