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Atmospheric Circulation Changes in a Warming Climate

Atmospheric Circulation Changes in a Warming Climate. Brian Soden Rosenstiel School for Marine and Atmospheric Science University of Miami. Isaac Held, Gabriel Vecchi Geophysical Fluid Dynamics Laboratory National Oceanic and Atmospheric Administration Princeton, NJ. Objective.

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Atmospheric Circulation Changes in a Warming Climate

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  1. Atmospheric Circulation Changes in a Warming Climate Brian Soden Rosenstiel School for Marine and Atmospheric Science University of Miami Isaac Held, Gabriel Vecchi Geophysical Fluid Dynamics Laboratory National Oceanic and Atmospheric Administration Princeton, NJ

  2. Objective 1) Identify those aspects of climate change which are: • Consistent across a large number of models • Are supported by simple physical arguments 2) Motivate observational studies to determine if these responses are currently detectable.

  3. IPCC AR4 Model Archive Held and Soden 2006

  4. Change in Convective Mass Flux DP/P – 0.07 DT Held and Soden 2006

  5. Spatial Structure of Weakened Circulation(multi-model ensemble mean) Background 500 Changes in vertical velocity oppose mean state (except Central Pacific) Weakening occurs primarily as a reduction in the Walker Cell, not Hadley Cell. Change in 500 • Other El Nino “like” patterns: • Eastward shift of precipitation • Reduction in SST gradient • Reduction in thermocline tilt Vecchi and Soden 2006

  6. GFDL CM2.1 Change in Daily Upward 500 Strongest updrafts become less common Weakest updrafts become more common Vecchi and Soden 2006

  7. Pacific Zonal SLP Gradients Decrease Full ocean GCMs Slab ocean GCMs Vecchi and Soden 2006

  8. Historical Trends in SLP: 1861-2000 • Observational record shows similar pattern of SLP change. • Only reproduced when anthropogenic forcings are added. Vecchi et al. 2006

  9. Model Projected Changes in Vertical Wind Shear Increased wind shear over much of Tropical Atlantic. Vecchi and Soden 2007

  10. Conclusions • The weakening of the tropical circulation is a robust projection of all climate models. • The slower circulation is manifest primarily as a weakened Walker Cell. • The weakening is associated with a shift of the mean climate to more “El Nino” like conditions in both atmosphere and ocean. • But it is not physically related to El Nino. • May have implications beyond tropical Pacific. • E.g., Tropical Atlantic wind shear

  11. EXTRA SLIDES

  12. Weakening of Surface Easterlies Vecchi and Soden 2006

  13. DP/P – 0.07DT Change in Convective Mass Flux P = Mc q DMc/Mc = DP/P – Dq/q Fractional Change in Convective Mass Flux Held and Soden 2006

  14. Zonal vs Meridional Decomposition of Weakened Circulation Weakening is manifest primarily as a reduction of the Walker cell, not Hadley cell Vecchi and Soden 2006

  15. Ocean Response Ensemble-mean response of tropical thermocline. Equatorial Pacific thermocline flattens and shoals. Ensemble-mean response of equatorial subsurface temperatures. Increased thermal stratification.

  16. Historical Trends in SST: 1861-2006 Structure of long-term changes in SST depend on dataset used. “La Niña-like” in Kaplan. “El Niño-like” in Reynolds. This is a problem that needs to be resolved. Vecchi and Soden 2006

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