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

How wide is the Tropics? Vary in tandem .

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

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

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

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

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

Annual mean tropopause hPa

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.

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.

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

∑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.

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

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

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

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

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

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.