Workshop on Sahel Climate Change Columbia University, March 19-21, 2007

1. Workshop on Sahel Climate Change Columbia University, March 19-21, 2007 The Dynamics of Monsoon: Lessons from the Annual Cycle Chidong Zhang RSMAS, University of Miami • Main points of this talk: • The fundamental circulation features of the West African monsoon are not unique to the monsoon season, but persist through the seasonal cycle in the region. • A new conceptual model for the West African monsoon with an emphasis on the lower-tropospheric circulation.

2. African Monsoon Fontaine and Janico (1992 ) Existing conceptual models for the monsoons Indian Monsoon Karabanov, Shem, Li, and Collins after Webster and Fasullo (2002)

3. Outline: • New conceptual model for the West African monsoon • In situ sounding data • Global reanalysis data • New onceptual model for the West African monsoon: Implications

4. Seasonal Migration of Rainfall over West Africa (10˚W-10˚E, TRMM, mm day-1) Phase II Phase III Phase I

5. New model Phase I 100 hPa 700 hPa SHL 100 hPa Phase II 500 hPa SHL Phase II Phase III Phase III 100 hPa SHL Phase I Existing model

6. Agadez Bamako Niamey Dakar Ouagadougou Douala Abidjan Bangui Libreville Wide Awake Field Luanda In situ sounding observations: – spares, susceptible to instrumental and human errors – free of model biases

7. Agadez Bamako Niamey Dakar Ouagadougou Douala Abidjan Bangui Libreville Wide Awake Field Luanda Seasonal cycles in precipitation, v and RH at individual sites (Zhang et al. 2006)

8. Agadez Bamako Niamey Dakar Ouagadougou Douala Abidjan Bangui Libreville Wide Awake Field Luanda Seasonal cycles in precipitation, v and RH at Abidjan:

9. Fountaine and Janicot (1992)

10. * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * Agadez Bamako Niamey Dakar Ouagadougou Douala Abidjan Bangui Libreville Wide Awake Field Luanda Seasonal cycles in v and RH at 700 hPa and precipitation: u, qe v, RH

11. Vertical structure of v in the three phases of the seasonal cycle: Phase I Phase II Phase III

12. Global reanalyses: ERA40, NCEP/NCAR (NCEP1) NCEP/DOE (NCEP2) 1981 - 2000, except when compared to soundings

13. Figure 11 Vertical-meridional cross-section of mean wind vectors averaged over West Africa (10˚E - 15˚W) during April (1991 - 2000) from reanalyses of (a) ERA40, (b) NCEP1, and (c) NCEP2. The vertical velocity is amplified 20 times for better illustration. Maximum vector amplitude is 6.4 m s-1.

14. Figure 14 Vertical-meridional cross-section of wind vectors averaged over the central Atlantic (15 - 30˚W) for October (1991 - 2000) from reanalyses of (a) ERA40, (b) NCEP1, and (c) NCEP2. The vertical velocity is amplified 20 times for better visual effects. Maximum vector amplitude is 11.5 m s-1.

15. Agadez Bamako Niamey Dakar Ouagadougou Douala Abidjan Bangui Libreville Wide Awake Field Luanda

17. Figure 15 Seasonal cycle (repeating once) in meridional wind at the equator averaged over West Africa (10˚E - 15˚W) from reanalyses of (a) ERA40, (b) NCEP1, and (c) NCEP2. Contour interval is 1 m s-1. Thick lines are for zeros and dashed for negatives (northerlies).

19. Phase II Phase III Phase I

20. Phase II Phase III Phase I

21. Phase II Phase III Phase I

22. Cook and Vizy 2006

23. Vertical-meridional cross-section of wind vectors and potential temperature NCEP1 (1990-2000)

25. Phase II Phase III Phase I New model Conjecture: The SHL and its associated shallow meridional circulation plays a key role in the seasonal cycle of the rainband over West African. Phase I 100 hPa 700 hPa SHL Phase II 500 hPa SHL Phase III 100 hPa SHL