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Haley Hansen Atmospheric Science

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Haley Hansen Atmospheric Science

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  1. Enhancement of Interdecadal Climate Variability in the Sahel by Vegetation InteractionNingZeng, J. David Neelin, K.-M. Lau, Compton J. Tucker, 1999Drought in the SahelNingZeng, 2003 Haley Hansen Atmospheric Science

  2. Stuff you should already know: • Evapotranspiration has 4 Components: • E = Eb + Ei +Es + TR • Evaporation from bare soil (Eb) • Evaporation from inception storage (Ei) • Snow sublimation (Es) • Transpiration (TR) Albedo: High albedo = More reflective; deserts, etc Low albedo = Less reflective; trees, etc

  3. Where and What is the Sahel? Desert Sahel Region Semi-arid Tropical

  4. Why is it so special? Very climatically sensitive! The Sahel has had many droughts Drying trend from 1950s – 1980s Drought in early 1990s and 2000s Why? Variations in sea surface temperature Land use change and vegetation-climate interaction enhance the changes caused by SST variations

  5. How Sea Surface Temperature Plays a Role Studies show Sahel rainfall linked to tropical SSTs in the Pacific, Atlantic and Indian oceans

  6. What About Vegetation? • Coupled atmosphere-land-vegetation model • Atmospheric component = Quasi-equilibrium Tropical Circulation Model (QTCM). • Simulates seasonal climate over the Sahel • Land surface component = Simple-Land (Sland) • Effects of varying vegetation through evapotranspiration and surface albedo Surface roughness and soil modification are not considered

  7. Equations!!! Plant leaf area index (LAI) Change in Vegetation Carbon assimilation coefficient Soil wetness Time scale (1 year) Extinction coefficient of photosynthetically active sunlight Vegetation amount (leaf biomass) Soil moisture dependence The Biomass Equation Does not include plant competition or species specific characteristics (such as resource allocation)

  8. More equations!!! These variables represent the same things they did in the first equation. Note the close relationship between photosynthesis and evapotranspiration. Canopy conductance for evapotranspiration Leaf-level maximum conductance Surface albedo of 0.38 for a desert, 0.08 for a forest Surface albedo This shows that vegetation feeds back into the atmosphere by modifying evapotranspiration and surface albedo

  9. Why are These Equations Useful? 3 model runs were used: The coupled land-vegetation model is driven by monthly SST from 1950 - 1998

  10. The Results Simulated soil moisture Increases inderdecdal drying trend 9-year mean showing the low-frequency variation LAI anomaly Drying trend, but weak interannual variation and much weaker interdecal signal Enhances rainfall variability, most like observed conditions

  11. Feedback Processes • Vegetation modifies precipitation through a chain of feedback loops Current understanding of vegetation dynamics and available modeling tools cannot accurately assess these relations Important to note: Vegetation lags behind rainfall

  12. Ensemble model runs • Nine additional model runs with different initial atmospheric and soil moisture conditions • This was used to show how vegetation lags behind rainfall • Amplitude determined by wet period from 1950-67 and dry period from 1968-67 • All the models underestimate observed trend • Desertification may be the cause of this Increase in variance when interactive vegetation is added

  13. But wait! There’s more equations! Approximates vegetation feedback to rainfall through changes in surface albedo and evapotranspiration Linearized biomass equation Rainfall sinusoidally forced at frequency w and amplitude F, representing large-scale SST-induced change in atmospheric circulation All the equations! Max phase lag Little phase lag, precipitation enhanced

  14. Why does this matter?! 1987 = really dry 1988 = fairly wet 1987 = really dry 1988 = still dry Lack of vegetation is increasing the albedo and decreasing evapotranspiration

  15. Why should we care? • Atmospheric dust • Reflects sunlight and changes cloud properties • This could alter the global carbon cycle and climate • African dust sources count for about half the global total • Drought may have increased global dust total by one-third

  16. Summary • The Sahel region has a unique location and delicate climate • Drought in the Sahel is initially caused by SST variations • The drought is amplified by vegetation feedback • Changes in vegetation are caused by both natural and anthropogenic causes More work needs to be done to determine if the drought is caused by natural or anthropogenic sources. Also, more studies should be done to determine whether or not humans are responsible for sea surface variations, and what kind of impact the Sahel has on climates elsewhere.

  17. Questions? Comments? Concerns?