West African monsoon response to greenhouse gas and sulphate aerosol forcing under two emission scenarios

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1 Climte Dynmics (2006) 26: DOI /s C. Cminde Æ L. Terry Æ E. Misonnve West Africn monsoon response to greenhouse gs nd sulphte erosol forcing under two emission scenrios Received: 11 June 2004 / Accepted: 6 Octoer 2005 / Pulished online: 14 Jnury 2006 Ó Springer-Verlg 2006 Astrct The impct of incresed greenhouse gses (GHG) nd erosols concentrtions upon the West Africn monsoon (WAM) is investigted for the lte twenty-first century period using the Me te o-frnce ARPEGE-IFS high-resolution tmospheric model. Pertured ( ) nd current ( ) climtes re compred using the model in time-slice mode. The model is forced y glol se surfce tempertures provided y two trnsient scenrios performed with lowresolution coupled models nd y two GHG evolution scenrios, SRES-A2 nd SRES-B2. Compring to renlysis nd oserved dt sets, the model is le to reproduce relistic sesonl cycle of WAM despite cler underestimtion of the Africn Esterly Jet (AEJ) during the orel summer. Men temperture chnge indictes glol wrming over the continent (stronger over North nd South Afric). Simulted precipittion chnge t the end of the twenty-first century shows n increse in precipittion over Sudn-Shel linked to strong positive feedck with surfce evportion. Along Guine Gulf cost, rinfll regimes re driven y lrge-scle moisture dvection. Moreover, results show men precipittion decrese (increse) in the most (less) enhnced GHG tmosphere over this region. Modifiction of the sesonl hydrologicl cycle consists in rin increse during the monsoon onset. There is significnt increse in rinfll vrince over the Shel, which extends over the Guine cost region in the moderte emission scenrio. Enhnced precipittion over Shel is linked to lrge-scle circultion chnges, nmely wekening of the AEJ nd n intensifiction of the Tropicl Esterly Jet. C. Cminde (&) Æ L. Terry Æ E. Misonnve CERFACS, 42 Avenue G. Coriolis, Toulouse-Cedex, Frnce E-mil: cminde@cerfcs.fr Tel.: E-mil: terry@cerfcs.fr E-mil: Eric.Misonnve@cerfcs.fr 1 Introduction Since the pre-industril period, greenhouse gses (GHG) concentrtion hs exponentilly incresed. In prticulr, cron dioxide (CO 2 ) concentrtion hs incresed in the tmosphere from 280 ppmv t the eginning of the industrilistion (since 1850) to 367 ppmv t the end of the twenty-first century (IPCC 2001). Emissions from fossil fuel comustion, deforesttion re responsile for the oserved GHG emission enhncement, resulting in n increse of the het energy mount t the surfce nd thus y stronger vlues of glol oserved surfce tempertures (Follnd et l. 1992; Snter et l. 1996). According to demogrphy nd humn ctivities projection on the next century, glol wrming will continue incresing with sustntil consequences on climte (IPCC 2001). Climte over West Afric is dominted y the Monsoon system. This sesonl reversl of the wind flow (linked to the temperture difference etween se surfce nd continent) rings moisture inlnd from the tropicl Atlntic Ocen, nd is responsile for the mjor prt of precipittion during the orel summer (July-August- Septemer or JAS) over Shel nd Sudn. The grrin ctivities nd incresing locl popultion strongly depend on the riny seson return. For instnce, the persistent drought oserved during the lst few decdes hd drmtic grrin nd socil consequences. Severl studies hve een performed to determine the climte processes tht could potentilly explin this decdl precipittion deficit. Severl nlyses highlight the links existing etween vrious se surfce temperture (SST) ptterns nd precipittion vriility over West Afric (Plmer 1986; Follnd et l. 1986; Jnicot nd Fontine 1996). At internnul time scle, Plmer (1986) suggests mjor influence of El Nin o Southern oscilltion (ENSO) signture on precipittion over Shel. Rowell (2001) explins in detil the teleconnection mechnism etween the Tropicl Pcific nd West Afric. During wrm ENSO events (El Nin o), the interction of tmo-

2 532 C. Cminde et l.: West Africn monsoon response to greenhouse gs nd sulphte erosol spheric equtoril Kelvin wve from the Tropicl Pcific with n off-equtor Rossy wve (response to the zonl SST grdient etween the Est Indin Ocen nd West Pcific) reinforces lrge-scle susidence over Afric. It induces weker convective ctivity (direct effect) nd reduces monsoon flow (indirect effect) over West Afric. At decdl time scle, Lm (1978, ) nd Hstenrth (1984, 1990) highlight the influence of the so-clled tropicl Atlntic SST dipole on precipittion vriility over Shel. An impct of Indin Ocen SST nomlies on the multi-decdl trend contrsting wet nd dry yers over West Afric is lso suggested y Shinod nd Kwmur (1994) nd more recently y Bder nd Ltif (2003) nd Ginnini et l. (2003). At sesonl time scle, West Africn climte is very sensitive to the continentl surfce oundry conditions (Chrney nd Shukl 1981). Zheng nd Elthir (1998) document the impct of vegettion distriution on West Afric monsoon (WAM) system. They detil the impct of Guine cost deforesttion on surfce net rdition nd oundry lyer entropy reduction, leding to weker convection over the Shel. Modelling studies using pleoclimtic time-scle experiments suggest the possile expnsion of vegettion cover of the Shrn lnd re under incresed CO 2 concentrtions forcing which trigger incresed rinfll over West Afric through vegettiontmosphere feedcks (Clussen et l. 2002; Brovkin 2002). Aledo nd surfce roughness lso ply n importnt role (Lvl nd Picon 1986) on WAM. An enhncement of surfce ledo is ssocited with stronger Tropicl Esterly Jet (TEJ) nd weker esterlies t low ltitude, nd thus lower precipittion over West Afric. Recent studies hve een performed to highlight the impct of GHG emission scenrios on WAM (Hulme 1994; Mynrd et l. 2002; Johns et l. 2003). The results gree on significnt wrming over West Afric for the end of the twenty-first century. However, the results out the hydrologicl cycle modifiction re not s consistent. Mynrd et l. (2002) show n enhncement of precipittion over West nd North Afric [ccording to the Specil Report on Emission Scenrio (SRES-B2) from the Intergovernmentl Pnel on Climte Chnge (IPCC)]. Hulme (1994) suggests rinfll deficit over the northern sector of the continent (IPCC SRES-A2 emissions projections). In these studies, the numericl experiments used clssicl low-resolution CGCM (coupled generl circultion models), nd did not consider uncertinty sources due to the selected GHG emission scenrio nd the internl climte vriility. Peth et l. (2003) study distinguishes SST versus nthropogenic climte chnge signls upon West Africn rinfll. Their model (only forced y oserved SST) ws le to reproduce the oserved drought tendency following the 1960s over the Shelin re. The most interesting feture of this work is tht the dditionl effect of GHG forcing in the model induces significntly positive rinfll nomlies in recent yers, s oserved since the 1990s ove West Afric. This result is consistent with their climte chnge scenrio simultions predicting n intensifiction of the hydrologicl cycle over the Guine cost nd Shel region. They suggest tht the underlying physicl mechnism linking the rditive forcing to the monsoonl rinfll enhncement works vi wrming of the tropicl Atlntic Ocen. Although numericl experiments performed using CGCMs re the most common tools to study future climte chnges, the trditionl corse grid resolution is such tht it does not cpture the complex regionl precipittion nd temperture ptterns over the Africn continent. In ddition, climte chnge informtion in terms of impcts is required t finer sptil scles thn the clssic GCM grid one. One solution to otin enhnced regionl informtion is to use higher resolution tmosphere stnd lone GCMs to simulte specific periods of interest ( time-slices ) using ocenic oundry forcing from coupled GCM trnsient scenrio simultions (Cusch et l. 1995). This pproch llows circumventing the expensive cost of running the centurylong coupled simultions. In this study, two time slices hve een selected nmely (present climte) nd (pertured climte). The purpose of this work is first to determine the impct of different greenhouse gses nd sulphte emission scenrios on the West Africn climte, nd then to highlight the physicl processes responsile of the WAM system modifiction using the ARPEGE-IFS vrile resolution model in time-slice mode. In ddition this work is first ttempt to estimte the reltive uncertinties of WAM chnges due to oth SST oundries forcing (coming from low-resolution CGCM trnsient scenrio simultions) nd selected emission scenrios. Furthermore, West Africn precipittion vriility modifiction is investigted in the scenrios experiments. In Sect. 2, the model, the numericl experiments nd the oservtion dt set will e descried. A vlidtion of the simulted climte for the current period over West Afric will e done in Sect. 3. Section 4 first investigtes the impct of different GHG emission scenrios on WAM system, nd highlights the hydrologicl cycle modifiction nd the ssocited tmospheric dynmics chnge. Then, West Africn precipittion internnul vriility chnge is detiled. Results will e summrized in Sect. 5, nd future perspectives will e given. 2 Model nd numericl experiments description 2.1 Model detils The Me te o-frnce ARPEGE-IFS vrile resolution version used in this study is descried in detil in Gielin nd De que (2003). The model uses semi-lgrngin dvection scheme, two time level discretiztion nd 30 min step. The spectrl trunction is T106 nd hyrid coordintes (Simmons nd Burridge 1981) re used for the verticl discretiztion with 31 verticl levels. The

3 C. Cminde et l.: West Africn monsoon response to greenhouse gs nd sulphte erosol 533 rditive forcing, including the effect of 4 GHG (CO 2, CH 4,N 2 O nd CFC) in ddition to wter vpour nd ozone nd of five erosol types (lnd, se, urn, desert nd sulphtes), is clculted y the Morcrette scheme (Morcrette 1990). The stretched grid hs the pole locted in the Mediterrnen Se (40 N, 12 E) with stretching fctor of 3. The most refined horizontl resolution is while the corse grid covers most of the Pcific with resolution close to Note tht the resolution is reltively high over West nd North Afric (1 1 ). The ISBA (Douville et l. 2000) soil vegettion scheme is included. A four lyers het diffusion scheme is used, nd the soil hydrology representtion holds four reservoirs: cnopy interception, snow, shllow surfce nd root lyer reservoirs. 2.2 Experimentl design For the present climte ( ), n ensemle of three simultions hs een performed. The model is forced y monthly-oserved SST from Reynolds dt set (Smith et l. 1996) nd y oserved GHG concentrtion evolution, ech simultion differing y its initil tmospheric condition. The ensemle men for the present period will e referred to s CTRL in the following. For future climte ( ) two GHG emission scenrios (A2 nd B2) re exmined, estimted from different demogrphic, socil nd economic evolution fctors s given y IPCC. Low resolution coupled simultions forced y the SRES-A2 nd SRES-B2 GHG emission scenrios hve een first performed with two CGCMs, respectively, HdCM3 nd OPA-ARPEGE. For more detils out the trnsient simultions, the reder is invited to refer to Johns et l. (2003) for HdCM3 nd Royer et l. (2002) for OPA-ARPEGE. Then, monthly SST fields re creted y superimposing the oserved internnul vriility (Reynolds dt set) over the lst 30 yers on the men chnge of glol SST coming from the coupled model simultions over the period. Finlly, the vrile resolution ARPEGE model is forced y these monthly SST fields nd y the two GHG emission scenrios. Similrly to the present climte experimentl set-up n ensemle of three simultions is performed for ech configurtion (SRES-A2 with oundry ocenic forcing coming from HdCM3, SRES-B2 with SST forcing provided y OPA-ARPEGE). The ensemle men of these simultions will e referenced herefter in the text s HCA2 (MFB2). These two configurtions hve een selected to contrst strong forcing cse (A2 nd SST from Hd- CM3) versus wek one (B2 nd SST from OPA- ARPEGE). Differences in the SST forcing fields come from different scenrio nd different physicl prmeteriztions. In the following, the SST field forcing provided y coupled model M (HC for HdCM3, MF for OPA- ARPEGE) with GHG emission scenrio G (A2 or B2) will e referenced s SST (M, G). The men SST (HC, A2) chnge pttern exhiits strong wrming over Tropicl Atlntic Ocen wheres SST (MF, B2) pttern revels quite uniform wrming over this sin during orel summer. Cross-tmospheric experiments (with SST (HC, B2) nd SRES-B2 GHG emission scenrio, nd SST (MF, A2) with SRES-A2 scenrio) hve een performed to determine the reltive impct of different SST nd GHG emission scenrios forcing impct on WAM system. These tmospheric experiments will e respectively referenced in the text s MFA2 nd HCB2. The entire experimentl set-up is summrized in Tle Oservtion dt set In order to ssess the performnce of the model in reproducing the present climte severl oservtion dt sets re exmined. Simulted precipittion nd lnd surfce temperture over Afric re compred to the glol resolution CRU (Climtic Reserch Unit) dt set for the period ( grid/cru_ts_2_0.html, Mitchell et l. sumitted). Pressure nd dynmic fields will e compred to the NCEP-NCAR (Ntionl Centre of environmentl prediction) monthly renlysis dt set over the period. ( for more explntion, Klny et l. 1996) 3 Model vlidtion In this section, the model ility to reproduce the oserved climte over the present period will e considered through comprison etween one numericl experiment (DA9, Tle 1) chrcterising one possile relistion of the climtic system nd the NCEP-NCAR renlysis nd CRU dt sets. 3.1 Monsoon dynmic (U, V, P) The oserved men pressure pttern during JAS (Fig. 1) depicts well-known lrge-scle structures: the Azores (over North Atlntic Ocen) nd Ste Helene (South Atlntic) highs nd low-pressure res over Shr nd Brzilin Nordeste. The model is le to reproduce the min pressure structures (Fig. 1); nevertheless, northest shift nd n underestimtion of the estwrd extension of the Azores nticyclone core, with stronger vlues of pressure over Ste Helene cn e noticed. At 925 hp, the oserved wind field (Fig. 1) during JAS exhiits the estlishment of the monsoon regime over West Afric. A strong southwesterly flow coming from the Tropicl Atlntic enters the continent (Gulf of

4 534 C. Cminde et l.: West Africn monsoon response to greenhouse gs nd sulphte erosol Tle 1 Experimentl set-up description Ensemle men experiment Experiment(s) SST forcing GHG emission scenrio Period HCA2 3 (DE6-DE7-DE8) HdCM3 SRES-A MFB2 3(DC9-DC2-DC7) OPA-ARPEGE SRES-B MFA2 1 (DE9) OPA-ARPEGE SRES-A HCB2 1 (DE5) HdCM3 SRES-B CTRL 3 (DA9-DE3-DE4) Reynolds dt-set Estimted over the lst 40 yers Guine) nd weker dry northesterly Hrmttn flow comes from the Shr. The model (Fig. 1) underestimtes the southwesterly monsoon flow entering West Afric. Note lso tht simulted wind field vlues re weker over the continent (underestimtion of the Hrmttn flow). Moreover, the wind direction is well reproduced y the model over the continent nd Gulf of Guine. Nevertheless, the trde wind convergence re is distriuted long 8 N in NCEP wheres the model depicts tilted convergence zone. Simulted wind fields centred on the Azores nticyclone show divergent circultion wheres oserved vlues exhiit more rottionl configurtion. The TEJ (ner 200 hp) is therml wind linked to the temperture difference existing etween high Tietn plteu nd the Indin Ocen. The min wind core is locted over Indin Ocen nd extends westwrd over the Shel (Fig. 2). The TEJ core is underestimted y the model (y 50%) nd shifted to the southest. Its westwrd extension over West Afric is weker (underestimted y 60%) in the numericl experiments (Fig. 2). Fig. 1 Men se level pressure (shding) in hp with superimposed men wind vector field (m/s) t 925 hp (JAS ) for NCEP renlysis () nd DA9 ()

5 C. Cminde et l.: West Africn monsoon response to greenhouse gs nd sulphte erosol 535 c d Fig. 2 Men zonl wind (JAS ) for oservtions from NCEP t 200 hp nd c 600 hp, nd DA9 simultion t 200 hp nd d 600 hp The Africn esterly jet (AEJ) is moderte jet strem (centred on 15 N over the Shelin re) cused y the therml contrst existing etween West Afric continent nd the Tropicl Atlntic ocen (Fig. 2c). It plys dominnt role in orgnizing the mesoscle convective systems nd generting synoptic-scle Africn esterly wves. The northwrd sesonl migrtion of the AEJ core during orel summer (Grist nd Nicholson 2001) is well simulted (not shown). Nevertheless, the model underestimtes the extension nd the AEJ intensity core y 60% (Fig. 2d). 3.2 Precipittion nd temperture The oserved precipittion pttern is distriuted (Fig. 3) on wide ltitude nd from 5 S to 18 N [linked to the inter tropicl convergence zone (ITCZ) position during JAS], nd high precipittion res re confined over Guine cost (from Senegl to Lieri), Nigeri cost nd Ethiopi (denoting regionl effect of high Ethiopin plteu on rinfll). The precipittion pttern is well reproduced in the present climte simultion (Fig. 3) despite locl differences (underestimtion of strong precipittion res long the Guine cost nd Niger). The shpe of the simulted ITCZ is comprle to oservtions ut extends slightly too fr to the North (this GCM is hs lredy een shown for other models y Moron 2003). Mximum tempertures cover the entire Shr nd Mghre while minim re oserved over South Afric (Fig. 4). Simulted temperture pttern (Fig. 4) grees resonly with oservtions despite smll cold is over West Afric. In conclusion, results show tht the ARPEGE-IFS model is le to cpture the min fetures of the WAM system during JAS, in spite of the significnt is concerning the AEJ representtion. 4 Impct of different greenhouse gses emission scenrios on WAM system The purpose of this prt is to document the modifiction of the simulted WAM system in response to different scenrios for greenhouse gses nd sulfte emissions t the end of the twenty-first century. 4.1 Men temperture chnges The distriution of surfce ir temperture difference in JAS etween the control (CTRL) nd the two scenrios experiments (HCA2, MFB2) is shown in Fig. 5. Glolly, wrming is stronger over continent thn over se (not shown) due to the strong therml inerti of the ocens, nd y quick response of lnd tempertures to higher longwve rdition forcing in n enhnced GHG tmosphere. These ptterns exhiit glol enhncement of temperture over Afric, with mxim locted over Mghre, Egypt nd South Afric region. The temperture increse is weker over Shel Sudn, in greement with the modifiction of long-wve surfce het flux

6 536 C. Cminde et l.: West Africn monsoon response to greenhouse gs nd sulphte erosol Fig. 3 Averge precipittion (mm/dy) in JAS ( ) from oservtion from the CRUTS2 dt set nd DA9 numericl experiment. Grid ox 1 defines the Shelin re (10 N 20 N, 20 W 15 E), ox 2 the Sudn region (10 N 20 N, 15 E 40 E), nd grid ox 3 delimits the Guine cost re (5 N 10 N, 8 W 8 E) sptil distriution (not shown). Note tht the temperture chnge signl is sufficiently strong ll over the Africn continent to e significnt t the 99% level s estimted y t test (not shown). Contrsting HCA2 (Fig. 5) nd MFB2 (Fig. 5) minus CTRL Africn surfce tempertures ptterns, stronger vlues cn e seen in the most enhnced GHG tmosphere excepted over South Afric re nd Egypt where there s no striking regionl differences. Tempertures predicted y HCA2 experiment re wrmer thn those simulted y MFB2 over Sudn nd Shel (y 1 C) during the orel summer. 4.2 Hydrologicl cycle Men chnges Whtever the GHG emission scenrio nd the SST forcing, the men precipittion difference over Afric during JAS (Fig. 6, left column) shows glol enhncement of the hydrologicl cycle over Shel nd Sudn. Significnt weker precipittion is oserved over Cmeroon, Congo nd Gon. This result is consistent with studies done y Mynrd et l (2002). The simulted rinfll increse over the Shelin region (0.4 mm/dy) is the sme in the two experiments. Precipittion chnge ptterns over Gulf of Guine region seem to e significntly different depending on the selected emission scenrio nd SST forcing. Reduced precipittion is depicted over Guine cost (Fig. 6) in HCA2, wheres ccording to MFB2 (Fig. 6) there is no significnt precipittion difference. Rinfll enhncement over Sudn is slightly stronger in MFB2 numericl experiment y 0.2 mm/dy. The HCA2 precipittion pttern might e interpreted s northwrd shift of the ITCZ s chrcterised y the men chnge in outgoing long-wve rdition (OLR, not shown).

7 C. Cminde et l.: West Africn monsoon response to greenhouse gs nd sulphte erosol 537 Fig. 4 Men lnd surfce temperture ( C) in JAS ( ) from oservtion (CRUTS2) nd DA9 experiment Precipittion sesonl cycle Figure 7 shows the men precipittion sesonl cycle over three regions of interest: the Shel (Fig. 3, ox 1), Sudn (ox 2) nd the Gulf of Guine (ox 3). Over Sudn nd Shel, oserved mximum precipittion occurs during JAS, with pek structure corresponding to the most northwrd position reched y the ITCZ during its migrtion. The oserved men sesonl cycle of precipittion over Guine shows two mxim occurring during June nd Septemer with flt profile during the estlishment of the monsoon regime (JAS). The fst rinfll increse (Mrch June) corresponds to the northwrd migrtion of the ITCZ nd the second pek (Septemer Octoer) is linked to its retret. The sesonl cycle of simulted precipittion grees with oservtions over Shel, Sudn nd Guine. Nevertheless, there is slight overestimtion of rinfll during the pre-onset (My June) nd the collpse (Octoer Novemer) of the Africn monsoon whtever the loction. Note lso tht precipittion is overestimted (y 0.5 mm/dy) over the Sudn. Different precipittion regimes hve een previously highlighted: increse of precipittion over Shel nd Sudn (more mrked with MFB2), nd rinfll deficit (no significnt difference) over the Gulf of Guine in HCA2 (MFB2). Whtever scenrio nd SST forcing considered, slight intensifiction of the sesonl hydrologicl cycle during the onset (My June) nd the estlishment (JAS) of the West Africn monsoon regime is otined over Shel nd Sudn. The simulted rinfll enhncement over Shel is similr in the two scenrio experiments. Note lso tht the rinfll increse concerning the Sudn region is stronger in MFB2 during July August. Over the Guinen re, n increse of precipittion is shown during the onset (first pek during My June) of the monsoon (more intense ccording HCA2). During JAS nd the collpse (Second pek, Septemer Octoer) of the monsoon regime, there is no significnt chnge ccording to the MFB2 experiment wheres HCA2 men sesonl hydrologicl cycle simultes slight rinfll decrese over Guine (consistent with Fig. 6) Physicl process ssocited Atmospheric wter vpour hs short life-cycle, out 10 dys stored in the tmospheric column. Furthermore

8 538 C. Cminde et l.: West Africn monsoon response to greenhouse gs nd sulphte erosol Fig. 5 Men lnd surfce temperture (K) chnges, HCA2-CTRL, MFB2 -CTRL during orel summer (JAS) wter vpour content udget is lnced t glol scle. Thus, considering the time-scle we focus on (sesonl men), to chrcterise the process responsile for precipittion chnge in the experiments, it will e considered tht the smll vritions of tmospheric wter content re negligile. Thus, the difference etween precipittion (P) nd evportion (E) eing equl to the tmospheric wter content t given instnt, is lnced y tmospheric moisture convergence. Contrsting E nomlies ptterns (Fig. 6c, d) etween the two experiments, results show n enhncement of surfce evportion over Sudn nd Shel in greement with n increse of precipittion over this re (positive feedck). The evportion increse is greter over Sudn thn over Shelin region. Nevertheless, the enhncement of surfce evportion over Sudn-Shel is weker in HCA2 (Fig. 6c) compred to MFB2 (with similr pttern, Fig. 6d). A wekening of surfce evportion is simulted over Cmeroon, Congo nd Gon (more mrked in HCA2 CTRL). Figure 8 shows the nomlous verticlly integrted moisture convergence (P E) with superimposed lowlevel (integrted etween 1000 nd 850 hp) specific humidity dvection chrcterizing the monsoon flow over West Afric. Importnt differences etween HCA2 nd MFB2 experiments cn e seen long the Gulf of Guine nd Shel. In HCA2 (Fig. 8), strong significnt vlues of moisture divergence re depicted long the Guine cost with weker monsoon flow entering the continent, nd no significnt differences over Shel nd Sudn. This pttern corresponds to northwrd shift of the ITCZ nd is consistent with the precipittion pttern (Fig. 6). Despite the wekening of the monsoon flow coming from Guine gulf, low-level humidity trnsport coming from Tropicl Atlntic Ocen is stronger over the shelin re. The men chnges in MFB2 (Fig. 8) show qusiopposite results ove the Gulf of Guine: stronger lowlevel monsoon flow entering the continent with strong enhncement of verticlly integrted moisture convergence. Over Sudn, the enhnced monsoon rinfll is ssocited with strong increse in surfce evportion. In

9 C. Cminde et l.: West Africn monsoon response to greenhouse gs nd sulphte erosol 539 Fig. 6 Left: Precipittion nomlies (mm/dy) etween HCA2 (), MFB2 () scenrio prediction ( ) nd control experiments (CTRL) during JAS. Right: Idem for evportion nomlies (mm/ dy), HCA2-CTRL (c), MFB2 CTRL (d). Bold line res denote response res with the significnce t the 99% level s estimted y t test c d HCA2 ( ) MFB2 ( ) CTRL ( ) CRUTS2 ( ) Shel (10N 20N, 20W 15E) 0.0 Jn Fe Mr Apr My Jun Jul Aug Sep Oct Nov Dec Sudn (10N 20N, 15E 40E) 0.0 Jn Fe Mr Apr My Jun Jul Aug Sep Oct Nov Dec Guine (5N 10N, 8W 8E) 0.0 Jn Fe Mr Apr My Jun Jul Aug Sep Oct Nov Dec Fig. 7 Men sesonl cycle of precipittion (mm/dy) over Shel, Sudn nd Gulf of Guine pertured climte (whtever the scenrio experiment selected) the physicl process responsile of the precipittion reinforcement ove Sudn is strong locl positive feedck of evportion, low-lyer humidity dvection plying minor role. Aove Shel, the convective ctivity is more intense in n enhnced GHG tmosphere whtever the scenrio considered. The summer monsoon rinfll is enhnced due to n increse of moisture dvected from Tropicl Atlntic Ocen nd lso to enhnced surfce evportion. Over Guine cost, the precipittion increse (decrese) is primry linked to n increse of moisture dvected y the lrgescle flow, locl evportion nomlies eing negligile. The climtic precipittion signl is very different (weker/stronger monsoon flow) etween the two experiments selected here ove the Guinen re. These results re in greement with studies of Lm (1983). Gong nd Elthir (1996) who chrcterized evportion over Tropicl Atlntic s primry moisture source for West Afric Monsoon. Considering HCA2 wind pttern, meridin dipole structure of the 600 hp zonl wind exhiits n increse over Equtoril Guine with decrese over the South Atlntic sin long the Africn cost (Fig. 9). A strong increse of the zonl wind t 200 hp over South Afric nd decrese over the South Indin sin cn e noticed too (Fig. 9c). In MFB2 experiment, wekening of the south prt of the AEJ core with northwrd extension (Fig. 9) hs een chrcterized. A westwrd extension of the TEJ (Fig. 9d) nd its reinforcement cn e noticed too. This chnge (wekening of the AEJ nd intensifiction of the TEJ) hs een linked to the precipittion increse over Shel in previous studies (Kidson et l. 1977). MFB2 ptterns re in greement with studies done y Mynrd et l. (2002) who chrcterised more vigorous hydro-

10 540 C. Cminde et l.: West Africn monsoon response to greenhouse gs nd sulphte erosol Fig. 8 Verticlly integrted moisture convergence nomlies (P E) in mm/dy (shding) for JAS with superimposed low lyer (integrted etween 925 nd 850 hp) reltive humidity dvection in g/kg m/s (vector field). HCA2-CTRL (), MFB2-CTRL (). Bold line res denote response res with the significnce t the 99% level s estimted y t test Recent studies sed on pleoclimtic dt show tht internnul nd decdl scle climte vriility my experience chnge s result of nturl longer scle vriility nd nthropogenic greenhouse forcing. Considering the experimentl setup of this study, ttention will e given on the dditionl impct of the men SST chnge (the oserved internnul SST vriility eing superimposed in the future climte simultions) nd the GHG emission scenrio upon West Africn precipittion vriility. The present climte ensemle simultion (CTRL) is le to resonly reproduce the oserved rinfll internnul vriility over Shel ut does not cpture the multi-decdl trend contrsting wet ( ) nd dry ( ) yers over West Afric nd especilly over the Shel region (not shown). Moreover, the time period eing reltively short (30 yers) in the GHG emission scenrio experiments the min focus will e on internnul vriility chnges. The precipittion dt is fil logicl cycle, n enhncement of the monsoon flow, wekening of the AEJ nd n intensifiction of Southern Hemisphere Hdley cell using low resolution coupled experiments with SRES-B2 GHG emission scenrio. The men dynmic chnge in HCA2 cnnot e linked to precipittion chnge over West Afric. As summry, whtever experiment selected, n increse of locl surfce evportion, relted to the surfce wrming is responsile of the summer rinfll increse over Sudn. The dvected humidity flow coming from Tropicl Atlntic Ocen is responsile of precipittion regime over the Gulf of Guine. It is consistent with the fct tht in HCA2 (MFB2) experiment, the wekening (strengthening) of the incoming monsoon flow is primry responsile of the rinfll deficit (excedent) over Guine cost. Aove Shel, oth surfce evportion feedck on precipittion nd enhnced humidity trnsport re responsile of the simulted rinfll increse. MFB2 experiment is chrcterized y wekening (strengthening) of the AEJ (TEJ) while no significnt dynmicl chnges pper in HCA Internnul vriility chnges

11 C. Cminde et l.: West Africn monsoon response to greenhouse gs nd sulphte erosol 541 c d Fig. 9 Men zonl wind difference (m/s) etween HCA2 (top), MFB2 (ottom) nd CTRL t 600 (left column) nd 200 hp (right column) during orel summer (JAS). Bold line res denote response res with the significnce t the 99% level s estimted y t test tered using highpss filter (with 8-yer cut-off) in order to study vriility chnges. Reltive vrince difference is displyed etween ech scenrio nd the control ensemle on Fig. 10. Results show significnt vrince increse etween the scenrio experiments nd the control simultions over West Afric (Fig. 10, ). Focusing on the (HC, A2) ensemle (Fig. 10), lrge vrince increse (from 10 to 40%) is displyed over the shelin re, wheres no significnt differences cn e seen over the Guinen cost region. Moreover mximum chnges (30 50%) re loclized etween 10 N nd 18 N. Considering the (MF, B2) ensemle (Fig. 10), the vrince increse is stronger thn in the (HC, A2) ensemle nd extends further south over the Guinen re (10 20%). Mximum chnges (30 40%) re displyed over the nd 5 15 N. A cross section of zonl men vrince summrizes ll these results (Fig. 10c). Wheres the (MF, B2) vrince distriution is clerly wider nd incresed over the whole West Africn domin compred to the control ensemle, tht of (HC, A2) does not significntly differ from the present climte one over the Guinen region (5 10 N), nd significntly increses over the Shel. A principl component nlysis (PCA) to filtered summer precipittion (for ech ensemle) over West Afric (0 N to 20 N, 16 W to 20 E) is used (Fig. 11) to nlyse these vriility chnges. The nlysis relies only upon the first precipittion mode ecuse no significnt chnges re found for the second one etween the control nd the scenrio experiments. The first precipittion mode exhiits positive rinfll nomlies long the Guine cost in the control ensemle (Fig. 11). This mode hs een well documented in previous work (Ginnini et l. 2003) nd is clerly ssocited with wrm SST nomlies in the southestern tropicl Atlntic Ocen. In (MF, B2) ensemle, the EOF sptil structure clerly extends northwrd with incresed mplitude (Fig. 11). Considering (HC, A2) the distriution of the first precipittion mode is clerly shifted northwrd, with centres of ction corresponding to mximum vrince chnges (Figs. 10, 11c). The first mode explined vrince significntly increses in the scenrio ensemle (30%) compred to the control one (20%). While the (MF, B2) SST pttern shows homogeneous wrming over the Tropicl Atlntic sin (Fig. 12), (HC, A2) revels n increse of the crossequtoril SST grdient (Fig. 12). Strong wrm SST nomlies (2.5 C) cn e seen off Muritnin cost, nd weker ones (1.2 C) long the equtoril Atlntic cold tongue nd Angol cost (Fig. 12). Decdl vritions of the Atlntic cross-equtoril SST grdient re

12 542 C. Cminde et l.: West Africn monsoon response to greenhouse gs nd sulphte erosol c Precipittion vrince (mm/dy squred) CTL HCA2 MFB N 10N 15N 20N 25N 30N Ltitude (degrees) Fig. 10 Upper pnel: Reltive chnge in precipittion internnul vrince (%) for (HC, A2) (Fig. 10) nd (MF, B2) (Fig. 10). Lower pnel: Zonl men precipittion vrince (mm/dy squred) verged etween 15 W 10 E for ech ensemle linked to meridionl shift of the ITCZ nd thus to precipittion regimes over Shel nd Guine (Hstenrth 1990). This SST pttern is ssocited with dipole contrsting positive (negtive) rinfll nomlies over Shel (Guine). The impct of men SST difference on West Africn precipittion vriility could e interpreted s follows. The increse of the cross-equtoril Atlntic SST grdient shifts the men position of the ITCZ northwrd, thus cusing the rinfll vrince to increse over Shel in the A2 ensemle (s confirmed y EOF nd vrince nlysis). Furthermore, there is strengthening of the Atlntic SST forcing influence upon the rinfll internnul vriility in the (HC, A2) scenrio (not shown). Considering (MF, B2) ensemle, the increse of filtered rinfll vrince is extended over the whole West Africn domin, while the men SST wrming is quite uniform over the Atlntic sin. An increse in the men monsoon flow is depicted in (MF, B2) wheres enhnced divergence is shown over Guine in (HC, A2) (Fig. 8). This dynmic configurtion is consistent with n increse (no significnt modifiction) of rinfll vriility in B2 (A2) ensemle over the Guinen re. Moreover, normlized rinfll histogrm hs een performed for July to Septemer monthly mens, over the period for ech scenrio ensemle nd over for the control one. The rinfll distriution over Shel (Fig. 11d) is clerly shifted in (HC, A2) compred to the control ensemle. There is significnt increse of intense monthly precipittion nd of the men. (MF, B2) histogrm over the shelin re exhiits wider structure thn in the control ensemle. Rinfll men nd extremes re significntly incresed. Wheres (HC, A2) distriution clerly exhiits trns-

13 C. Cminde et l.: West Africn monsoon response to greenhouse gs nd sulphte erosol 543 d e c Fig. 11 First EOF mode of High-pss filtered precipittion (JAS) over West Afric. The first leding mode is displyed for the CTRL ensemle () over the period, (MF, B2) () nd (HC, A2) (c) ensemle over the period. Normlized rinfll histogrm computed from July to Septemer montly mens for ech scenrio simultions type over Shel (d) nd Guine (e)

14 544 C. Cminde et l.: West Africn monsoon response to greenhouse gs nd sulphte erosol Fig. 12 Men se surfce temperture chnges (K) with respect to oserved SST for (HC, A2) () nd (MF, B2) () during JAS ltion compred to the control ensemle, (MF, B2) distriution chnges in the shpe. Over Guine (Fig. 11e) there is decrese of the men nd intense rinfll in (HC, A2) nd no cler modifiction of the histogrm in (MF, B2) ensemle. All these results re consistent with men precipittion chnges for ech scenrio s descried elow in Sect Deling with hydrologicl cycle vriility modifiction relevnt to climte chnge is not n esy tsk. Conceptul studies concerning glol chnges of rinfll extremes in n incresed GHG tmosphere (Trenerth 1999) show tht the surfce wrming cused y incresed downwelling infrred rdition enhnces oth evportion nd the wter-holding cpcity of the tmosphere. As consequence, the tmospheric moisture content increses, nd thus, extreme events re fvourized (ll precipittion system feeding on the ville moisture in the tmosphere). Considering the scenrio experiments of this study, significnt increse of men nd extremes evportion nd tmospheric moisture content is reveled over West Afric (not shown). Moreover, the tmospheric moisture content nd evportion increse is stronger in the most enhnced GHG tmosphere (HC, A2). Over the shelin region, ll these results re consistent with Trenerth s conceptul study relting rinfll extremes increse nd climte chnge. Nevertheless, decrese of the men nd intense rinfll hs een highlighted over the Guinen re, suggesting tht other mechnisms influence regionl scle chnges Sensitivity nlysis To ssess the tmospheric model sensitivity to different SST nd GHG forcing, two dditionl experiments re considered. One simultion with the scenrio B2 (A2) nd SST fields provided y HdCM3 (OPA-ARPEGE) model hs een performed nd will e referenced s HCB2 (MFA2) (Tle 1). Depending on which GHG emission scenrio considered (A2 or B2), the hydrologicl cycle nomlies response of ARPEGE-IFS tmospheric model forced y the SSTs coming from HdCM3 (HC) coupled model exhiits the sme pttern: slight increse of precipittion over Sudn-Shel nd weker precipittion over the Gulf of Guine (Fig. 13). Wek significnt precipittion differences cn e shown over Senegl, Ethiopi nd Guine for MFA2 minus MFB2 pttern (Fig. 13). Figure 13c, d shows the sensitivity of the model results to the choice of SST

15 C. Cminde et l.: West Africn monsoon response to greenhouse gs nd sulphte erosol 545 c d Fig. 13 Precipittions nomlies (mm/dy) during JAS etween the different cross-forced experiments. Bold line res denote response res with the significnce t the 95% level s estimted y t test forcing for given emission scenrio. Over Shel nd Sudn, the impct of OPA-ARPEGE SST forcing on the summer rinfll increse is stronger (Fig. 13c). Over the Gulf of Guine nd Cmeroon the precipittion decrese linked to HdCM3 SST forcing (Fig. 13d) is shown (northwrd shift of the ITCZ). For given coupled model, the men SST chnge sptil pttern is similr despite the choice of the GHG emission scenrio (A2 or B2, not shown). The only impct of the scenrio is in term of mgnitude (stronger tempertures in A2, weker ones in B2). In this study, SST (HC, A2) (Fig. 12) pttern is coherent with northwrd shift of the ITCZ (see previous section). It leds to the precipittion increse over Shel nd the decrese of the hydrologicl cycle over Guine. The increse of precipittion over Sudn-Shel for MFA2 nd MFB2 experiments cn e primry linked to locl process (evportion enhncement), SST (MF, B2) wrming eing homogeneous over Atlntic sin. To conclude, the simulted hydrologicl cycle sensitivity to SST forcing coming from the choice of the coupled model is stronger thn the emission scenrio selected. At multi-decdl time scles, previous work highlight lrge-scle SST pttern s the min fctor tht drives precipittion trend over West Afric (Ginnini et l. 2003). The fct tht climte vriility over Afric or more generlly within the ltitude nd is primry linked to SST ptterns is coherent with these results. 5 Conclusion The impct of nthropogenic emission scenrio on WAM system is still controversil. If mny studies gree on glol wrming over Afric, there is not still consensus out precipittion chnge nd hydrologicl cycle modifiction, prticulrly t regionl scle. In this study, chnges in the Africn monsoon system t the end of the twenty-first century hve een investigted using the ARPEGE-IFS vrile resolution model, for two GHG emission evolution scenrios (SRES-A2 / SRES-B2) nd two SST forcings. Results show significnt wrming over the Africn continent, especilly over Mghre nd South Afric, the temperture increse eing stronger with SRES-A2 scenrio. Chnges in precipittion ssocited with the glol wrming re chrcterized y n increse of rinfll over Shel Sudn (with strong increse in surfce evportion), nd decrese (no significnt differences) of precipittion long the Guine Gulf considering HCA2 (MFB2) numericl experiments. Weker monsoon flow (with strong moisture divergence) entering the continent, nd slight northwrd shift of the ITCZ hs een highlighted with HCA2 experiment wheres MFB2 pttern depicts n enhncement of precipittion over Shel with stronger monsoon flow (ssocited with

16 546 C. Cminde et l.: West Africn monsoon response to greenhouse gs nd sulphte erosol strong moisture convergence). The ltter result is consistent with wekening of the AEJ nd strengthening of the TEJ. West Africn precipittion vriility chnges in the scenrio experiments hve een highlighted. A significnt increse of the rinfll vrince over the Shel is shown in the (HC, A2) ensemle (consistent with the men increse of the Atlntic crossequtoril SST grdient, nd ssocited northwrd shift of the ITCZ). In (MF, B2), the precipittion vrince increses over the whole West Africn domin. Impct of glol wrming on the sesonl cycle of precipittion is chrcterized y n increse of rin events during the onset of the West Africn monsoon. Note tht precipittion intr-sesonl vriility is not tking into ccount in this work. Sultn et l. (2003) hve shown tht rinfll nd convection over West Afric re significntly modulted t two intr-sesonl time-scles, leding to vritions of more thn 30% of the sesonl signl. Results show tht uncertinties linked to the choice of the CGCM tht provides the SST forcing re predominnt compred to the direct impct of the emission scenrio in this time-slice pproch. In this study, vegettion surfce chrcteristics re fixed to the current period vlues over the period Bsed on Zheng nd Elthir (1998) who show the importnce of Guine cost deforesttion on precipittion regimes over West Afric, future studies must focus on this spect, performing new experiments with fullcoupled ocen tmosphere iosphere experiments. Numericl experiments hve een crried out with the stretching pole centred in Mediterrnen Se. Although resolution is reltively high on the West Africn region to e confident in the results, it would e interesting to select the pole of stretching over the Guinen gulf centre t the equtor. Recently, Mynrd nd Royer (2004) focus on the impct of lnd cover chnge on greenhouse-wrmed Africn climte. 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