1. Tobias Bayr and Dietmar Dommenget The Inhomogeneous Tropospheric Warming as the Driver of Tropical Sea Level Pressure and Walker Circulation Changes Helmholtz Centre for Ocean Research Kiel, Germany

2. SLP trend in CMIP3 multi model ensemble (A1B Scenario) Motivation: What drives the Tropical SLP Changes in Global Warming? Vecchi and Soden (2007) 2

3. Weakening of the tropical circulations Held and Soden (2006), Vecchi and Soden (2007): ● specific humidity increases 7 % per K warming following the Clausius–Clapeyron equation, relative humidity stays constant ● precipitation increases only by 2 % per K warming; limiting factor: radiativ cooling ● vertical transport decreases about roughly 5 % per K warming  weakening of the tropical circulations in global warming w1 > w2 Comparable with a widening of a water pipe in a circulation 3

4. SLP trend in Vecchi and Soden (2007) What drives the Tropical SLP Changes in Global Warming? SLP trend in Bayr and Dommenget (2013) 4

5. Homogeneous and inhomogeneous surface warming Total surface warming Homogeneous warmingInhomogeneous warming 5

6. Relative warming over Oceans Land Land-Sea Warming Contrast Inhomogeneous surface warming Bayr and Dommenget (2013) 6

7. Relative Trends of Tropospheric Temperature Bayr and Dommenget (2013) surface 850 – 500 hPa 500 – 300 hPa 300 – 100 hPa 1000 – 850 hPa 7

8. Bayr and Dommenget (2013) Tropospheric Temperature and SLP Trends pattern correlation = -0.90 8

9. Mechanism: Bjerknes circulation and Hydrostatic Bayr and Dommenget (2013) T 1 = T 2 p 1 = p 2 mass flow warmingcooling a)b)c) p 1 = p 2 p 1 < p 2 T 1 > T 2 9

10. Tropospheric Temperature and SLP Trends 10 Bayr and Dommenget (2013)

11. Zonal Streamfunction in CMIP5 multi model ensemble 11 Mean state in 20C

12. Zonal Streamfunction in CMIP5 multi model ensemble 12 Difference 21C-20C (contours: mean state) patterncorrelation = -0.48

13. 13 Weakening of the zonal circulation cells?

14. ECHAM5 AGM Experiments: Homogeneous vs Inhomogeneous warming Total surface warming Homogeneous warmingInhomogeneous warming 14

15. ECHAM5 AGM Experiments: Homogeneous vs Inhomogeneous warming Inhomogeneous Homogeneous Diff in Total warming run Homogenous + Inhomogeneous

16. Conclusion

17. Thank you for your attention! Reference: Bayr, T., and D. Dommenget (2013), The Tropospheric Land-Sea Warming Contrast as the Driver of Tropical Sea Level Pressure Changes, Journal of Climate, 26, (4), pp. 1387-1402.