Widening of Tropical Belt in a Changing Climate: Model Simulations versus Observations

1. Widening of Tropical Belt in a Changing Climate: Model Simulations versus Observations Qiang Fu Dept. of Atmos. Sciences University of Washington

2. Enhanced midlatitude warming in satellite measurements Temperature trends 1979-2005 Fu, Johanson, Wallace and Reichler (2006, Science)

3. Changing meridional temperature gradients in the troposphere Temperature trends 1979-2005 • Meridional temperature gradients • reduced (equatorward), weaker westerlies • increased (poleward), stronger westerlies • Total widening of Hadley cell ~2° Fu, Johanson, Wallace and Reichler (2006, Science)

4. Enhanced midlatitude warming in satellite measurements Temperature trends 1979-2005 (JJA) Fu, Johanson, Wallace and Reichler (2006, Science)

5. Implications of Hadley cell widening Poleward movement of Hadley cell edges ~ poleward displacement of the subtropical dry zone boundaries Changes in rainfall patterns at subtropical dry zone margins May contribute to desertification of marginal lands

6. Hadley cell Meridional overturning circulation driven by differential heating of atmosphere Transports energy and momentum poleward Rising, moist air near Equator Large scale sinking of dry, warm air ~30NS

7. Locating Hadley cell edges Stream function Evaporation – Precipitation (E-P) Hadley cells OLR contours Upper tropospheric jets

8. Locating Hadley cell edges Days/year with height > 15 km Total ozone (e.g., TOMS total ozone, 3/11/1990 Upper tropospheric jets

9. Observed Hadley cell widening Consensus: Hadley cell has widened by ~3° since 1979 Seidel, Fu, Randel, and Reichler (2008, Nature Geoscience) Also see Hudson et al. (2006), Hu and Fu (2007), Seidel and Randel (2007), Archer and Caldeira (2008)

10. Mechanisms for Hadley cell widening Global warming: wider Hadley cell in warmer world Held (2000): Aquaplanet GCMs (Frierson et al. 2007) Simulations of 21st century climate (Lu et al. 2007) Warming of the Indo-West Pacific ocean (Lau et al. 2008) Strengthening of polar stratospheric vortex(Polvani and Kushner 2002) Tropospheric warming/stratospheric cooling Altered wind shear across tropopause, increasing phase speed of baroclinic eddies(Chen and Held 2007)

11. Objective: Comparison of tropical widening between model simulations and observations Tools

12. Description of models used to investigate Hadley cell widening CMIP3 Coupled Model Intercomparison Project AMIP2 Atmospheric Model Intercomparison Project Forcings included in 20C simulations

13. Description of observations used to investigate Hadley cell widening Latitude where stream function = 0 at 500 hPa from reanalysis NCEP/NCAR NCEP/DOE ERA-40 Latitude where OLR = 250 W m-2 from satellite HIRS ISCCP GEWEX Hu and Fu (2007)

14. Results: time series of Hadley cell width Interannual timescales: variability similar in observations and individual models Hadley cell width 1979-2005 Observations: increase 20th century simulations: no change Stream function OLR Johanson and Fu (2008, submitted) • Hadley cell width 2000-2099 • A1B: increase • By 2099, Hadley cell much wider than pre-industrial control

15. Observed widening not explained by natural variability Mean widening Multi-dataset ensemble mean trends from observations Ensemble mean trends from samples of 25-yr time periods in PIC Range of widening Trend distribution from observational datasets Trend distribution from samples of 25-yr time periods in PIC Observed 3° on average Range 2.3-4.5° Pre-industrial control (PIC) No tenancy to shift Hadley cell towards wider or narrower state Widening occurs as often as narrowing Hadley cell width variation within ±1.5° Mean Hadley cell widening Distributions of Hadley cell widening

16. 20th century simulations do not reproduce observations Mean widening Multi-dataset ensemble mean trends from observations and 20C-A1B from 1979-2005 Range of widening Trend distributions from observational datasets Trend distributions from 20C-A1B ensemble members Widening in 20C-A1B from 1979-2005 Less than 1/5th observed values At least 97.5% of the 20C-A1B trends from ensemble members show less widening than observed Mean Hadley cell widening Distributions of Hadley cell widening

17. Observed widening not explained by pattern of SST warming Mean widening Multi-dataset ensemble mean trends from observations and AMIP2 from 1979-1999 Range of widening Trend distributions from observational datasets Trend distributions from AMIP2 ensemble members from 1979-model end year Widening in AMIP2 less than 1/4 observed values Only 2 of 57 realizations had widening larger than 3º Next largest trend less than 2º Mean Hadley cell widening Distributions of Hadley cell widening

18. Observed widening not explained by global warming 1. Hu and Fu (2007) 2. Johanson and Fu (2008) 3. Lu et al. (2007) 4. Frierson et al. (2007) Rate of widening per degree of global warming is ~10 times larger in observations than models Global warming alone can't explain recent widening as simulated by GCMs

19. Observed widening not explained by stratospheric ozone depletion Strengthening of polar stratospheric jet → wider Hadley cell(Polvani and Kushner 2002) Strengthening of S. Hem jet in recent decades largely attributable to ozone depletion(Thompson and Solomon 2002, Science; Gillett and Thompson 2003, Science) No consensus among model simulations of 20th century

20. GCM results with interactive chemistry and good representation of stratospheric dynamics Lamarque & Solomon (2008)

21. Conclusions Observed widening of about 3º since 1979 Not due to natural variability Cannot be explained by historical changes in forcings as simulated by GCMs Cannot be explained by pattern of SST warming Occurs at a faster rate than in projections of future widening Further investigation required to resolve discrepancy between observations and GCM simulations