The West African Monsoon and its socio-economical impacts

Aïda Diongue-NiangDirection de la Météorologie Nationale

Sénégal

First THORPEX Science SymposiumMontréal, 6-10 December 2004

The West African Monsoon Region

Strong meridional Land-Surface gradientsLand-ocean-atmosphere system

April

August

Rainfall Variability in West Africa

Strong interannual to decadal variability. Long period of drought conditions from 70’s, just at the independence from colonialism.Impacts Direct (water resources,

pasture, agriculture, power production…)Indirect (rural to urban immigration, health problem, Increase of food importation…)

Rainfall: intraseasonnal variabilityThe seasonal accumulatedrainfall is the integration ofsmall-scale rainfall events: MCS:90% in sahelianrainfall (Laurent et al 1998)

Dry years characterized by lower frenquency ofconvective events, theiraverage magnitude remaining unchanged(Lebarbé et al 2002)

Monsoon onset arrives in two peaks with abrupt shift from guinean to soudano-sahelian region (Sultan andJanicot 2000, lebarbé et al 2002)

Month of the year Month of the year

From Lebarbé et al 2002

Rainfall: intraseasonnal variability (2)

Seasonnal rainfallcharacterized by dry spells.Short dry spells more frequent in the core ofthe rainy season: important for planning desherbing, fertilizingLong dry spells more frequent at thebegining and the end, Real or false monsoononset 0

1

2

3

4

5

6

1 à <=5JRS 6 à <=10 jrs 11 à <=15 jrs sup a 15 jrs

Dry Spell Frequency (1950-2003) Diourbel, Sénégal

MaiJuinJuilAoutSeptOct

Agriculture application concerns 70% of people over the Sahel

Variability of cropproduction does notdepend only on variability ofaccumulated seasonalrainfallIt depends also on thedate of sowing and on the availablity of waterrequirements of the crop(see Sivakumar 1992, Diop, 2000)

Note that they are otherfactors which contributepositively or negatively to ensure a good watersupplyof the plant (e.g. soilcaracteristics, humanparameters, fertilizing)

Groundnut production and seasonal rainfall Diourbel, Sénégal

0

200

400

600

800

1000

1200

1960/61 1966/67 1972/73 1978/79 1984/85 1990/91 1996/97 2002/2003

Locust

Groundnut

Rain

Issues

Prediction of the monsoon onset in relation to the date of sowing Prediction of dry spells in relation to water requirements of thecrop. For a given stage of development, prediction thatsufficient rain will or not occur within 7, 10, 15 days to meetwater requirements of the crop.

Decision making Date of sowingAlternative solution as supplementary irrigation

The WAM System: land-ocean-atmosphere with many interacting scales

African easterly jet

Convective systems

Synoptic Easterly waves

Tropical easterly jet

Interactions between main atmospheric features overWest Africa

From Redelsperger, Diongue,…(2002) QJRMS

??

Vertical transports Reverse

shear

HeatingVortex

Vertical transports

AnvilsOutflows

Baroclonic &Barotropicconversions

Horiz & Vert. shear

convergence

Can we forecast accurately the WAM system?

Prediction of the WAM SystemModelling studyWith a NH high resolutionmodel, two-way gridnesting(Méso-NH), it has been possible:To reproduce the life cycle of a sahelian squall line consideringconvective scale and largerscaleGood agreement withobservations structure 2D and 3D

(Diongue et al, 2002)

Perspective on operational numericalprediction resolving explicitelythe convection

W > 1 m/s

15 UTC 15 UTC

H H

22 UTC 22 UTCZ (g/kg)

500 km

Hydrometeores (g/kg) Wind (m/s)

Analysis and Forecast the WAM system

NWP

JET 2000 campaign over West Africa

25-30 August 2000http://www.env.leeds.ac.uk/JET2000Thorncroft et al 2003 in BAMS Assimilation in real time of extra

data by ECMWF model and theUnified Model of the UK Met. Office

Improvement of the AEJ structure and intensity and theboundary layer humidty Even without the inclusion ofextra data the representation ofthe AEJ is quite good BUT the5-day and 10-day forecastexhibit strong departure fromanalysis and observation.

…Analysis and Forecast the WAM system

Data denial experimentwith ECMWF IFS:T511/L60, 4D-Var

On 28thAnalysis on 23th 5 day ForecastAnalysis on 23th Analysis with

no satellite dataAnalysis on 23th Analysis with

no synop dataAnalysis on 23th Analysis with

no upper-air dataAnalysis on 23th Analysis with

no wind information

Synop and TEMP observations on 28th

…Analysis and Forecast the WAM system

Control=no extra data 5 days forecast U (m/s)

No satellite No upper air data

S N10N

Radiosonde data dominates the model analysis despite their few numberRemoving one source of information : not sufficient to degrade the AEJ to the level of the 5 day forecast.(Tompkins, Diongue-Niang, Parker and Thorncroft, 2005)

Concluding remarks

The variability of the West African Monsoon system has a high social and economical impact.For many applications (e.g. agriculture), there is a highneed of subseasonal information (short and medium range).

Define and Implement relevant monitoring systems.Conduct more numerical studies to define predictionstrategies.To integrate reseach outcomes and operational products in decision making

Perspectives

Through AMMA (African Monsson Multidisciplinary Analysis)http://medias.obs-mip-mip.fr/amma

International scientific Programme for West Africa with SOP field campaign in 2006

African network for AMMA: AMMANET WP: Tools-Methods-Demonstration/NWP

Through THORPEX which interest can be summarized as « improvingpredictions of high impact weather on 1-day to 14-day timescale for benefit ofsociety and economy »

See Chris Thorncroft talk on Friday