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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:
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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.
African Monsoon Fontaine and Janico (1992 ) Existing conceptual models for the monsoons Indian Monsoon Karabanov, Shem, Li, and Collins after Webster and Fasullo (2002)
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
Seasonal Migration of Rainfall over West Africa (10˚W-10˚E, TRMM, mm day-1) Phase II Phase III Phase I
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
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
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)
Agadez Bamako Niamey Dakar Ouagadougou Douala Abidjan Bangui Libreville Wide Awake Field Luanda Seasonal cycles in precipitation, v and RH at Abidjan:
* * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * 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
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) 1981 - 2000, 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 (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.
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.
Agadez Bamako Niamey Dakar Ouagadougou Douala Abidjan Bangui Libreville Wide Awake Field Luanda
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 II Phase III Phase I
Phase II Phase III Phase I
Phase II Phase III Phase I
Cook and Vizy 2006
Vertical-meridional cross-section of wind vectors and potential temperature NCEP1 (1990-2000)
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