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Helmholtz Centre for Ocean Research Kiel, Germany

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. Motivation: What drives the Tropical SLP Changes in Global Warming?.

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Helmholtz Centre for Ocean Research Kiel, Germany

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  1. Tobias Bayrand Dietmar Dommenget The Inhomogeneous Tropospheric Warming as the Driver of Tropical Sea Level Pressure and Walker Circulation Changes Helmholtz CentreforOceanResearch Kiel, Germany

  2. Motivation: What drives the Tropical SLP Changesin Global Warming? SLP trend in CMIP3 multi model ensemble (A1B Scenario) 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 Comparable with a widening of a water pipe in a circulation w1 > w2 3

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

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

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

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

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

  9. Mechanism: Bjerknes circulation and Hydrostatic a) b) c) mass flow T1 > T2 T1 = T2 T1 > T2 p1 = p2 p1 = p2 p1<p2 warming cooling Bayr and Dommenget (2013) 9

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

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

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

  13. Weakening of the zonal circulation cells? 13

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

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

  16. Conclusion • SLP response in-homogenous Ttropos(direct Bjerknes Circulation) • In-homogenous Ttropos warming is dominated by the land-sea warming contrast • CMIP multi model ensemble shows a weakening of the Zonal Circulation Cells, with a big spread of individual models • In-homogeneous warming strengthens the Zonal Circulation Cells • Homogeneous and In-homogeneous warming have contrary effects on the Zonal Circulation Cells and can be an explanation of the spread of responses in the individual climate models

  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.

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