1. Jian Lu Collaborators: Clara Deser Tomas Reichler
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?
Vary in tandem .

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)
Underlying mentality: if a trend be detected, one tends to link implicitly to a anthropogenic source.
Are the integral parts of one phenomena.

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
“SST+RAD”: Obs SST/SIC + radiative forcings including: GHG, O3, solar and sulphate and volcanic aerosols
“SST+RAD” – “SST”: Effects of radiative forcings

7. Tropopause-based metric
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
One should take extra caution when using the statistic of daily tropo to define a width of the tropics.
Any average, in either space or time, can scramble the tropical values of tropo with the extratropical values and give rise to an artificial intermediate tropopause level, which
May never happens in reality.
Cyclonic breaking -- equatorward shift of jet
Anti-cyclonic wave breaking --- poleward shift of jet -- extend the tropical tropopause regime poleward.
I will not talk about this, I bet nobody has a 3-D picture of what is going on during the
Isothermal surface overturns.

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
Undergoes switches between tropical and extratropical regimes.
Count how many days does the tropopause reside in the tropical / extra regimes.

11. PDFs of daily tropopause P(bin,y)
120 hPa following Seidel and Randel
latitude
To define the width the tropics, first we adopt Seidel and Randel 07’s criterion, for each latitude, counting how many days per year the tropopause dwells on levels higher than 120hPa.
Pressure bins

12. ∑P(bin≤120hPa, y) 1999 1958 days 200 days
Accumulated days on which the tropopause is above 120hPa.
We use the latitude where there are more than 200 d/per year for the daily tropo to be above the 120hPa level to demarcate the edge of the tropics.

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

14. Broadening of tropics ANN NH SH Agung El Chichon Pinatubo
Lungs inhale and exhale in tandem. SH

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

16. Significance of the Widening
Trend
Trend

17. Fingerprints of radiative forcings
solar
volcano
natural
GHG O3
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.
Culprit;
Based on our understanding of the impacts of the different forcing agents, it is very likely the anthropogenic O3 and well-mixed GHG forcing.