20 th century trends in the tropical atmospheric circulation

1. 20th century trends in the tropical atmospheric circulation Amy Clement Rosenstiel School of Marine and Atmospheric Sciences, University of Miami Meeting of the Working Group on Space-Based Lidar Winds Miami, Florida February 6 – 9, 2007

2. Weakening of the tropical atmospheric overturning circulation in a warming world (Held and Soden 2006; Vecchi and Soden 2007) Precipitation increases at a smaller rate implying a weaker circulation assuming: P=Mq Water vapor increases with surface temperature at a rate predicted by C-C in climate models IPCC AR4 models under the A1b scenario. Differences are 2080-2100 minus 2020-2000

3. Is there evidence for a weakening circulation over the 20th century ? • Walker circulation • Hadley Circulation

4. 20th century changes in the Walker circulation: An alternative hypothesis Ocean thermostat mechanism (Clement et al. 1996, Cane et al. 1997): In response to a surface heating, the eastern tropical Pacific warms up by less than the west because of cooling by upwelling. This east-west asymmetry is amplified by the Bjerknes coupled feedback leading to a stronger Walker circulation.

5. Evidence?

6. Evidence? Linear trend in SLP from 1861-1992 (Vecchi et al. 2006) Zhang and Walker (2006)

7. 20th century changes in the Hadley cell GFDL AM2

8. 20th century changes in the Hadley cell GFDL

9. Chen et al. (2002): Increased incidence in high OLR/low SW Increasing subsidence from 1985-2000

10. Linear trend HIRS OLR (W/m2/decade) 1987-2004 Linear trend SSMI water vapor (mm/decade) 1987-2004 Increased subtropical subsidence?

11. Conclusions • Models suggest that in a warming world, the tropical overturning circulation should weaken because precipitation increases more slowly than water vapor • Walker circulation over the 20th century • While atmospheric mechanism predicts a weakening circulation, ocean thermostat mechanism predicts a strengthened Walker circulation in a warmer world • SST data are inconclusive • SLP data suggest a weakening Walker circulation • Hadley circulation over the 20th century • NCEP1 and ERA40 reanalyses show a significant increase in the Hadley cell strength (but not NCEP2) • This trend does not appear in models • Independent observations (radiative fluxes and water vapor) suggest increased subsidence in parts of the subtropics, but period of record is too short to distinguish internal decadal variability from long-term trends