Past and future changes in Sahel rainfall: Possible mechanisms

1. Past and future changes in Sahel rainfall: Possible mechanisms Kerry H. Cook Department of Earth and Atmospheric Sciences Cornell University Ithaca NY

2. Present some of the dynamical processes that are responsible for variability in the Sahel on all time scales paleoclimate – the African Humid Period decadal (Samson Hagos) interannual intraseasonal

3. The African Humid Period with Christina Patricola

4. African Humid Period AHP Present Day Vegetation for (a) present day (b) and African Humid Period according to Hoelzmann et al. (1998) with grassland - 7, shrubland - 8, savanna - 10, evergreen broadleaf forest - 13, and desert -19.

6. Enhancement of the westerly low-level jet is a primary moisture source. Note that the southerly low-level southerly flow is unchanged.

7. The African easterly jet is not a part of the AHP climate

8. The Monsoon Jump with Samson Hagos

9. Coastal

10. “Sahel”

11. Smoothed rainfall in mm/day from TRMM (top) and FEWS (bottom) 2004 Coastal Sahel

12. Daily rainfall in mm/day from TRMM 2002, 2003, 2005, 2006

13. Precipitation difference: “Sahel” – “Coast”

14. Precipitation difference: “Sahel” – “Coast” monsoon onset 2002: July 14 2003: June 24 2004: June 16 2005: July 8 2006: July 10

15. The regional model captures the monsoon jump

17. X

18. Pre-monsoon onset A permanent sensible heating maximum exists from about 10N-12N: relatively low albedo => shortwave radiation maximum and net total radiative heating maximum This sensible heating drives a shallow meridional circulation (Zhang et al. 2006) low-level moisture convergence moisture transport into the middle layer (825 -525 hPa), divergence The radiative forcing increases through the spring and, near the middle of May, the gradually increasing moisture supply from the boundary layer begins condensing in the middle layer => condensation and precipitation increases in the continental interior

19. Monsoon Onset The condensational heating in the 825 - 525 hPa layer introduces a meridional pressure gradient in this layer which results in an inertial instability => coastal region becomes unfavorable for convergence => maximum precipitation abruptly shifts from the coast into the Sahel

20. Eastern Sahel: Another "Monsoon Jump" Two-Stage Monsoon Onset over Ethiopia with Emily Riddle

23. Low-level 910 mb winds Pre-onset Mar 1 – Mar 31 Transitional Apr 20 – May 15 Post-onset Jun 1 – Jun 30

24. The precipitation dipole response to SSTAs in the Gulf of Guinea with Edward Vizy

25. Surface Temperature Anomalies A prominent mode of interannual variability: ~ 25% of the years 1950 – 2000 are identified as dipole years (12 years) Extremely high correlation with warm SSTAs in the Gulf of Guinea during dipole years 1984 Precipitation Anomalies

26. A north/south cross-section along the Greenwich meridian Streamlines (v, wx10-2) and meridional velocity (m/s)

27. A north/south cross-section along the Greenwich meridian Vertically-confined monsoon inflow A north/south cross-section along the Greenwich meridian Streamlines (v, wx10-2) and meridional velocity (m/s)

28. 2nd selection criterion: Reasonable monsoon circulation Subsidence over the Gulf of Guinea Streamlines (v, wx10-2) and meridional velocity (m/s)

29. Southward mid-tropospheric flow (African easterly jet) Saharan high thermal low Streamlines (v, wx10-2) and meridional velocity (m/s)

30. Top: Climatological circulation From a regional climate model. Bottom: Circulation anomalies associated with warming in the Gulf of Guinea and the dipole precipitation mode. Anomalously high rainfall along the Guinean coast occurs in association with an increase in the moisture content of the monsoon inflow. Subsidence over the Gulf of Guinea suppresses the precipitation anomaly over the ocean.

31. With warm SSTAs in the Gulf of Guinea, the southward outflow from the Saharan high has a larger meridional extent, and is located closer to the surface. These differences in the outflow generate subsidence and drying over the Sahel due to shrinking of both planetary and relative vorticity.

32. Cold Air Surges andMonsoon Breaks with Edward (Ned) Vizy

33. What is a cold surge? • Mid-tropospheric ridge/trough pattern • Shallow dome of cold air with a sharp temperature gradient along it’s leading edge • Typically moves along topography, e.g., east of the Rockies and Andes Fig 2. from Garreaud (2001): Conceptual model of a cold surge moving from mid-latitudes

34. The climatology summer mid-tropospheric geopotential height field does have the ridge/trough pattern Topography (m) and June-August climatological 500 hPa geopotential heights (m) and winds (m/s) from the NCEP2 reanalysis

35. The climatological summer mid-tropospheric heightfield has the ridge/trough pattern eastern Mediterranean Saharan high Topography (m) and June-August climatological 500 hPa geopotential heights (m) and winds (m/s) from the NCEP2 reanalysis

36. A B C D E F • Local rate of change of temperature (negligible) • Mean diabatic heating and cooling term (calculated as a residual from the NCEP2) • Mean vertical advection of potential temperature term • Mean horizontal advection of temperature term (Zonal + Meridional components) • Vertical transient term • Horizontal transient term

37. A B C D E F 850 hPa JJA Thermodynamical Budget Analysis B C E+F D D D

38. Strong mid-tropospheric subsidence over the eastern Mediterranean Sea June-August Climatological Vertical-p velocity along 35N NW Africa E. Med Sea

39. Daily TRMM rainfall rates (mm/day) and 850 hPa wind convergence (contoured) for a JULY 2005 cold air surge event

40. Precipitation climatology in the current generation of climate models 1949 – 2000 JJAS

44. Coastal

45. “Sahel”

46. Daily rainfall in mm/day from TRMM (top) and FEWS (bottom) 2004 Coastal Sahel

47. Smoothed rainfall in mm/day from TRMM (top) and FEWS (bottom) 2004 Coastal Sahel

48. Daily rainfall in mm/day from TRMM 2002, 2003, 2005, 2006

49. Daily rainfall in mm/day from FEWS 2002, 2003, 2005, 2006

50. Precipitation difference: “Sahel” – “Coast” monsoon onset 2002: July 14 2003: June 24 2004: June 16 2005: July 8 2006: July 10