Workshop on Sahel Climate ChangeColumbia University, March 19-21, 2007

The Dynamics of Monsoon: Lessons from the Annual Cycle

Chidong Zhang

RSMAS, University of MiamiMain 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 monsoonsIndian Monsoon

Outline:

New conceptual model for the West African monsoonIn situ sounding dataGlobal reanalysis dataNew onceptual model for the West African monsoon: Implications

Phase IPhase IIPhase IIISeasonal Migration of Rainfall over West Africa(10W-10E, TRMM, mm day-1)

Existing model

In situ sounding observations: spares, susceptible to instrumental and human errors free of model biases

Table 1 List of data available to this study.

Site

Latitude

Longitude

Period (year)

Agadez, Niger

1658N

759E

1995 2003

Dakar, Senegal

1440N

1728W

1973 2003

Niamey, Niger

1332N

205E

1973 2003

Bamako, Mali

1239N

8W

1976 2003

Ouagadougou, Burkina Faso

1220N

140W

1996 2003

Abidjan, Ivory Coast

519N

42W

1973 2000

Bangui, Central African Republic

422N

1835E

1994 2001

Douala, Cameroon

43N

942E

1977 2003#

Libreville, Gabon

023N

927E

2000 2003

Wide Awake Field,

Ascension Island

758S

1424W

1973 2001

Luanda, Angola*

848S

1314E

1973 1978

* In some months available data are less than 15.

# Except for 1991

Seasonal cycles in precipitation, v and RH at individual sites(Zhang et al. 2006)

Seasonal cycles in precipitation, v and RH at Abidjan:

Fountaine and Janicot (1992)

v, RHu, qeSeasonal cycles in v and RH at 700 hPa and precipitation:

Vertical structure of v in the three phases of the seasonal cycle:Phase IPhase IIPhase 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 (10E - 15W) 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 - 30W) 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.

Figure 15 Seasonal cycle (repeating once) in meridional wind at the equator averaged over West Africa (10E - 15W) 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).

Cook and Vizy2006

Vertical-meridional cross-section of wind vectors and potential temperatureNCEP1 (1990-2000)

Conjecture:

The SHL and its associated shallow meridional circulation plays a key role in the seasonal cycle of the rainband over West African. 100 hPaPhase IIIPhase I700 hPaSHL500 hPaPhase IISHLNew modelSHL100 hPa