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

SLP trend in CMIP3 multi model ensemble (A1B Scenario)

Motivation:What drives the Tropical SLP Changesin Global Warming?

Vecchi and Soden (2007)

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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

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SLP trend in Vecchi and Soden (2007)

What drives the Tropical SLP Changesin Global Warming?

SLP trend in Bayr and Dommenget (2013)

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Homogeneous and inhomogeneous surface warming

Total surface warming

Homogeneous warming Inhomogeneous warming

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Relative warming overOceans Land

Land-Sea Warming Contrast

Inhomogeneous surface warming

Bayr and Dommenget (2013)

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Relative Trends of Tropospheric Temperature

Bayr and Dommenget (2013)

surface

850 – 500 hPa

500 – 300 hPa

300 – 100 hPa

1000 – 850 hPa

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Bayr and Dommenget (2013)

Tropospheric Temperature and SLP Trends

pattern correlation = -0.90

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Mechanism:Bjerknes circulation and Hydrostatic

Bayr and Dommenget (2013)

𝑑𝑝𝑑𝑇 =0.5 𝜌𝑔 h𝑇  ≈−1.7

h𝑃𝑎𝐾

T1 = T2

p1 = p2

mass flow

warming cooling

a) b) c)

p1 = p2 p1 < p2

T1 > T2 T1 > T2

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Tropospheric Temperature and SLP Trends

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Bayr and Dommenget (2013)

Zonal Streamfunction in CMIP5 multi model ensemble

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Mean state in 20C

Zonal Streamfunction in CMIP5 multi model ensemble

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Difference 21C-20C (contours: mean state)

patterncorrelation = -0.48

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Weakening of the zonal circulation cells?

ECHAM5 AGM Experiments:Homogeneous vs Inhomogeneous warming

Total surface warming

Homogeneous warming Inhomogeneous warming

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ECHAM5 AGM Experiments:Homogeneous vs Inhomogeneous warming

Inhomogeneous

HomogeneousDiff in Total warming run

Homogenous + Inhomogeneous

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

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.