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.

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

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

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

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

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

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

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

Fountaine and Janicot (1992)

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

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

Global reanalyses: ERA40, NCEP/NCAR (NCEP1) NCEP/DOE (NCEP2) , except when compared to soundings

Figure 11 Vertical-meridional cross-section of mean wind vectors averaged over West Africa (10˚E - 15˚W) during April ( ) 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.

Figure 14 Vertical-meridional cross- section of wind vectors averaged over the central Atlantic ( ˚W) for October ( ) 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.

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

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).

Phase I Phase II Phase III

Phase I Phase II Phase III

Phase I Phase II Phase III

Cook and Vizy 2006

Vertical-meridional cross-section of wind vectors and potential temperature NCEP1 ( )

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 Phase II Phase III 100 hPa Phase III Phase I 700 hPa SHL 500 hPa Phase II SHL New model SHL 100 hPa