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Chapter A.5The Sahelian Climate

Yongkang Xue · Ronald W. A. Hutjes · Richard J. Harding · Martin Claussen · Steven D. PrinceThierry Lebel · Eric F. Lambin · Simon J. Allen · Paul A. Dirmeyer · Taikan Oki

A.5.1 Introduction

The Sahel has experienced several drought periods inthe past 500 years, however no available records show adrought as persistent and severe as the one that startedin the 1960s (Nicholson 1978). Many thousands of peo-ple died and many more suffered severe disruption oftheir lives in the severe phases of this drought (e.g. in1984). The human dramas and socio-economic conse-quences resulting from drought-induced famines in theSahel region have presented a strong motivation for re-search into the causes of the drought. Ever since, thecauses for this prolonged drought have been soughtsomewhere between two extreme views. One view con-siders land-surface degradation resulting from popula-tion pressure in excess of the region’s carrying capacityas the main driver. This implies the existence of positiveland-surface/atmosphere feedbacks mostly internal tothe region, and in principle could incite the developmentof mitigation strategies to reverse the trend. The otherview attributes the drought to unfavourable anomalouspatterns in sea-surface temperature (SST) in the oceans.This implies the existence of a driver external to the re-gion and by nature beyond human control, and requiresthe development of adaptation strategies to make theregion’s societies less vulnerable to droughts.

To position ourselves in this scientific debate, sev-eral important questions must be answered. We focuson the questions related to the first view, as it is still themore controversial of the two.

1. Can land-surface dynamics in principle lead to adrought of the magnitude witnessed over the lastdecades?

2. What is the role of soil moisture dynamics? Does soildegradation alter these dynamics?

3. What is the role of internal vegetation dynamics? Isvegetation a passive victim of the drought or an ac-tive modulator?

If the answer to the first question is positive, and withknowledge of issues raised in the other two, we can go astep further:

4. Are observed land-surface anomalies, if any, suffi-ciently strong to induce climate anomalies of the ob-served magnitude?

5. If so, do anthropogenic activities indeed form a signifi-cant driver in the observed land-surface dynamics?

Only with sufficient knowledge of all these issues maywe be able to assess the relative importance of land sur-face versus SST forcing as the cause of droughts in theregion, and may we hope to answer questions such as:

6. Is the current drought a temporary variation or apersistent and perhaps irreversible trend?

We will address aspects of all these questions in thenext sections. First we will characterise the Sahelian re-gion in ecological and climatological terms on one hand,and socio-political terms on the other. Then we will re-view the scientific efforts that have built the current un-derstanding of land-surface/hydrology/climate interac-tions in the region, which includes discussion of the roleof past and present field experiments in the region, andthe results and evidence from modelling studies. Finally,after analysing the mechanisms, we will come back tothe questions formulated above and assess where westand.

A.5.1.1 Background

The Sahel is a tropical, semi-arid region (approximately1.5 × 106 km2) along the southern margin of the Saharadesert formed from large parts of six African countriesand smaller parts of three more. The word “Sahel” wasderived from the Arabic for a “shore”, and this was pre-sumably how the vegetation cover seemed to early trad-ers who entered the region from the Sahara desert tothe south. Today it is a bioclimatic zone of predomi-nantly annual grasses with shrubs and trees, receiving amean annual rainfall of between 150 and 600 mm y–1.There is a steep gradient in climate, soils, vegetation,fauna, land use and human utilisation, from the almostlifeless Sahara desert in the north to savannas to thesouth. The similarity of climate and land cover in the

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60 CHAPTER A.5 · The Sahelian Climate

E-W direction contrasts dramatically with the strongN-S gradient. The uniformity of this geographical pat-tern is partly a result of an absence of rapid changes intopography but also because of the zonal nature of theclimate with desert to the north and ocean or humidforests to the south. The geographical patterns of rain-fall, vegetation cover, soils, human settlement and landuse all share this zonal arrangement and are stronglycorrelated, so that cause and effect are hard to disentan-gle. The continental scale land mass and relative flatorography (excluding eastern Sahel) warrant that land-surface/atmosphere interactions play a major role in theregional climate and also allow such interactions to bedetected relatively easily in model simulations. In fact,tropical northern Africa is the first region where conti-nental scale land-surface/atmospheric interaction stud-ies were conducted (Charney 1975).

The monsoon climate system increases further thepotential for interaction between the lower atmosphereand the land surface. The Sahel summer climate is domi-nated by the West African monsoon system. The basicdriver of the monsoon circulation is provided by ther-mal contrast between the continent and adjacent oce-anic regions (Holton 1992). Any changes in the contrastmay affect substantially the monsoon flow. Furthermore,the region is located in the tropics. In the tropics, thedynamical flow instabilities are relatively weak (com-pared to mid-latitudes). The boundary conditions modi-fy the diabatic heating (latent and radiative heating)which in turn changes circulation and rainfall at sea-sonal and interannual time scales.

Soils in this region are dominated by a sand sheet ofvarying depth, usually resulting in unstructured, free-draining soils with low nutrient content and with a strongtendency to form an impervious “cap”. This low soil nu-trient content is sometimes considered a more seriousconstraint on rangeland quality and production than lowrainfall (Breman and de Wit 1983). As a result of the crustformation, much rainfall runs off rather than infiltratingand is subsequently concentrated in local depressions.

These pools provide surface water for livestock allow-ing traditionally nomadic herdsmen to take their cattle,sheep, goats, camels and donkeys away from permanentwater points such as boreholes, wells and rivers during

the rainy season. As the people of the Sahel are depend-ent almost entirely on subsistence livestock productionand agriculture they are therefore particularly vulner-able to the frequent droughts that occur in the region.

A.5.1.2 Climate Anomalies and Climate Changein the Sahel

Rainfall is the controlling factor in the life of the Sahel.A brief rainy season occurs, caused by the northwardmovement of the Intertropical Convergence Zone (ITCZ)in the northern summer, which causes humid air fromthe Gulf of Guinea to undercut the dry north-easterlyair. West-moving squall lines and local convective ac-tivity cause mixing of the two air masses and results inshort, torrential thunderstorms that increase in fre-quency and rainfall amount towards the south wherethe humid air mass is deeper. The rainy season varies inlength from two to four months, but “rain days” can beseparated by weeks of no rain.

Rainfall in the Sahel is characterised by high spatialand temporal variability, both within and between sea-sons, and by a high north-south gradient in the region.The climatological annual rainfall gradient over theSahel is about 1 mm km–1 (Lebel et al. 1992). Not only isyear to year variability high but also longer dryer orwetter periods may continue over a number of years.

Although the Sahel is usually defined with referenceto a range of annual average rainfall, it is not always theamount of rainfall that is the most important character-istic, rather the spatial and temporal variation in rain-fall, especially interannual variations, which determinesmuch of the type and pattern of vegetation, fauna andhuman utilisation. Using satellite remote sensing theSahel can be delimited by the zone of high interannualvariations in vegetation activity. These characteristicshave been discussed by a number of studies (e.g. Lamb1978a,b; Nicholson and Palao 1993; Lebel et al. 1997).

Since the late 1960s, a persistent summer drought haslasted for more than 30 years. Although in the 1990s therainfall was not as scarce as the 1980s, it was still belowthe climatological average (Fig. A.35). One of the impor-tant features of sub-Sahara drought is the displacement

Fig. A.35.July-August-September rain-fall anomalies for the Sahelregion (10–20° N, 15° W to 40° E)

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of isohyets in the 1980s, retreating southward in thenorthern Sahel during dry years and also shifting slightlyto the south in the southern Sahel. This redistributionreduces the rainfall in the Sahel and increases rainfall tothe south of 10° N. Figure A.36b shows the zonal averagerainfall over the African continent from Nicholson’s ob-servational data. It indicates that the maximum rainfallwas not reduced very much and it was the shifting alonethat gave rise to the rainfall deficiency in the Sahel.

At small scales the data, even though relatively scarceby standards of temperate countries, do allow for detailedstatistical analysis of spatial and temporal dynamics incertain areas. Le Barbé and Lebel (1997) have shown thatthe drought in Niger is associated mostly with a decreaseof the number of rain events at the height of the rainyseason (July/August). The mean event rainfall and thelength of the rainy season hardly contributed to the de-crease in annual rainfall. The rainy season duration andtiming between dry (1970–1990) and wet (1950–1970) pe-riods are similar.

A number of studies has also shown that many riversin the region exhibit substantial reduction in dischargebetween the 1950s through the 1980s (for example, Savenije1995; Oki and Xue 1998) as a consequence of the reducedrainfall. Annual discharge declined to less than one-thirdat some stations during the 1980s. The discharge of theRiver Niger at Niamey was reduced from an annual aver-age of 1 060 m3 s–1 over the period 1929–1968 to 700 m3 s–1

over the period 1969–1994, e.g. a decrease of 34%. At thesame time the average discharge during the month oflow water decreased from 64 m3 s–1 to 11 m3 s–1. Figure A.37shows the river runoffs during 1950–1970 and 1971–1990

from the Koulikoro station based on the data from theGlobal Runoff Data Centre. This station, with a basin sizeof 12 104 km2, is located in the upstream region of theRiver Niger, and represents the mean runoff from thesouth-western Sahel region. The figure shows that theriver runoff is substantially reduced during 1971–1989,and this reduction occurs mainly during the summermonsoon season.

In addition, several investigations have found that thetemperature has increased in the Sahel during dry years(Tanaka et al. 1975; Schupelius 1976; Kidson 1977). Tanakaet al. (1975) reported a surface air temperature increaseof about 2 K. The observed temperature data, providedby the Oak Ridge National Laboratory (Vose et al. 1992),has also shown that the summer surface temperatureover the Sahel region is greater by 1–2 °C during the sum-mers of the 1980s compared with the 1950s (Xue 1997).

Fig. A.36. Zonally averaged rainfall distribution (mm d–1). a Three-month mean for control run (solid line) and desertification run(dashed line); b solid line is the mean for 1950 and 1958, dashed lineis for 1983 and 1984 (from Xue and Shukla 1993)

Fig. A.37. Comparison of simulated and observed runoff. a Controlrun and observed mean from 1951–1970; b desertification run andobserved mean from 1971–1990 (after Oki and Xue 1998)

A.5.1 · Introduction


In prehistoric times, the Sahelian region had a muchmore dramatic desertification episode than that whichpersists today. The climate was humid in the early andmid-Holocene from 10 000 to c. 4 000 years B.P. (Alexandreet al. 1997; Claussen and Gayler 1997). During that period,swamp forest vegetation was established in the interdunedepression in the Nigerian Sahel. The vegetation limit inthe region reached at least 23° N. A dry phase during thelate Holocene (between c. 4 000 and 1 200 years B.P.) fol-lowed and came to be replaced by wetter climate condi-tions from c. 1 000 years B.P. The modern shrubs and sa-vanna were developed c. 700 years B.P. Subtle changes inthe Earth’s orbit, strongly amplified by regional and glo-bal climate-vegetation interactions, are currently believedto be the main causes for this change in the region’s cli-mate 4 000 years B.P. (Kutzbach et al. 1996a).

Historical data reveal several other drought periodsfrom the 16th century in the Sahelian area (data frombefore the 16th century are very scare), but no recordsshow a drought as persistent and severe as the one thatstarted in the 1960s (Nicholson 1978). Only the droughtduring the 1840s might be as severe as the one in the1980s.

A.5.1.3 The Complex Processes of Land-use Changein the Sahel

The Sahel region has a variety of land uses that gener-ate goods and services for the population: fuelwood innatural vegetation areas, food for subsistence and mar-ket demands in croplands, livestock in the pastoral area.Pastoralism is particularly extensive. Biomass produc-tion relies on the natural productivity of grasslands.Burning is used in most rangelands to preserve savannasagainst woody invasion and to reduce weeds and pests.Production from shrubs and trees, and crop residues isalso important for consumption by livestock. In exten-sive farming systems in African savannas, four years ofcultivation is the usual time limit before the land is putinto fallow.

The Sahelian region has undergone significant chang-es in land cover over the past decades. These changesresult from interactions between short-term climatefluctuations and longer-term anthropogenic impacts, inparticular, agricultural expansion, agricultural intensi-fication, and rangeland modifications (Stephenne andLambin 2001). Expansion of cultivation into previouslyuncultivated areas or migration into unsettled areastakes place with the help of newly developed technol-ogy. Agricultural expansion thus leads to deforestationand to a reduction in pastoral land. Pastoral land alsoexpands into natural vegetation areas. Note that graz-ing livestock move into fallows and cropland during thedry season. Expansion of cropland and pastoral land isthus driven by changes in human and animal popula-

tions that increase the demand for food crops and for-age, through variability in rainfall that modifies landproductivity, and policy changes that modify the rulesof access to resources and market changes. The rate andreversibility of change are, however, much higher forrainfall compared to demographic, institutional or eco-nomic changes.

Once specific land thresholds are reached, additionaldemand for food crops results in agricultural intensifi-cation. In Sahelian agriculture, intensification mostlytakes place as a shortening of the fallow cycle, compen-sated by the use of labour, agricultural inputs (mainlynatural fertilisers) and selection of seeds with, so far,only a minor use of mechanisation and irrigation (Diop1992; Gray 1999). A shortening of the fallow cycle with-out input rapidly depletes soil fertility.

The increase in livestock population combined withshrinking pastoral land results in an increasingly sed-entary society, where livestock relies more on crop resi-dues for consumption. This puts increasing grazing pres-sure on pastoral land. Conflicts between pastoralists andsedentary farmers sometimes take place when land scar-city becomes acute.

State policies throughout sub-Saharan Africa areframed in the belief that rangelands are over-stockedby pastoralists, leading to rangeland degradation (Obaet al. 2000). The resulting management strategies aimto control, modify, and even obliterate traditional pat-terns of pastoralism (Ellis and Swift 1988). Rangelandsare maintained by the interaction of human and bio-physical drivers, and reducing or excluding the humanuse may trigger significant changes in these ecosystems.Indeed, reduced grazing is associated with a loss of spe-cies diversity, a decline in vegetation cover and reducedplant production. A weakened indigenous pastoral sys-tem may lead to economic decline or to urban migra-tion with rural remittances.

There has been rapid population growth in the re-gion over the past 50 years. For example, the populationof five Sahelian countries (Senegal, Niger, Mali, Sudanand Chad) rose from 20 million in 1950 to 55 million in1990 and is projected to rise to over 135 million by 2025(World Resources Institute 1994). This rise in popula-tion, as well as political constraints on nomadism, willcontinue to cause extensive land-cover conversion. Thispopulation growth has occurred at a time of generallyreduced rainfall but the proposition that this has led towidespread desertification is not fully established.

Desertification is a loosely defined concept but it istaken here to mean the degradation associated with in-creased soil erosion caused by wind and water, soilcompaction and reduced water holding capacity, re-duced infiltration of rainfall and vegetation production,and loss of palatable species, all leading to a decline incrop yields, livestock production, and fuel-wood supply(Le Houérou and Hoste 1977; Dregne 1983; Rapp 1987;

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Le Houérou 1989; Goudie 1990; Barrow 1991; Verstraeteand Schwartz 1991). Semi-arid rangelands are highly dy-namic and resilient, moving through multiple vegeta-tion states by shifting chaotically in response to humanand biophysical drivers. This intrinsic variability ofrangeland ecology makes it difficult to distinguish di-rectional change (e.g. loss of biodiversity, soil degrada-tion) from readily reversible fluctuations, such that in-terpretations of “degradation” and “desertification” mustbe viewed cautiously.

The distinction between desertification and degrada-tion on the one hand, and drought on the other, is not al-ways clearly drawn. Although there is consensus on localdegradation in the Sahel (e.g. Lindqvist and Tengberg1993; Mabbutt and Floret 1980), opinions on widespreaddegradation in the Sahel are quite controversial. A num-ber of studies showed that overgrazing of the naturalrangelands by livestock, northwards extension of culti-vation and increased fuel-wood extraction, coupled withsevere drought, are leading to widespread land degrada-tion (Gornitz 1985; Dregne and Tucker 1988; Skoupy 1987;Lanly 1982; Dregne and Chou 1992; Akhtar-Schuster 1995;Benjaminsen 1993; Gray 1999). However, some studies,based on the satellite data from the late 1970s, have chal-lenged views on large scale degradation (e.g. Watts 1987;Forse 1989; Tucker et al. 1991; UNCED 1992; Nicholsonet al. 1998; Prince et al. 1998). Measuring rates of drylanddegradation is thus a complex challenge and requires longtime-series of rainfall data, remote sensing-based indi-cators of surface conditions and field observations of soilattributes, floristic composition, etc.

The issue is in urgent need of reconsideration withmore appropriate data (Helldén 1991). The actual extentand severity of regional and subcontinental-scale deg-radation is an important issue that affects policy withrespect to economic development strategies and aid pro-grammes, and also the degree to which changes in theland cover need to be considered in the context of globalclimate change modelling (Rasmusson 1987; Schlesingeret al. 1990). Prince et al. (1998) have used the ratio of netprimary production (NPP) to precipitation – the rain-use efficiency (RUE) – to map degradation. In the Sahel

the results suggested that NPP was remarkably resilient,a fact that was reflected in only little variation in the RUEduring the period of study. Thus, in much of the region,NPP seems to be in step with rainfall, recovering rapidlyfollowing drought and not supporting the fears of wide-spread, subcontinental scale desertification taking placein the nine-year period that is studied (Nicholson et al.1998). In fact, the results show a small but systematic in-crease in RUE for the Sahel as a whole from 1982 to 1990,although some of the areas contained within the regiondid have persistently low values.

A.5.2 Observational Studies of SahelianLand-surface/Atmosphere Interactions

To investigate interactions of land surfaces with the at-mosphere and impact of land-cover change in the Sahelregion, requires regional measurements of land-surfaceenergy and water balances, and of boundary layer andfree-atmosphere dynamics. They provide useful infor-mation about Sahelian surface hydro-meteorology andare essential to validate models.

Two large field campaigns were conducted during thelate 1980s (Sahelian Energy Balance Experiment, SEBEX)and 1991–1992 (HAPEX-Sahel). Building on these, twoadditional studies are currently operational (SALT andCATCH). Most of these projects incorporate substantialremote sensing programmes. Figure A.38 shows the lo-cations of these four field experiments. Table A.2 liststhe web sites for these experiments.

A.5.2.1 The Sahelian Energy Balance Experiment(SEBEX)

This experiment was conducted by the U.K. Institute ofHydrology and the International Crops Research Insti-tute for the Semi-Arid Tropics (ICRISAT) Sahelian Centerat two sites near Niamey, Niger, from 1988 through to 1991(Wallace et al. 1992) and covering three contrasting Sahe-lian land types. One site was a fenced area of fallow sa-

Table A.2. Relevant websites relating to Sahelian studies

HAPEX-Sahel online database (Hydrological Atmospheric Pilot Experiment) http://www.orstom.fr/hapex/index.htm#1

CATCH project(Couplage de l´atmosphère tropicale et du cycle hydrologique) http://www.lthe.hmg.inpg.fr/catch/welcomefr.html

SALT project (Savannas in the Long Term) http://medias.obs-mip.fr/www/francais/lettre/10/Dossiers/pf2225/pf2225.htm

WAMP project (West African Monsoon Project) http://www.met.rdg.ac.uk/~swsthcri/tropical.html

CLD project (Climate and Land Degradation) http://www.nwl.ac.uk/ih/cld/

Sahel standardised rainfall index (Mitchell) http://tao.atmos.washington.edu/data_sets/sahel/#values

Sahel standardised rainfall index (Hulme) http://www.cru.uea.ac.uk/tiempo/floor2/data/sahel.htm

El Niño impacts in Africa http://www.essc.psu.edu/~osei/west/westaf1.html

A.5.2 · Observational Studies of Sahelian Land-surface/Atmosphere Interactions


vanna, which supported a comparatively high density ofnatural vegetation, and a second was an area of degradednatural forest, which consisted of a mosaic of dense veg-etation and large areas of bare soil. There was also agri-cultural land that was used for growing traditional cropssuch as millet. One major objective of the project was toobtain direct measurements of available energy, evapo-ration and sensible heat flux from different land surfaces,and to improve understanding of the impact of changesin vegetation on the energy and water balance. The meas-urements covered both the Sahelian dry and rainy sea-sons. Although there were minor differences amongmeasured variables, all datasets include hourly evapora-tion rates, sensible heat flux, soil heat flux, friction veloc-ity, short-wave and long-wave radiation flux, net radia-tion at surface, and meteorological data.

A.5.2.2 The Hydrological and Atmospheric PilotExperiment in the Sahel (HAPEX-Sahel)

The HAPEX-Sahel experiment was executed in Niger,West Africa during 1991–1992 (Goutorbe et al. 1994,1997a). The objective of this experiment was to improvethe parameterisation of surface hydrological processesin semi-arid areas at scales consistent with general cir-culation models, and to develop methods for monitor-ing the surface hydrology at such large scales. With ob-jectives similar to the SEBEX experiment, the experi-ment was carried out at a much larger scale, i.e. in a1° square (2–3° E, 13–14° N) in Niger. The vegetationwithin the experimental area was typical of the south-ern Sahelian zone: arable crops, fallow savanna andsparse dryland forest. The observations included a pe-riod of intensive measurement during the transition

period of the rainy to the dry season, complemented bya series of long-term measurements. Analyses of datafrom satellite remote sensing combined with groundmeasurements were carried out to estimate energy fluxesfor the GCM grid scale.

Three super-sites were instrumented with a varietyof hydrological and meteorological equipment to pro-vide detailed information of surface energy exchangeat the local scale. Two less heavily instrumented siteswere also established. These were to extend the spatialcoverage over the square. In addition to these sites, anetwork of automated weather stations was establishedover the full 1° square. They provided information onthe regional-scale variability of the main climatic vari-ables. Meanwhile, balloon and aircraft measurementswere also carried out to provide information about thedevelopment of the atmospheric boundary layer.

The experiments in the Sahel were extensive andcomplex and the main experimental results are pre-sented in a Special Issue of the Journal of Hydrology(Goutorbe et al. 1997a). The main contributions fromthese experiments of interest to the climate modellingcommunity are:

� Measurements of the energy balance of the main covertypes were made, including mean surface albedo, skintemperature and evaporation, for millet cropland andnatural vegetation. Millet is the dominant crop grownby subsistence farmers, in areas cleared of natural sa-vanna vegetation. The millet crop had a significantlyhigher surface albedo and skin temperature and lowerevaporation rate (Table A.3; Gash et al. 1997), indicat-ing some of the changes in land-surface propertieswere likely to have been a result of the extension ofagriculture.

Fig. A.38. Location of past and present land-surface experiments in the Sahel region, projected on USGS land-cover classification.SEBEX comprised two sites near Niamey (Niger), HAPEX Sahel a 1° gridbox in the same area. CATCH includes the latter, while addinga catchment of similar area in Benin, both nested inside two larger regions. SALT includes eight 40 km2 along two gradients

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� Detailed measurements of the bare soil patches withinthe vegetation covers were made. The energy and wa-ter balance of the bare soil areas turned out to be cru-cial to the response of the overall landscape.

� The extensive raingauge network has yielded consid-erable insight into spatial rainfall and soil moisturepatterns in the Sahel (see e.g. Lebel et al. 1997; Taylorand Lebel 1998). These data are beginning to be usedto understand the aggregate description of the Sahe-lian landscape (e.g. Taylor and Blyth 2000).

� Extensive site and spatial datasets have been pro-duced which are providing an invaluable resource forcurrent and future Sahelian research.

A.5.2.3 Coupling Tropical Atmosphere andHydrological Cycle (CATCH)

While HAPEX-Sahel produced key results, some impor-tant shortcomings made it difficult to place those re-sults in a broad climatic context. First, the lack of ap-propriate atmospheric measurements at the regionalscale prevented a link between the mesoscale and thelarge circulation. Second, intensive experiments for alimited time are not on their own sufficient to describethe interannual and decadal variabilities of the watercycle. Finally, the mechanisms that control the varia-bility of West African climate and hydrology must bestudied.

Therefore, CATCH addresses scales larger than thosein HAPEX-Sahel. A nested approach encompasses thefollowing:

� West Africa as a whole, to study the structure and thevariability of large atmospheric entities (e.g. easterlywaves);

� the so-called CATCH regional window (0–5° E; 6–15° N)will be used as a reference area to compare the out-puts of various atmospheric models (global to meso-scale) with observations;

� two focus areas were selected for fine resolutionmeasurements and process studies: the HAPEX-Sahelsquare (2–3° E; 13–14° N) and the upper Ouémé catch-ment (1.5–2.5° E; 9–10° N);

� super-sites, covering in the order of a hundred km2,allow for small-scale studies, especially fluxes at thesoil-atmosphere interface.

CATCH is organised around four themes:

1. an atmospheric theme addresses climatic variabilityrelated to east African waves and global teleconnec-tions aiming at improving seasonal precipitation fore-casts, and spatio-temporal dynamics of convectiveprocesses in relation to planetary boundary layer(PBL) dynamics;

2. a biospheric theme focusing on the role of small-scalespatial and structural (trees v. herbaceous vegetation)heterogeneity in evapotranspiration and carbon cy-cling;

3. a hydrospheric theme focusing on precipitation/run-off dynamics, as well as on non-saturated zone mois-ture dynamics at larger scales;

4. a land-atmosphere interaction theme focusing on anintegrated (modelling) analysis of coupling of theother themes.

Recently, CATCH has evolved into a larger interna-tional project centered on the study of the whole WestAfrican monsoon system and of its links with the watercycle. This project, named AMMA, is endorsed byCLIVAR, GEMEX and GCOS.

A.5.2.4 Savannas in the Long Term (SALT)

While AMMA is a project rather strongly oriented to-wards atmospheric and hydrological processes, SALT ad-dresses more ecological issues. Set up as a transect (andas such endorsed as an IGBP-GCTE Transect), its obser-vational programme includes eight sites covering thetwo major eco-climatic gradients in the region: the southto north aridity gradient, and a west to east continen-tallity gradient. SALT’s primary objective is to under-stand and predict current and future dynamics andchanges in West African savanna ecosystems under cli-matic and anthropogenic pressures. It addresses savannadynamics in terms of species composition and spatialstructure in relation to the cycles of water, energy, car-bon and nutrients. A special focus is on how these areaffected by land degradation and erosion.

Apart from ground measurements, remote sensingplays an important role in SALT to scale up vegetationdynamics, to monitor fire dynamics and to study aero-sol dynamics.

Table A.3.Mean surface albedo, surfaceskin temperature and evapo-ration at the HAPEX-SahelSouthern Super-Site, throughAugust and September, 1992and simulated differencesbetween degraded and controlexperiments over the test area

Vegetation type Surface albedo Surface skintemperature (˚C)

Evaporation(mm m–1)

Millet 0.27 29.5 77

Fallow savanna 0.19 28.4 115

Observed difference 0.08 1.1 –38

Simulated differences 0.08 0.8 –24

A.5.2 · Observational Studies of Sahelian Land-surface/Atmosphere Interactions


A.5.2.5 Satellite Data

Validation and calibration studies using the SEBEX andHAPEX-Sahel data have helped to improve the models’simulations in semi-arid areas and to understand theimportant features and mechanisms of land-surface/at-mosphere interactions in the Sahel area (Goutorbe et al.1997b; Kabat et al. 1997b; Xue et al. 1996a, 1998). How-ever, considering the high spatial variability in land-sur-face conditions in the Sahel, and given that these twoexperiments cover relatively small areas and short peri-ods of time, both the observed and derived land datafrom HAPEX-Sahel are not wholly representative of veg-etation conditions over the entire Sahel area.

During the past two decades, new satellite data havebrought the Earth systems science community to thebrink of a new era for land-surface data information(Shuttleworth 1998). In the earlier stage, vegetation mapswere the main products from satellite data (DeFries andTownshend 1994). Recently, vegetation parameters withmore physical meaning, such as leaf area index (LAI) andbiomass, and surface meteorological, ecological, and hy-drological variables, such as surface temperature, albe-do, soil moisture, can be derived with increasing accu-racy from space (for example, Los et al. 2000; Goetz 1997;Goward et al. 1999), due to more optimised sensor char-acteristics, better atmospheric correction procedures andbetter algorithms. Such datasets will increasingly feed di-rectly to models for land-atmosphere interaction studies.

During HAPEX-Sahel, a very wide range of aircraftand satellite measurements was made (Prince et al. 1995).Recently, a subset of a global biophysical land-surfacedataset at 8 km spatial and 15-day temporal resolutionsfor 1982–1998 derived from AVHRR by NASA GoddardSpace Flight Center (GSFC) has been introduced to Sahelstudies by Kahan (2002). This dataset (hereafter referredto as Los data) is an extension of their previous 1° data-set (Los et al. 2000), and includes LAI and fraction ofphotosynthetically active radiation (FPAR) absorbed byvegetation. Using this dataset and vegetation cover in-formation, other vegetation properties, such as green leaffraction, surface roughness length, vegetation cover per-centage, are obtained.

This dataset was used to specify the land-surface con-ditions over a 1° square area in the Sahel. With a forcingdataset from ISLSCP Initiative I CD-ROM (InternationalSatellite Land Surface Climatology Project; Meeson et al.1995), uniform over the one degree box, the off-line ver-sion of the Simplified Simple Biosphere Model (SSiB; Xueet al. 1991, 1996b) was integrated from 1 January 1987through 31 December 1988 at every 8 km pixel. Fig-ure A.39 shows the July, August, September (JAS) meansfor latent heat and surface air temperature for the bothyears 1987 and 1988 from this 1° area, respectively. Thefigures show that even with a uniform meteorological

forcing in a 1° box, vegetation distribution (with 8 kmresolution) alone can produce a north-south tempera-ture gradient due to significant spatial variation in sen-sible and latent heat fluxes. Furthermore, using the sameLos dataset, the model also simulated the interannualvariations well. The year 1987 (a dry year) has higher sur-face temperature and lower latent heat flux than 1988 (anormal year).

A.5.3 Coupled Modelling of SahelianLand-Atmosphere Interactions

A.5.3.1 Brief Overview

A number of studies with different models have beenconducted over the past 20 years to examine the role ofbiospheric feedbacks in the Sahel drought of the sameperiod. In the pioneering work by Charney et al. (1977)on the effects of albedo on the climate, he found that in-creases in albedo caused a reduction in precipitation. Thisdiscovery has been confirmed by a number of studieswith different models (for example, Chervin and Schnei-der 1976; Sud and Fennessy 1982; Laval and Picon 1986).The effects of soil moisture and evaporation have alsobeen investigated (e.g. Walker and Rowntree 1977; Sudand Fennessy 1984). Most studies showed that less initialsoil moisture leads to less precipitation. Furthermore, thecombined effects of surface albedo and soil moisture havebeen studied (e.g. Sud and Molod 1988; Kitoh et al. 1988).Kitoh et al. (his Table 2) found that the combined effectsalmost equalled the sum of the albedo or soil moistureeffects alone. Besides these two parameters, other fac-tors have also been investigated (Cunnington and Rown-tree 1986). These modelling studies consistently demon-strated that the land surface may have a significant im-pact on the Sahel climate.

In the studies that were carried out during the 1970sand 1980s, simple surface layer models of the bucket typewere used for sensitivity studies. In most cases, only asingle land-surface parameter was tested each time. Theprimary factors of the desertification under investiga-tion were surface albedo and soil moisture. In most ofthese sensitivity studies, the area associated with land-surface changes and the extents of the changes in thesurface characteristics, such as albedo and soil moisture,were somewhat arbitrary. Atmosphere-biosphere inter-actions are, however, much more complex in the realworld and involve many more parameters and processes.

Therefore, sophisticated land-surface models areneeded to assess realistically the impact of desertifica-tion on the Sahel drought. A proper evaluation of thesurface feedback to climate can be obtained only whenall comparable components of the energy and waterbalances are considered. A number of surface schemesthat include a realistic representation of the vegetation

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responses have been applied to the Sahel (e.g. the Bio-sphere-Atmosphere Transfer Scheme, BATS, Dickinsonet al. 1986; the Simplified Simple Biosphere Model, SSiB,Xue et al. 1991; the Meteorological Office Surface EnergyScheme, MOSES, Cox et al. 1998). They have been cou-pled with atmospheric models to conduct a number ofSahel drought studies (Xue et al. 1990; Xue and Shukla1993; Xue 1997; Clark et al. 2001). Different from the stud-ies in the early 1980s, some of these studies not onlytested the sensitivity of the regional climate to land-sur-face processes but were also intended to link the land-surface processes to observed decadal climate anoma-lies and, therefore, explored the possible mechanismsinvolved more realistically.

Most recently, an additional level of complexity isbeing explored by the coupling of dynamic vegetationmodels to atmospheric models. Though still in its earlydevelopment stage, this field of activity has already pro-duced a number of interesting results regarding land-surface climate interactions in the Sahel. Some studies

investigated the climate equilibrium states (e.g. Claussenand Gayler 1997; Wang and Eltahir 2000b). Using a GCMwith an interactive vegetation model, Claussen andGayler (1997) found multiple equilibrium states in cli-mate/vegetation dynamics in northern Africa whichcould not be found in other parts of the world. This dis-covery was confirmed by Wang and Eltahir (2000a) witha coupled two-dimensional climate/dynamic vegetationmodel. In another study using a coupled atmosphere anddynamic vegetation model, Zeng et al. (1999) reproducedthe decadal precipitation variability in Sahel, whichcould not be simulated if interactive vegetation proc-esses were not included in their model.

Mesoscale models are also introduced to Sahel land-atmosphere interaction studies. Thermal anomalies ina landscape may trigger convection and lead to rainfall.The causes of these contrasts in surface energy parti-tioning can be attributed to contrasts in soil moisture(Taylor et al. 1997b) and/or to contrasts in vegetationdensity (Hutjes and Dolman 1999). Whilst models illus-

Fig. A.39.1° × 1° fields of latent heat(Wm–2) and surface air tem-perature (°C) in the Sahel intwo successive years (1987:dry; 1988: wet) as simulatedby SSiB. The images show theimpact of the land-surfacevariability obtained from sat-ellite observations (at 8 kmresolution). Climate forcingswere uniform for the entiregrid cell

A.5.3 · Coupled Modelling of Sahelian Land-Atmosphere Interactions


trate this possibility, it remains to be proven from ob-servations that mesoscale heat flux gradients are strongenough to influence rainfall patterns by this mechanism.

In addition to land-surface effects, observational andmodel studies have revealed that SST anomalies are high-ly relevant to the seasonal and interannual rainfall varia-bility in northern Africa (Lamb 1978a,b; Palmer 1986;Semazzi et al. 1988; Folland et al. 1991; Lamb and Peppler1991; Shinoda and Kawamura 1994; Rowell et al. 1995; Xueand Shukla 1998). Some of the modelling studies on thissubject will be briefly introduced in Sect. A.5.3.2 andA.5.3.4 for comparison.

A.5.3.2 Large Scale Force-Response Studiesof the Sahelian Climate Anomaly:The Relative Importance of Land-surfaceProcesses and Sea-surface Temperatures

Starting in the late 1980s, numerical experiments withcoupled biosphere-atmosphere models have been con-ducted to derive a better understanding of the mecha-nisms of land-atmosphere interactions in northern Af-rica. More realistic simulations of climate anomalieshave been achieved by using land-surface descriptionswhich can be changed to represent changes closer to thereal world than was the case in the earlier experiments.In the following we will use the results obtained by Xueand collaborators to describe in detail the up-to-dateachievement in simulating the climate anomalies andthe most important pathways through which land-coverchange affects Sahelian climate. Meanwhile, other recentSahel studies with models of interactive vegetation andmesoscale models will also be presented.

To explore the impact of land degradation in the Sahelon seasonal climate variability and the water balance,the coupled Center for Ocean-Land-Atmosphere Study’sGCM (COLA GCM, Kinter et al. 1988) and SSiB land-sur-face model (Xue et al. 1991, 1996b) was integrated using a“normal” vegetation map (control simulation) and sev-eral vegetation maps where savanna or shrubs withground cover were changed to shrubs with bare soil in aspecified area (degraded simulation). Climatological SSTswere used as the lower atmospheric boundary conditionsover the oceans. Because of the internal variability inGCMs, the model in Xue’s study (1997) was integratedfrom 1 June, 2 June, and 3 June for three years, respectively,and ensemble means were used to identify climate impacts.

The world vegetation map was read into the coupledsurface-atmosphere model to provide the land-surfaceconditions required by SSiB. Twelve vegetation types arerecognised by SSiB, including trees, short vegetation,arable crops and desert. Different vegetation and soilproperties, including surface albedo, LAI, soil hydraulicconductivity and surface roughness length, are defined

for each vegetation type. For land-surface degradationsimulations, the normal vegetation types in the Sahelarea were changed to the types that would result fromland degradation, altering the prescribed vegetation andsoil properties. Since there are no quantitative data avail-able for the 1950s’ land-cover information in the Sahel,the selections of the degradation areas were based onsome available information (e.g. Dregne and Chou 1992)but do not necessarily represent the reality that occurredduring the past 50 years. As a result, these simulationsmust be regarded as sensitivity studies.

The differences between the ensemble mean July-Au-gust-September (JAS) rainfall of the degraded and con-trol simulations are shown in Fig. A.40b. The rainfall isreduced in the degraded area but increases slightly to thesouth. This dipole pattern is consistent with the observedpattern for dry climate anomalies in Fig. A.40a, whichshows the JAS rainfall differences between the 1980s andthe 1950s. The simulated rainfall in a test area (from 9° Nto 17° N, and 15° W to 43° E), including most of the speci-fied degraded area, is reduced by 39 mm month–1, closeto the 45 mm month–1 observed reduction.

At the beginning of the Sahelian dry season (October-November-December, OND), when the Intertropical Con-vergence Zone (ITCZ) moves to the south, little rainfallis observed or simulated in the Sahel. However, the areasof reduced rainfall related to land degradation shift tothe south of the Sahel, with a positive anomaly over east-ern Africa (Fig. A.40d), consistent with the observed ONDrainfall anomaly (Fig. A.40c). Thus, the effect of land de-gradation is not limited to the summer rainy season withinthe Sahel but extends to the autumn and into East Africa.

The JAS surface air temperature is higher in the de-graded simulation than the control, consistent with theobserved JAS temperature difference between the 1980sand the 1950s. In the test area, the simulated surface airtemperature increases by 0.8 K, close to the observedincrease over the same area, 1.1 K, and consistent withthe difference measured in the field (Table A.3).

The impact of land-surface degradation on river dis-charge variability in tropical northern Africa has alsobeen investigated. The simulated soil moisture, surfacerunoff and subsurface drainage in degradation experi-ments decrease, consistent with the reduction in rainfall.To compare the runoff at grid points in GCM simula-tions with observed river discharge, a linear river rout-ing scheme is applied. The lateral flow direction is takenfrom Total Runoff Integrating Pathways (TRIP; Oki andSud 1998). The simulated river discharges from controland degradation experiments at the Koulikoro station,located at the top end of the River Niger, represent themean runoff from south western Sahel region and areused to compare with the observed 1951–1970 mean andthe 1971–1990 mean, respectively (Oki and Xue 1998).Some stations in the Sahel cannot be used because their

69

data are severely affected by irrigation. The mean month-ly discharges are simulated fairly well and the contrastbetween control and degraded simulations correspondsto the observed difference between 1951–1970 and 1971–1990 (Fig. A.37).

Since we may never know the exact extent and de-gree of real land degradation in the last 50 years, an-other set of tests was designed to investigate how theextent of specified land-surface changes may affect theresults. Five subregions were degraded in turn: north-ern Sahel, southern Sahel, West Africa, East Africa, andthe coast area along the Gulf of Guinea (Clark et al. 2001).In general, degradation results in reduced rainfall overthe changed surface. But considerable differences be-tween the areas indicate that the location of the degradedarea is important. Degradation in northern Sahel andWest African Sahel results in the largest and most sig-nificant reductions of rainfall. In particular, degrada-tion of northern Sahel causes widespread reduction ofrainfall across tropical northern Africa, both within andoutside the degraded area. Meanwhile, the deforestationin the coast area produces little rainfall variation.

The simulated climate anomalies are caused by thespecified land degradation, which affects the atmospheremainly through modulating the hydrological processesand energy balance at the surface. There are about 20 pa-rameters in a biosphere model. To investigate the effectsof each parameter in this degradation study, numericaldegradation experiments have also been conducted, inwhich not the vegetation type but rather the individualparameter was changed in the degraded area (Xue et. al.1997). The results show that surface albedo, surface aero-dynamic resistance, stomatal resistance, LAI, and hydrau-lic conductivity of the soil have the largest impact on thesimulation results. The surface albedo change caused byland degradation has a strong effect on the surface en-ergy and water balances, so it must be specified realisti-cally in Sahel studies. The albedo changes used in theSahel study were reasonable (0.09–0.1) and close to theobserved albedo difference between the cropland andnatural vegetation in the Sahel (Table A.3). Despite theimportance of albedo, changes in other vegetation andsoil properties (such as LAI) also contribute significantlyto rainfall and surface temperature anomalies.

Fig. A.40. a Observed JAS rainfall difference (mm month–1) between 1980s and 1950s; b JAS rainfall difference in ensemble mean, de-graded minus control; c same as (a) but for OND; d same as (b) but for OND. In the degraded simulations the vegetation types in theheavy lines were changed to shrubs with bare soil (adapted from Xue 1997)



Land degradation also manifests itself in the changesin large-scale circulation and easterly wave propagation.One of the important features of sub-Saharan droughtis the displacement of isohyets in the dry years. The iso-hyets retreat southward in northern Sahel during dryyears, which reflects the changes in ITCZ position. Insouthern Sahel, the isohyets also shift slightly to thesouth. This movement reduces the rainfall in the Saheland increases rainfall to the south of 10° N. In the modelsimulations, both shifting and reduction of the maxi-mum rainfall play roles in reducing the rainfall in theSahel (Fig. A.36). Both the strength and the depth of themonsoon flow may change.

Reed (1986), based upon the observed data from 1967to 1982, concluded that the sub-Saharan baroclinic zoneand near equatorial rain belt jointly spawned about thesame number of disturbances over Africa each year, butthat fewer strong and/or highly convective disturbancesoccurred near the coast during periods of extendeddroughts. In the degraded experiments (Xue and Shukla1993), the intensity of disturbances was greatly reducedbut not the number of the disturbances. However, re-cently available observational data from Niger (Le Barbéand Lebel 1997) show that the drought in Niger is mostlyassociated with a decrease in the number of rain eventsin July and August. The mean event rainfall and thelength in the rainy season hardly contributed at all tothe decrease in annual rainfall.

At the continental scale, the oceans play an impor-tant role, and the relative strength of forcing of landsurface versus SSTs has been a subject of some studies.In particular, the relationship between SST and seasonalto interannual rainfall variations in the Sahel region haslong been an important scientific subject. The first com-prehensive observational evidence for a possible rela-tionship between the Atlantic SST anomalies and theSahelian rainfall anomalies was presented by Lamb(1978a,b). Lamb showed that a relationship exists be-tween displacements of Atlantic SST patterns and rain-fall patterns. Tropical Atlantic SST patterns are knownto accompany extreme sub-Saharan rainfall conditions,especially towards the west. This discovery was furtherconfirmed, and extended by several subsequent studies(Hastenrath 1984, 1990; Druyan 1991; Lamb and Peppler1991), to explain long-term changes. In addition, weakerbut significant correlation was found between El Niño/Southern Oscillation indices and sub-Saharan rainfallespecially towards eastern (horn of Africa) and south-eastern Africa. But there is considerable disagreementin the literature on the ENSO signal in the Sahel (e.g.Wolter 1989; Nicholson and Kim 1997).

Several other observational and modelling studiessuggested that the global SST anomalies also played animportant role in producing rainfall anomalies over theSahel and the adjoining regions (Folland et al. 1986; Pal-

mer 1986; Folland et al. 1991; Palmer et al. 1992; Rowellet al. 1995). Folland et al. (1991) found that the relativelymodest variations observed in the large-scale patternsof SST had a substantial impact on the variations of Sahelrainfall. Tropical oceans, on the whole, had considerablymore influence than extra-tropical oceans. They alsofound that warmer SST in the Southern Hemisphere rela-tive to that in the Northern Hemisphere had been asso-ciated with Sahel drought. Rowell et al. (1995), using re-alistic SST as the boundary condition for the UK Mete-orological Office GCM, found that the model was ableto simulate the interannual and decadal variations ofSahelian rainfall very well.

These results were not confirmed by the simulationsby other modelling groups (Sud and Lau 1996). In anexperiment with the COLA GCM and the same SSTs usedby Folland et al. (1991), Xue and Shukla (1998) found that,although the results generally supported the conclusionof Folland et al., they were quite model-dependent. Forexample, in the 1958 case, the COLA GCM could not pro-duce the observed rainfall anomaly pattern as did theU.K. Meteorological Office model. The simulated anoma-lies had relatively larger sensitivity to the initial condi-tions in this study than those in their desertificationstudy (Xue and Shukla 1993). Furthermore, the simu-lated rainfall anomalies were smaller than observedones. The results from the desertification experiments(Xue and Shukla 1993; Xue 1997) showed that despite thedramatic changes in prescribed land-surface conditionsin the desertification experiment, the simulated rainfallanomaly was still smaller than observations. In theCOLA GCM, neither SST nor land anomaly forcing alonecould reproduce the observed rainfall anomalies. It islikely, therefore, that both SST and land-surface anoma-lies play a role in Sahelian rainfall.

The teleconnection between Sahel climate and otherparts of the world is an important scientific subject. Ob-servational evidence has shown a correlation betweenthe rainfall anomalies in the Sahel and the rainfall anoma-lies in other tropical regions (e.g. India) and subtropicalregions (e.g. Europe, Ward 1995). But thus far, there arefew studies on this subject.

Xue’s study (1997, Fig. A.40d) shows there may be apotential link between Sahelian and East African climate,and some impact of Sahel desertification on circulationin some tropical regions. Further study is necessary toinvestigate this issue. Another example of possible tele-connections influencing Sahelian rainfall comes from apalaeoclimate study in which vegetation changes asso-ciated with agricultural expansion and deforestation inMediterranean Europe and North Africa toward the endof the Roman Classical Period (some 2 000 years ago)appear to contribute to long-term climate change in theSahel. Reale and Dirmeyer (2000) and Reale and Shukla(2000) reconstructed the distribution of vegetation in

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lands surrounding the Mediterranean Sea from a vari-ety of historical and palynological sources. The Medi-terranean forests of southern Europe, the Middle East,and the mountains of North Africa and the Roman agri-cultural belt in North Africa were restored. That distri-bution was substituted for present-day vegetation in theCOLA GCM and the model has been integrated for bothcases. The most striking difference in the simulations isthat rainfall over sub-Saharan Africa is greatly affected.There is a stronger northward propagation of the ITCZand its associated rain into the Sahel during boreal sum-mer in the Roman Classical Period case.

A.5.3.3 Mesoscale Interactions between SahelianPrecipitation and Land-surface Patterns

The heterogeneity of vegetation distribution within aGCM grid box has not been taken into account in previ-ous GCM simulations. Observational evidence showedthat spatial heterogeneity of surface types in the HAPEX-Sahel area was high (Prince et al. 1995), with a distinctnorth-south gradient. Taylor et al. (1997a) found soilmoisture variations to be the important cause of varia-tions in surface energy partitioning. Hutjes and Dolman(1999) also found variations in vegetation density to bemore dominant in surface energy partitioning than soilmoisture in certain cases. The later work also focusedon issues of aggregation of surface characteristics. Basedon a vegetation classification map with 30 m resolution(Prince et al. 1997), Arain et al. (1997) aggregated veg-etation parameters to the model resolution of 2 km, us-ing either those of the dominant class or averaging eachparameter appropriately weighted by subgrid relativeareas of each vegetation class. Either method producedsimilar domain-averaged heat fluxes but the variationin fluxes differed significantly. Moreover, only aggre-gated vegetation parameters could, at least qualitatively,reproduce observed rainfall, whereas using dominantclass parameters at the land surface did not produce anyrainfall at all.

Observational evidence suggests positive surfacefeedback leads to persistent rainfall gradients, which aremuch stronger and more localised than the large-scaleaverage. Taylor and Lebel (1998) found strong positivecorrelation between daily and antecedent rainfall dif-ferences at a range of time scales. In semi-arid regionsthis mechanism may lead to preferred and seasonallypersistent rainfall patterns. The influence of antecedentrainfall on storm development is particularly noticeablewhen intense large-scale storms passed over areas ofmarked gradients in evaporation. Preliminary 2D-simu-lations of a single storm with resolved convective proc-esses by Taylor and co-workers (pers. comn.) could re-produce rainfall enhancement over wet soils surrounded

by dry ones. In a similar 3D-study of two consecutivestorms, Hutjes and co-workers found that (pers. comn.)rain did not fall over the area wetted by the first storm,but fell instead on the still dry land.

Both preliminary studies need to have a larger num-ber of ensembles for statistical confidence favouring ei-ther mechanism. These are two forms of a land-surface/atmosphere feedback that occurs at a much smaller scalethan in previous studies, and therefore needs to be in-cluded in future regional studies of climate and meso-scale change.

A.5.3.4 Climate System Interactions in the Sahel

In all the studies described above, vegetation types werefixed during the period of model integration. This maybe realistic for time periods of days up to a few yearsbut obviously becomes less so for longer, decadal to cen-tennial or even millennial scale studies, although we donot know yet how much the impact could be. The logi-cal conclusion from the studies discussed above is thatthere is a need for a realistic representation of land-sur-face conditions in land-atmosphere models. At the sametime it stresses the need to test the reverse link: responsesof land-surface conditions to environmental changes.

In the past decade, a number of groups attempted toquantify the missing climate-vegetation feedbacks bycoupling equilibrium biogeography models “asynchro-nously” to GCMs and regional models (for example,Prentice et al. 1992; Neilson and Marks 1994; Woodwardet al. 1995). These vegetation models attempt to simu-late global vegetation patterns and predict the potentialchange in vegetation distribution associated with chang-es in other components of the climate system. This kindof study involves an iterative procedure in which theGCM calculates a climate implied by a given land cover;and the vegetation model calculates the land cover im-plied by a given atmospheric-ocean condition. This proc-ess is repeated until a mutual climate-vegetation equi-librium is reached (for example, Claussen 1994).

In another type of model, the land cover is treated asan interactive element, by incorporating dynamic glo-bal vegetation models (DVGMs) within climate modelsin a physically consistent way (for example, Friend et al.1993). During recent years, several studies were con-ducted to develop physically consistent biosphysical/bio-geochemical models (for example, Sellers et al. 1996c;Dickinson et al. 1998). Attempts have also been made todevelop dynamic biosphere models (Foley et al. 1998)and to use interactive vegetation models for regionalclimate prediction (for example, Pielke et al. 1999; Luet al. 2001; Eastman et al. 2001a,b).

Claussen and Gayler (1997) used the first type of veg-etation model developed by Prentice et al. (1992) cou-



pled to ECHAM (climate model based on the ECMWFmodel, version 3.2) to simulate climate evolution overthe long time scales. As discussed in Sect. A.5.1.2, the cli-mate in the Sahara during the early and middle Holocenewas much wetter than today, and vegetation was foundfar north of the present desert border. Using an interac-tively coupled model, this shift in vegetation has beenrecaptured. On the other hand, when only an atmos-

pheric model and present-day land-surface conditionsare employed, the enhancement of simulated precipita-tion was insufficient to diagnose an encroachment ofxerophytic woods/shrub into the Sahara north of ap-proximately 20° N.

Using the coupled model, Claussen (1994, 1997, 1998)also found that, starting from different initial conditions,the system came to multiple equilibrium states in cli-

Fig. A.41.The role of the land surface incontrolling precipitation vari-ability in the Sahel, as inferredfrom the study of Zeng et al.(1999). Top panel: the observedanomalies in rainfall (blackbars) and NDVI (green dots);lower three panels: modelledanomalies in rainfall (blackbars) and vegetation (greendots) arising from ocean-at-mosphere interactions (“AO”),ocean-atmosphere-land inter-actions (“AOL”) and ocean-atmosphere-land-vegetationinteractions (“AOLV”)

73

mate/vegetation dynamics in northern Africa. Underpresent-day conditions of the Earth’s orbital parametersand SST, two stable equilibria of vegetation patterns werepossible. One solution corresponds to present-day sparsevegetation in the Sahel and desert in the Sahara (desertequilibrium) while the second solution yielded savanna,which extended far into the western part of the Sahara(green equilibrium).

Wang and Eltahir (2000a,b) used a different model-ling concept to show how different initial vegetation con-ditions may lead to different equilibrium climate patterns.They used a zonally symmetric two-dimensional regionalmodel, synchronously coupled to the Integrated Bio-sphere Simulator (IBIS, Foley et al. 1996) biosphere mod-el. Their surface conditions outside the tropics were fixedto NCEP (National Centers for Environmental Prediction)re-analysis climatology. IBIS is a DVM of the second typewhich models vegetation growth and competition explic-itly. Using this model they found that the equilibrium cli-mate pattern, as in Claussen (1994), was sensitive to theinitial vegetation distribution. Starting with desert cov-ering all of west Africa, the vegetation at equilibrium var-ied from tall grass near the coast to short grass and desertnorthward. In contrast, starting with forest all over westAfrica, the equilibrium vegetation consisted mostly of for-ests covering most of west Africa, with a narrow grass-land band in the north and a much higher productivityand rainfall than in the first case.

In addition, they showed that the new equilibriumstates depended on the initial disturbances in modelsimulation. The system was able to recover completelyin terms of vegetation distribution and rainfall whenthe grass biomass removal was less than 60% between12.5 and 17.5° N. With 60–75% biomass removed, the sys-tem converged to a new equilibrium, about 40% drierand 65% less productive. When even more biomass wasremoved the system collapsed to a 60% drier and nearly100% less productive equilibrium. These new drierequilibria were associated with both weaker large-scalecirculation and suppressed local convection. The rea-son for this weakening/suppression was found to differbetween wet and dry seasons. In the dry season, a deg-radation process that leads to higher albedo, reducednet radiation, and cooler land surface was the dominantprocess, while in the wet season reduced evaporationand warm land surface was the dominant process. Be-cause the lack of the zonal wave disturbance in a 2-Dmodel may have contributed to the existence of multi-ple equilibrium solutions, further studies with a 3-Dmodel are necessary.

Recent work by Zeng and Neelin (2000) shows howinterannual variability in forcing (SST) affects the mul-tiplicity of stable climate patterns. Using a simple DVM,conceptually of the second type, they showed how inabsence of SST variability (using the same SST clima-

tology each year again) the equilibrium vegetation wassensitive to initial vegetation distribution, consistentwith the findings of both Claussen and Wang. However,when real SSTs, exhibiting variability on various timescales, were used to force the model, the equilibrium cli-mate pattern becomes nearly independent of initial con-ditions.

In another study, Zeng et al. (1999) also showed thatthe interaction between all three vegetation, soil andoceans components, best reproduces observed climatein terms of interannual precipitation variability over theSahel. In a model run with soil moisture and vegetationfixed, precipitation showed little variability comparedto the climatology of the last 50 years. Decadal variabil-ity is nearly absent and the amplitude of interannualvariability is small. A run with soil moisture feedback,but with each year having the same imposed vegetationdynamics, improves precipitation variability somewhatin both respects (timescale and amplitude). Allowingvegetation to respond to precipitation/soil moisture re-produced the best-observed precipitation decadal vari-ability and with an amplitude of the correct magnitude(Fig. A.41).

A.5.4 Understanding Mechanisms

All the experiments described above demonstrate theimportance of land-surface processes in Sahel climate.Here, we analyse the mechanisms involved in the atmos-phere/land-surface interactions and their relative im-portance for the Sahel region. The following discussionof the dominant physical mechanisms is outlined inFig. A.42, with heavy lines representing the main proc-esses. The major differences in the energy balance be-tween degradation and control experiments are alsolisted in Table A.4 for comparison.

Vegetation degradation and soil drying increase thealbedo. In the southern Sahel, when the albedo increases,more solar radiation would be reflected back into theatmosphere. However, there are fewer clouds as a resultof less evaporation associated with the higher albedo.Therefore, a negative feedback occurs, and the solar ra-diation that reaches the ground is not significantlychanged. In the northern Sahel, however, although ahigher albedo increases the short-wave radiation thatreflects into the atmosphere, it cannot reduce clouds anyfurther because there are not many clouds in the firstplace. The values given in Table A.4 show that in thenorthern Sahel, where clouds are scarce, a cloud/radia-tion feedback is, therefore, not important and the in-creased albedo leads to reduced surface heating. In thesouthern Sahel, in contrast, the cloud/radiation interac-tion is particularly strong and the net short-wave radi-ation absorbed by the surface does not change. The net

A.5.4 · Understanding Mechanisms


long-wave radiation at the surface is reduced becausethe higher surface temperature increases outgoing long-wave radiation. The reduced cloud and water vapour inthe degradation simulations also reduce the incominglong-wave radiation at the surface. However, this re-duction is smaller compared with other energy compo-nents.

The changes in sensible heat flux are associated withthe changes in short-wave radiation. The sensible heatflux is reduced when the short-wave radiation decreasessubstantially, such as in the northern Sahel. Otherwise, itincreases to balance the reduction of latent heat flux inthe surface budget such as in the southern Sahel. Evapo-ration in Table A.4 is substantially lower in both regions,correlating with the simulated rainfall reductions. Evapo-ration decreases partially because of the reduced net ra-

diation, but more importantly because of lower LAI, sur-face roughness length, higher stomatal resistance, andchanges in other vegetation and soil properties in thedegradation simulations. The reduced evaporation issimilar to that observed (Table A.3) when replacing thedenser natural vegetation by thinner crops. Again, thereis a difference between northern and southern Sahel.When there is more evaporation initially (southernSahel), the effect is stronger.

Because of the large reduction in evaporation (see theletter E in Fig. A.42), less moisture is transferred to theatmosphere through the boundary layer. Table A.4 showsthat this results in less convection and lower atmosphericlatent heating rates at 500 mb, where the largest reduc-tion occurs. (Fig. A.42, F). The lower convective latentheating rate is responsible for more than 70% of the re-

Table A.4.Simulated ensemble meandifferences between controland degradation experimentsover different regions

Fig. A.42.Schematic diagram of land-surface degradation interac-tions and feedback processes.The dark lines represent themain process. The dashed linebetween the surface and MFCindicates the uncertainty oftheir interaction

Parameter South Sahela (JAS)b North Sahelc (JAS)

Precipitation (mm month–1) –56 –29

Latent heat flux / evaporation (W m–2 / mm month–1) –23 / –26 –18 / –21

Sensible heat flux (W m–2) 10 –8

Net short-wave flux (W m–2) –1 –16

Net long-wave flux (W m–2) –12 –10

Total diabatic heating at 500 mb (K d–1) –0.69 –0.59

Convective latent heating at 500 mb (K d–1) –0.47 –0.41

Moisture flux convergence (mm month–1) –29 –12

Surface temperature (K) 1.1 0.4

Albedo (W m–2) 0.10 0.09

a From 9 to 13˚N and 15 to 43˚E.b JAS: July, August, September.c From 13 to 17˚N and 15 to 43˚E.

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duction in the total atmospheric diabatic heating rate inthese experiments. The reduced total diabatic heating ratein the atmosphere is associated with relative subsidence,which in turn weakens the African monsoon flow, reducesmoisture flux convergence (Fig. A.42, G), and lowers rain-fall (Fig. A.42, F). How precisely monsoon flow and mois-ture convergence are affected depends on the geographi-cal position of the land-surface anomaly as shown byClark et al. (2001).

Early Sahel studies considered albedo as the majorfactor causing the long-term drought. But the dramaticalbedo changes assigned in these studies and the cool-ing caused by high surface albedo are not supported byobservations, and as a result the credibility of the land-surface effect has long been challenged (for example,Ripley 1976). In the experiments discussed above, thealbedo changes were reasonable (close to the Nicholsonet al. (1998) estimation). The dominant factor in diabaticcooling of the upper troposphere and enhanced subsid-ence appears to be the reduction of convective heatingrates as a result of reduced latent heat flux and moistureflux convergence. The change in radiative heating ratewas rather a secondary factor, although it may triggerother anomalous processes.

Once rainfall is affected additional feedbacks startplaying a role. Reduced rainfall reduces evaporation pro-ducing a positive feedback (Fig. A.42). These feedbacksexhibit different temporal scales associated with differ-ent “memory time scales” of the processes involved. Veryfast response time scales, such as hours or less, are nor-mally associated with evaporation of moisture in the topsoil layer and water intercepted by the vegetation. Thisprocess could be active during a storm (Taylor and Lebel1998). Longer time scales, such as weeks to a season, areassociated with plant transpiration, which reduce thedeeper soil moisture. Yet longer time scales, such as sea-sons to years, are related to plant development. The studyby Zeng et al. (1999) illustrates how the combination ofmultiple feedbacks, each with its own time constant, rec-reates a realistic interannual rainfall variability. When adrought is persistently long enough, the ecosystems maychange or “move”. This brings up issues such as ecosys-tem resilience as explored by Wang and Eltahir (2000b).At the longest time scales, such as centuries and longer,the additional interaction may result in “step changes”in response to gradually imposed forcings (Claussenet al. 1999).

In the above discussion, the surface evaporation is adominant factor. However, in some cases, even surfacetemperature change alone could also produce a robustfeedback process, such as in Charney’s study (1975) andWang and Eltahir’s study (2000b). We shall not discussthis process further here since the hydrological proc-esses are dominant in the desertification experimentspresented in this chapter.

By and large, the above analysis shows that land deg-radation, and in particular vegetation degradation,modifies the water and energy balance as well as thepartitioning of available energy over sensible or latentheat fluxes. These are first-order effects and their rela-tive importance may vary spatially and temporally. Inthe atmosphere, differences in radiation, latent and sen-sible heat inputs lead to altered heat and moisture con-tents within the atmospheric boundary layer. This ef-fect produces a feedback to the surface through atmos-pheric stability conditions and other environmental fac-tors, which control stomatal behaviour of plants, thuscreating a first potential loop. Meanwhile, the turbulencefluxes and thermal structure within the boundary layeralso affect convective heating, total diabatic heating,subsidence, and moisture convergence. All these proc-esses affect cloud formation, precipitation and thestrength of the monsoon flow. Clouds strongly affectradiation flux transfer and create the second potentialfeedback loop. When precipitation is affected, additionalfeedback loops are activated through soil moisturestorages, vegetation growth and phenology, and even-tually ecosystem changes.

A.5.5 Conclusion

Research on Sahelian land-surface/atmosphere interac-tions has been continuing for several decades. It startedwith a very simple model with even no water in it (Char-ney 1975). During the early 1980s, a number of studieswere conducted by simple bucket type of land modelscoupled with GCMs. The vegetation models were intro-duced from the late 1980s. All this research showed aconsistent result: the Sahelian climate is sensitive to thelocal land-surface processes.

In the introduction, we have raised several issues re-lated to land-surface/atmosphere interaction in the Sahelregion. This Chapt. A.5, by reviewing the relevant researchduring the past two decades, has aimed at understand-ing and clarifying these issues. The answer to questionone in the introduction is apparently positive. In princi-ple, land-surface changes can result in rainfall reductionsof the observed magnitude. The studies discussed inSect. A.5.3.2 illustrate this. If changes in vegetation prop-erties between the 1950s and the 1980s were comparableto the specified control/desertification differences in thatstudy, the results indicate that the degradation could leadto regional climate changes of the order of the differ-ences found between the 1950s and the 1980s, with in-creases in the surface air temperature, and reductions inthe summer rainfall, runoff, and soil moisture over theSahel region. The impact is not only limited to the speci-fied desertification areas and the JAS period but it alsoaffects the region south of this area and continues into

A.5.5 · Conclusion


the OND period in East Africa. All these results are inline with the observed climate anomalies, which suggestthat the climate anomaly during the 1980s could be dueto land-surface processes. While early studies attributedthe drought primarily to a albedo/radiation driven proc-ess chain, we now realise that an evaporation/convectiondriven process chain is probably more important.

An apparent shortcoming in sensitivity studies wasthat the specified land-cover changes and simulated land-surface energy and water balances at the surface werenot verified by observations. The SEBEX and HAPEX-Sahel field experiments provided very important infor-mation of land-surface processes in the Sahel region.These field measurements have led to a considerable in-crease in our understanding of the surface energy bal-ance at the Sahelian land surface and in particular theinterplay between the hydrological responses of soil andvegetation.

In general, these experiments have resulted in im-proved calibration and validation of the surface bio-sphere models. Our knowledge of physical and biologi-cal characteristics of soil and vegetation and energy andhydrological processes has improved significantly (e.g.Taylor and Blyth 2000; Xue et al. 1996a). These spatialanalyses are leading to a strategy for the aggregate de-scription of the landscape in these regions where boththe rainfall and the vegetation have high spatial vari-ability. However, since all these measurements were con-ducted in one region, it is difficult to apply the results tothe continental scale. Therefore, new field measurementsshould cover much larger areas and should include moretypical vegetation. In addition, long-term measurements,which include at least a few growing seasons, are neces-sary to understand the seasonal and interannual vari-ability in the region. Both the SALT and CATCH projectsseek to fill these needs.

Satellites have provided land-surface conditions sincethe late 1970s. The various results discussed before(Sect. A.5.2.5) demonstrate the great potential in apply-ing the satellite dataset for Sahel studies. These data pro-vide temporal and spatial variations of land-surface con-ditions over the entire globe and cannot be obtainedfrom any other method. They can also be and have beenused to specify land-surface conditions more realisti-cally in model simulations, although more research isneeded to develop and validate the methodology usedto convert satellite signals to vegetation characteristicsand other land-surface information. In addition, theyprovide useful information for potential validations ofdynamic vegetation models, which we believe has notyet been attempted. The full potential of satellite meas-urements should be explored further.

Our knowledge of the role of vegetation dynamics inthe Sahelian drought has also greatly improved recentlyby the use of dynamic vegetation models. Although such

models are still at the preliminary stage, several studieswith African climate as a prime subject have demon-strated their promising potential. Thus far, these stud-ies focused on time scales of decades and centuries. Fur-ther development in coupled dynamic vegetation-at-mosphere models and more realistic simulations at sea-sonal-interannual-decadal scales are necessary.

The field experiments, satellite data, off-line land-model validations, dynamic vegetation models and cou-pled models greatly help us to understand land-surfaceprocesses and the roles of soil and vegetation in theseprocesses, and the influence and mechanisms of land-surface/atmosphere interactions (Questions 2 and 3 inthe introduction).

As to the real land-cover change in the Sahel, we maynever know the real scope of the changes that occurredduring the past half century. However, it is undeniablethat the human population in Sahel has increased sub-stantially over the last 50 years. The rise in populationand projected further increases have caused, and willcontinue to cause, extensive conversion of land from natu-ral vegetation to agriculture, and increasing demand forfuelwood and grazing (Stephenne and Lambin 2001).Currently, work is under way to use this type of informa-tion to produce more realistic reconstructions of land-surface dynamics over the past 40 years and to make pro-jections in to the future (Taylor pers. comn.). This line ofresearch will eventually provide a better answer to thequestion of whether it is really population pressure lead-ing to land degradation that leads to desertification. Wenow know it could; we may soon know whether it did,and what might happen in the future. This should par-tially answer the Questions 4 and 5.

At this stage, we still cannot decide whether the cur-rent drought is a temporary phenomenon, what is theexact role of humans in its cause, or what kind of possi-ble remedy should be taken (Question 6). One area thatremains unclear is how well the normal and degradedvegetation scenarios specified in the Sahel represent thereal land-surface conditions in the 1950s and the 1980s,as reliable information is unavailable. With the dynamicvegetation models, we may be able to assess more real-istically the role of SST, land-surface processes, and otherprocesses in the long-term African drought.

However, we might come closer to answering thisquestion by looking at other regions undergoing simi-lar desertification trends, such as Inner Mongolia (seeChapt. A.8 and Xue 1996). To decide whether suchdroughts are “permanent”, at least on human time scales,we may look back into the distant past that some groupshave begun to explore (Claussen et al. 1999).

The present chapter demonstrates that changes insurface properties caused by intensification of land usemay have had serious consequences for the regional cli-mate. To avoid such consequences, the implementation

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of sustainable resource management policies must be apriority. Xue and Shukla (1996) demonstrated that wide-spread afforestation of the Sahel might reverse the re-gional climate changes caused by land degradation. Inthis chapter we mainly presented the biophysical proc-ess studies for the Sahel area. We believe other dimen-sions of the Sahelian droughts, e.g. water resources, food

security issues, and human activity (socio-economic andpolitical drivers of land-use change), should be muchmore tightly integrated with physical process studies.Thus new links in the process chains may be discovered,leading to better reconstruction of past and better pre-dictions of future climate changes and anomalies in thissensitive region.

A.5.5 · Conclusion

chapter a - the university of edinburgh · chapter a.5 the sahelian climate yongkang xue · ronald...

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