1. Long-Term Evolution of the Tropical Cold Point Tropopause Simulation Results and Attribution Analysis Thomas Reichler, U. of Utah, Salt Lake City, USA John Austin, UCAR-NOAA-GFDL, Princeton, USA

2. Motivation Tropical tropopause is closely related to climate change Examples: – Tropopause controls stratospheric water vapor – Tropopause is controlled by tropical upwelling, QBO, tropo- spheric and stratospheric temperature changes Exact cause-and-effect relationship between climate change and tropopause change is unclear Outline – Simulations with coupled chemistry climate model – Past and future evolution of tropical tropopause – Identify factors for change: 1.regression analysis 2.conceptual model

3. Methodology GFDL-AMTRAC: stratosphere resolving coupled chemistry climate model (2.5x2.5, L48) Simulation: 1960-2100, 3 members – PAST: 1960-1990, historical forcings (SST, GHG, ODS) – FUTURE: 1990-2100, IPCC-A1B and WMO (2003) forcings, SSTs from GFDL-CM2.1 Cold point tropopause – Interpolation after Reichler et al. (2003) – Lapse rate definition: 0 K/km – Zonal averages (22S-22N) and annual means

4. Tropical Tropopause Evolution Heights – PAST: Increase – FUTURE: Increase – 1960-2100: ca. 1 km or ca. 10% Temperatures – Climatologically important transition – PAST: Cooling – FUTURE: warming

5. Decadal Trends Tropics PAST FUTURE Global 1980-2004 OBS Height [m/dec.]7064 12364 Temperature [K/dec.]-0.130.25 -0.27-0.41

6. 1. Linear Regression Model Fit tropical tropopause parameters (temperature, pressure, height) to a linear regression model using the following four factors: AER Aerosols (60 hPa at equator) SSTTropical SSTs (22S-22N) O3Total ozone (globally averaged) UPWTropical mass upwelling (77 hPa), BDC These factors represent major processes known to influence the tropopause.

7. Regression Parameters Evolution

8. Regression Analysis: Heights Plots are decadally smoothed Contribution of each term to tropopause height 1. UPW - most important 2. SST - comes second

9. Plots are decadally smoothed 1.O3 - dominates PAST 2.SST - dominates FUTURE 3.UPW - probably strongly related to SST 4.AER - small impact Regression Analysis: Temperatures

10. 1.ΔT t > 0:height ↑ temperature ↑ 2.ΔT s < 0:height ↑ temperature ↓ Staten and Reichler (2008) show: Shepherd (2002): Constant lapse rates above (γ s ) and below (γ t ) tropopause Explain tropopause change by temperature change below (ΔT t ) and above (ΔT s ) 2. Conceptual Tropopause Model and

11. Testing the Simple Model PAST FUTURE Test impact of simulated temperature trends above ΔT s (0, LS) and below ΔT t (0, LT, UT) on tropopause itself γ s = -4 K/km, γ t = 6 K/km PAST LS (and UT) FUTURE (LS and) UT

12. Cause and Effect Analysis Change per century ΔZ trop ΔT trop ΔT UT ΔT LS O3UPWGHG PAST700-1.32.5-5↓↓↑↑↑ FUTURE6402.550↑↑↑↑ Tropopause increases mostly due to LS cooling (PAST) and UT warming (FUTURE) ΔZ trop Similar PAST: LS cooling dominates FUTURE: UT warming dominates ΔT trop GHG FUTURE ΔT UT O3, UPW, GHG PAST ΔT LS

13. Conceptual vs. Regression Model In the PAST, ozone depletion was most important for cooling and lifting the tropopause In the FUTURE, greenhouse gas increase will be most responsible for warming and lifting the tropopause ConceptualRegression PAST ΔT↓ O3 + UPW + GHG O3 ΔZ↑UPW FUTURE ΔT↑ GHG (SSTs) SST + O3 ΔZ↑UPW

14. Thank You More info: Austin and Reichler (2008, JGR)

15. Trend Analysis