On the origin of equatorial Atlantic biases in coupled general circulation models

Transcription

1 Clim Dyn (2008) 31: DOI /s z On the origin of equtoril Atlntic ises in coupled generl circultion models Ingo Richter Æ Shng-Ping Xie Received: 21 Septemer 2007 / Accepted: 4 Jnury 2008 / Pulished online: 30 Jnury 2008 Ó Springer-Verlg 2008 Astrct Mny coupled ocen tmosphere generl circultion models (GCMs) suffer serious ises in the tropicl Atlntic including southwrd shift of the intertropicl convergence zone (ITCZ) in the nnul men, westerly is in equtoril surfce winds, nd filure to reproduce the estern equtoril cold tongue in orel summer. The present study exmines n ensemle of coupled GCMs nd their uncoupled tmospheric component to identify common sources of error. It is found tht the westerly wind is lso exists in the tmospheric GCMs forced with oserved se surfce temperture, ut only in orel spring. During this time se-level pressure is nomlously high (low) in the western (estern) equtoril Atlntic, which ppers to e relted to deficient (excessive) precipittion over tropicl South Americ (Afric). In coupled simultions, this westerly is leds to deepening of the thermocline in the est, which prevents the equtoril cold tongue from developing in orel summer. Thus reducing tmospheric model errors during orel spring my led to improved coupled simultions of tropicl Atlntic climte. I. Richter (&) S.-P. Xie Interntionl Pcific Reserch Center, University of Hwii t Mno, 2525 Corre Rod, Honolulu, HI 96822, USA e-mil: irichter@hwii.edu S.-P. Xie Deprtment of Meteorology, University of Hwii t Mno, Honolulu, HI, USA 1 Introduction Tropicl Atlntic climte displys lrge est-west nd north-south symmetries, with cold tongue developing from the cost of southern Afric towrd the west long the equtor, nd zonl nd of high se surfce temperture (SST) north of the equtor tht nchors the intertropicl convergence zone (ITCZ). The southest trde winds previl on the equtor yer-round, colliding with the northest trdes long the ITCZ. During orel summer, these symmetries re most pronounced while during spring, the SST field is nerly symmetricl out the equtor with uniformly wrm SST covering the entire equtoril Atlntic. The influence of djcent continents is evident: in My, strong southerly surfce winds in the Gulf of Guine ssocited with the onset of the West Africn monsoon re instrumentl in inititing the sesonl development of the equtoril cold tongue, y inducing upwelling long the southern Africn cost nd in the open ocen just south of the equtor (Mitchell nd Wllce 1992; Okumur nd Xie 2004). The relistic simultion of tropicl Atlntic climte remins chllenge for stte-of-the-rt coupled ocen tmosphere generl circultion models (GCMs). Most models fil to reproduce the oserved estern equtoril cold tongue in orel summer, nd mny even plce cool SSTs in the western sin where wrm pool is oserved in nture. In the nnul men this is mnifested s reversl of the climtologicl SST grdient long the equtor s shown in n erlier coupled model intercomprison y Dvey et l. (2002). Despite sustntil model development in the intervening yers, the sic prolem remins in coupled GCMs, such s those prticipting in the Intergovernmentl Pnel on Climte Chnge (IPCC) Fourth Assessment Report (AR4). Figure 1 compres

2 588 I. Richter, S.-P. Xie: On the origin of equtoril Atlntic ises in coupled generl circultion models June July August (JJA) SST in simultions nd oservtions, with the former represented y n AR4 multi-model ensemle men from their Climte of the 20th Century runs. It is ovious from Fig. 1 tht the models fil to cpture the cold tongue in the estern equtoril Atlntic; insted, reltively cool SSTs re found in the western equtoril Atlntic. This constitutes reversl of the climtologicl SST grdient s reported y Dvey et l. (2002). Furthermore, coupled models produce too much precipittion south of the equtor during Mrch April My (MAM), which leds to spurious doule-itcz in the nnul men (Breugem et l. 2006; Deser et l. 2006; Stockdle et l. 2006). This doule-itcz syndrome lso c os CMIP CMIP - os Fig. 1 JJA SST ( C) for ICOADS oservtions, the CMIP model ensemle-men, nd c the difference -. Shding in the difference plot (c) indictes positive vlues persists in coupled simultions of the tropicl Pcific climte (Mechoso et l. 1995; de Szoeke nd Xie 2008). In ddition to the wrm is in the estern equtoril region, Fig. 1 shows n even more pronounced wrm is to the south long the cost of Southwest Afric. This type of error is lso seen in the southest Pcific nd is likely due to the under-representtion of strtocumulus nd costl upwelling (M et l. 1996; Lrge nd Dnsoglu 2006; Hung et l. 2007). The southest Atlntic is lso fetures prominently in sesonl forecst models tht do not suffer from serious equtoril ises, such s the NCEP Coupled Forecst System (CFS; Hung et l. 2007), suggesting tht the two types of ises my e distinct in their origins. Since modes of tropicl Atlntic vriility involve ocen tmospheric feedcks tht re dependent on the men stte (Xie nd Crton 2004; Chng et l. 2006; Keenlyside nd Ltif 2007), the filure of coupled GCMs to simulte relistic men stte odes ill for their ility to predict sesonl-to-internnul SST nd precipittion nomlies (Repelli nd Nore 2004; Stockdle et l. 2006). It lso clls into question the crediility of long-term projections for the region under climte forcing scenrios (Breugem et l. 2006). It is therefore importnt to investigte the cuses of GCM ises in the tropicl Atlntic nd to work towrd their elimintion. While coupled ocen tmosphere feedcks certinly ply role in producing the ove ises, previous studies hve shown tht even uncoupled tmospheric GCMs (AGCMs) with prescried climtologicl SSTs suffer significnt ises. Bisutti et l. (2006) find tht the southwrd shift of the ITCZ is common prolem in tmospheric GCMs nd tht it is often ccompnied y tendency to plce the rinfll over the SST mximum wheres oservtions plce it over the surfce convergence mximum. Chng et l. (2007) show tht the Ntionl Center for Atmospheric Reserch (NCAR) Community Atmospheric Model (CAM) underestimtes the equtoril esterlies, contriuting significntly to the ises when coupled with n ocen GCM (see lso DeWitt 2005). Richter et l. (2008) show tht tmospheric GCM precipittion ises over tropicl South Americ nd Afric hve the potentil to ffect the simultion of the South Atlntic nticyclone. In coupled model such ises in the southest trdes force n SST response, leding to sin-wide djustments in the ocen nd tmosphere through the wind-evportion-sst (WES) feedck (Xie 1996). The present study investigtes the sources of coupled model ises over the equtoril Atlntic, with focus on the lck of the estern cold tongue on the equtor nd the doule ITCZ in the models. In prticulr, we explore the hypothesis tht the surfce wind nd precipittion ises in coupled GCMs originte in their tmospheric component

3 I. Richter, S.-P. Xie: On the origin of equtoril Atlntic ises in coupled generl circultion models 589 nd re mplified y coupled ocen tmosphere feedcks. This hypothesis is tested y compring severl pirs of coupled nd tmosphere-only GCM simultions performed for the IPCC AR4. Our ojectives re to (1) identify common prolems in the coupled GCMs, nd (2) exmine to wht extent these errors cn e trced ck to shortcomings in the tmospheric component. While severl studies hve ddressed tropicl Atlntic ises, our pproch is unique in its utiliztion of multimodel dtse tht includes oth coupled GCMs nd their tmospheric component. This llows us to identify common error ptterns, their tmospheric sources, nd their mplifiction y coupled processes. Furthermore, while most studies hve focused on the nnul men or orel summer, our study exmines the sesonl evolution of errors nd identifies MAM s the crucil seson. The rest of the pper is orgnized s follows. Section 2 introduces the models nd their simultions. In Sects. 3 nd 4 we nlyze the models simultion of the equtoril cold tongue nd meridionl symmetry, respectively. Section 5 gives the summry of our results nd discusses generl implictions. 2 Models We exmine IPCC AR4 Climte of the 20th Century (lso known s the Coupled Model Intercomprison Project or CMIP-3) integrtions nd the simultions y their tmospheric component. In the tmosphere-only runs, the tmospheric GCMs re forced with oserved SSTs, following the Atmospheric Model Intercomprison Project (AMIP) protocols. Monthly climtologies re otined using yers (CMIP) nd (AMIP), the ltter eing the mximum common period for the AMIP simultions. The description of individul models is ville t documenttion/ipcc_model_documenttion.php (see lso Meehl et l. 2005). All model output ws interpolted to grid. Ensemle mens for CMIP nd AMIP simultions were clculted from the six models mrked y sterisks in Tle 1, ll developed t mjor modeling centers. Requiring AMIP simultions in the AR4 dtse reduces the numer of models for nlysis significntly. However, the composites shown here re very similr if lrger set of CMIP models is selected. Thus our ensemle verge using suset represents well the typicl ises in the coupled AR4 GCMs. The following oservtionl dtsets re used for comprison: Climte Prediction Center (CPC) Merged Anlysis of Precipittion (CMAP; Xie nd Arkin 1997); Interntionl Comprehensive Ocen Atmosphere Dtset Tle 1 Summry of models considered in this study nd the lels used to denote them in legends nd sctter plots Model/dtset Lel Model/dtset Lel Os mpi_echm5* l Renlysis ncr_ccsm3_0* m Ensemle c ukmo_hdcm3 n Cnrm_cm3* d ukmo_hdgem1* o Csiro_mk3_0 e ccr_cm2_0 p gfdl_cm2_0 f giss_om q gfdl_cm2_1 g giss_model_e_h r Ingv_echm4 h giss_model_e_r s ipsl_cm4 i ip_fgols1_0_g t miroc3_2_hires* j miu_echo_g u miroc3_2_medres* k ncr_pcm1 v The sterisk mrks models tht re prt of the AMIP nd CMIP ensemle mens (ICOADS; Worley et l. 2005) for SST, surfce winds nd se level pressure; Interntionl Stellite Cloud Climtology Project (ISCCP; Rossow nd Schiffer 1999) for net surfce shortwve rdition; Ntionl Center for Environmentl Prediction (NCEP) renlysis (Klny et l. 1996) for ltent het flux; nd Ntionl Ocenogrphic Dt Center (NODC) World Ocen Atls (WOA; Conkright et l. 2002) for the computtion of the 20 C isotherm depth. 3 Zonl ises Figure 2 shows nnul men SSTs long the equtor for 12 CMIP models. The spred is considerle with men temperture difference mong models of up to 3 K (UKMO HdCM3 vs. CNRM CM3). Common to ll models, however, re (1) reversed temperture grdient in the centrl Atlntic, nd (2) wrm is in the est (with the exception of the CNRM model, which suffers from generl cold is long the equtor). In generl, the deficiencies re similr to those shown in Dvey et l. (2002), leit slightly less severe. The ensemle men sesonl evolution of SST nd surfce wind ises long the equtor is depicted in Fig. 3. During MAM strong westerly is ppers in the centrl nd estern equtoril Atlntic. This is ccompnied y wek wrm (cold) SST is in the est (west). In the following seson the westerly is relxes while the SST is grows to mximum strength, with the error in est-west SST difference reching *4K during July August. In contrst to the oserved estern cold tongue, the modeled SST long the equtor increses towrd the est (Fig. 1).

4 590 I. Richter, S.-P. Xie: On the origin of equtoril Atlntic ises in coupled generl circultion models the equtoril Atlntic ises. These precipittion ises re not confined to the ocen ut extend to the djcent continents. In prticulr, simulted rinfll is deficient over equtoril South Americ nd excessive over equtoril Fig. 2 Annul men of SST ( C) long the equtor in selected CMIP models verged etween 2 S nd 2 N. The thick lck nd gry lines show ICOADS oservtions nd the ensemle men, respectively In serch for the source of the coupled model ises, we turn to the AMIP simultions. Figure 3 shows the sesonl evolution of ensemle men errors in surfce wind nd precipittion long the equtor. The figure clerly documents tht the westerly is is lredy present in the uncoupled GCMs, leit somewht weker. In contrst to CMIP, however, the westerly is only exists from Ferury to My, when it is ccompnied y positive selevel pressure is in the western equtoril Atlntic (Fig. 3c). This se level pressure error cn e further trced ck to the est-west contrst in precipittion. As Fig. 3 shows, est-west precipittion errors occur during pproximtely the sme period s the westerly is ut pper one month erlier. Equtoril precipittion is underpredicted in the west, off the South Americn cost, nd overpredicted in the est, off the Africn cost. Figure 4 shows the precipittion errors in the AMIP ensemle during MAM, which re chrcterized y meridionl dipole over the ocen (which will e discussed in detil in Sect. 4) tht is slightly tilted nd thus gives rise to zonl component in Fig. 3 Longitude-time sections of surfce winds (m/s, vectors) nd c SST ises ( C, contours) in CMIP. Bises of surfce winds (m/s, vectors) nd precipittion (mm/dy, contours) in AMIP. c Bis of se level pressure (P) long the equtor during MAM in AMIP. All fields re meridionlly verged etween 2 S nd 2 N. Positive vlues in nd re shded. Model ises re in reference to ICOADS (SST nd surfce winds) nd CMAP (precipittion). The fields shown represent the ensemle men over selected CMIP models () nd AMIP models (, c). The ensemle memers re indicted in Tle 1 c SLP is [P] long the equtor

5 I. Richter, S.-P. Xie: On the origin of equtoril Atlntic ises in coupled generl circultion models 591 Fig. 4 MAM precipittion (mm/dy, contours; positive shded) nd surfce wind (m/s, vectors) ises for CMIP, nd AMIP ensemle mens. The reference dt for precipittion nd winds re CMAP nd ICOADS, respectively CMIP AMIP Afric. This est-west dipole of precipittion errors drives the westerly wind ises over the equtoril Atlntic. Such precipittion nd wind errors re consistent with wekened Atlntic Wlker cell s reported for the CAM 3 y Chng et l. (2007). Okumur nd Xie (2004) lso note the westerly is in equtoril winds during the wrm seson in n tmospheric GCM. While the westerly is in oth AMIP nd CMIP models peks during MAM, the SST error is most pronounced from June to Septemer. This delyed SST response cn e explined s follows. The westerly wind error during MAM cuses the thermocline to deepen in the est (Fig. 5). The deepening peks in My with vlues in excess of 30 m. In the following seson, the westerly is wekens while the cross-equtoril southerly winds, which re crucil to equtoril upwelling (e.g., Okumur nd Xie 2004), re even slightly stronger thn oserved. Thus the wind ptterns would e otherwise fvorle for upwelling nd cold tongue development, ut the unrelisticlly deep thermocline prevents equtoril Atlntic cooling, resulting in pek in the wrm SST is during the oserved cold tongue seson (JJA). With the equtoril westerly is wekening, the thermocline depth slowly recovers from June nd reches pproximtely the oserved vlues in Septemer. In the full Fig. 5 Longitude-time sections of the CMIP ensemle-men ises of surfce wind (m/s, vectors) nd the 20 C isotherm depth (m, contours). The reference dtsets re ICOADS (surfce winds) nd NODC WOA (isotherm depth)

6 592 I. Richter, S.-P. Xie: On the origin of equtoril Atlntic ises in coupled generl circultion models fields (Fig. 6), the model thermocline egins with too lrge depth (90 m in the est) in My nd never ecomes s shllow s in oservtions: the minimum 20 C-isotherm depth is less thn 40 m in oservtions (July) ut 50 m in the model ensemle (Septemer). The southerly winds in the Gulf of Guine egin to weken in Septemer s the West Africn monsoon egins its withdrwl. As result of the excessively deep thermocline, the orel summer cooling in the Gulf of Guine (Fig. 6) is much sudued in the models, reching minimum SST of 26.5 rther thn the oserved 24.5 C. In oservtions, the thermocline shols gin in the estern equtoril Atlntic in Decemer ecuse the secondry ccelertion of the equtoril esterly winds excites the resonnt sin mode of equtoril ocen wve djustment (Okumur nd Xie 2006; Heler et l. 2007). The models underestimte this Decemer sholing of the thermocline. Figure 7 illustrtes the reltionship etween MAM surfce zonl wind (verged etween 2 S 2 N nd 40 W 10 W) nd JJA SST grdient (difference etween 40 W nd 8 E, verged 2 S 2 N) mong 19 CMIP models. The inter-model correltion is A similr nlysis for the zonl wind is in April My nd nomlous thermocline depth grdient in My June yields correltion of These correltions suggest tht in the coupled models studied here, there is close link etween MAM westerly errors, the erroneously deep thermocline in the est, nd the filure to develop n equtoril cold tongue in orel summer. The correltions lso indicte tht the Bjerknes feedck plys role in the development of the lrge SST ises during JJA. We isolte this coupled feedck y clculting the CMIP AMIP difference in tmospheric fields, which represents the tmospheric response to CMIP SST ises. Note tht CMIP SST ises result from AMIP tmospheric ises s well s this CMIP AMIP difference. Figure 8 shows SST errors, nd the CMIP AMIP differences in precipittion nd surfce wind. In the equtoril Atlntic, the estern wrming nd western cooling induce westerly wind nomlies, which is indictive of the Bjerknes feedck. The estern wrming is prtilly due to the deepened thermocline in response to MAM westerly wind ises while the SST-induced westerly nomlies help deepen the thermocline even more in the est. 4 Meridionl ises In ddition to the zonl ises discussed so fr, the AMIP models lso feture the well-known southwrd shift of the ITCZ (Hung et l. 2004; Bisutti et l. 2006). Figure 4 shows surfce winds nd precipittion during MAM, the seson when the oserved ITCZ reches its southernmost WOA Fig. 6 Annul cycles in the estern equtoril Atlntic (0 5 E, 2 S 2 N) of depth of the 20 C isotherm, nd SST. The solid nd dshed lines denote oservtions nd model ensemle, respectively. The oservtions re World Ocen Atls for isotherm depth nd ICOADS for SST position in the Atlntic, nd sometimes migrtes south of the equtor. The coupled GCMs plce the Atlntic ITCZ south of the equtor nd thus overpredict rinfll there, in ssocition with northerly error in cross-equtoril winds. As with the zonl ises, these errors exist, to lrge extent, in oth the CMIP (Fig. 4) nd AMIP (Fig. 4) simultions ut re more pronounced in the former. It is interesting to note tht the southwrd shift of precipittion is limited to the western equtoril Atlntic in AMIP ut extends cross the sin in CMIP, suggestive of SST effects in the ltter.

7 I. Richter, S.-P. Xie: On the origin of equtoril Atlntic ises in coupled generl circultion models 593 SST grdient, JJA zonl wind, MAM Fig. 7 Scttering mong CMIP models of MAM zonl surfce wind (m/s; verged etween 2 S 2 N nd W), nd JJA SST grdient etween 40 W nd 8 E (K; verged etween 2 S nd 2 N). Model lels re listed in Tle 1 We further exmine the coupled feedcks in Fig. 8, which shows the CMIP AMIP difference in surfce winds nd precipittion. The wrm SST ises re confined to the Southest Atlntic nd pek off the southern Africn cost (20 S) possily ecuse of the depressed thermocline tht results from the equtoril westerly is vi equtoril nd costl Kelvin wves. The SST error pttern resemles the Benguel Nino tht tends to pek during MAM in oservtions (Shnnon et l. 1986; Rouult et l. 2003). SST errors re smll on the equtor during MAM ecuse upwelling does not ply n importnt role during this seson, thus reducing the impct of simulted errors in thermocline depth nd cross-equtoril winds. The wrm SST errors in the Southest Atlntic mplify errors in the tmospheric fields y enhncing the spurious southern ITCZ rin nd nd y incresing northesterly wind errors ner the equtor. The CMIP AMIP wind difference is much stronger south thn north of the equtor, nd the wekened southesterly winds south of the equtor ct to reduce surfce evportion nd wrm the SST there, indictive of the WES feedck. The CMIP AMIP northwesterly wind difference on the equtor might lso help mintin the SST wrming in the Southest Atlntic y deepening the thermocline in the Benguel costl region. From Jnury to June, the WES nd thermocline feedcks re necessry to overcome the reduced solr rdition due to incresed convective clouds nd sustin the SST wrming just south of the equtor (Fig. 9). During July Decemer, on the other hnd, the shortwve feedck turns positive over the SE Atlntic ecuse the erroneously wrm SST inhiits the formtion of low-level clouds, Fig. 8 Difference of the CMIP nd AMIP ensemle mens of surfce winds (m/s, vectors), nd precipittion (mm/dy, shding nd white contours), nd SST ( C, contours) for MAM, nd JJA. Precipittion differences less thn -2 mm/dy nd greter thn +2 mm/dy re shded light nd drk gry, respectively. The intervl for the white precipittion contours is 2 mm/dy, the intervl for the lck SST contours is 1 C. Negtive SST contours re dshed which re oserved to previl during this seson (Klein nd Hrtmnn 1993). Figure 10 exmines the mplifiction of the MAM northwesterly wind error on the equtor in individul CMIP models. The MAM surfce wind errors re verged over the re W nd 2 S 2 N for oth AMIP nd CMIP runs. In ll the models the westerly is (Fig. 10) ecomes more severe in the coupled simultions, consistent with the Bjerknes feedck. The strength of this feedck, however, vries gretly from model to model, with the strongest response in the NCAR PCM, nd the wekest in the GISS model. These differences re proly relted to the men thermocline depth, which is shllow in the former ut very deep in the ltter model (not shown). The meridionl wind ises for the sme region (Fig. 10) show similr pttern. The mjority of the models feture n increse of the northerly is in their CMIP runs reltive to their AMIP runs ut there re severl exceptions, nmely the IPSL model, MIROC high nd low resolution models, nd the GISS model. All of these models lso hppen to feture reltively smll zonl wind is in their CMIP runs. The mplifiction of the zonl nd meridionl surfce wind ises is summrized in Tle 2.

8 594 I. Richter, S.-P. Xie: On the origin of equtoril Atlntic ises in coupled generl circultion models Tle 2 Zonl (u) nd meridionl (v) surfce wind velocity ises (m/s) in AMIP nd CMIP ensemles for MAM, verged over 40 W 10 W nd 2 S 2 N. Bises re in reference to ICOADS Ensemle Du Dv AMIP CMIP u SW [W/m 2 ] SST [K] v Fig. 9 Sesonl cycle in the SE Atlntic (10 W-10 E, EQ-20 S), for CMIP ises of net surfce shortwve rdition (W/m 2, downwrd positive), nd SST (K). The reference dtsets re ISCCP (shortwve) nd ICOADS (SST) Atmospheric error ptterns differ etween MAM nd JJA in their meridionl structure. In MAM, the wrm SST is, nd the precipittion nd wind errors re ll locted south of the equtor while in JJA, wind nd precipittion ises re mximum on the equtor (Fig. 8). These differences in meridionl structure my reflect the dominnt modes of climte vriility: the meridionl mode nd Benguel Nino tend to occur during MAM while the Atlntic Nino domintes JJA in oservtions (Xie nd Cton 2004). The ltter is due to the sesonl sholing of the thermocline in the estern equtoril Atlntic. 5 Continentl precipittion ises Our AMIP nlysis in Sect. 3 suggests tht the dipole of rinfll deficit over equtoril South Americ nd excess over tropicl Afric sets up pressure grdient tht drives the westerly wind ises in the equtoril Atlntic. While mjor re-orgniztion of precipittion tkes plce over the ocen in CMIP, CMIP AMIP precipittion differences induced y SST errors re smll over lnd, generlly less thn 1 mm/dy in mgnitude (Fig. 8) except on the cost of Guine. Thus it ppers tht errors in continentl convection induce errors in simulted Atlntic climte ut not the other wy round. Fig. 10 Br chrts of surfce wind ises in individul models verged etween 40 nd 10 W for zonl winds, nd meridionl winds. The light nd drk rs represent AMIP nd CMIP models, respectively. The corresponding model nmes re displyed next to the rs The sesonl evolution of continentl precipittion ises in AMIP is depicted in Figs. 11 nd 12 s longitudend ltitude-time sections, respectively. On the equtor, deficient rinfll over South Americ develops from Jnury to August nd is pronounced during Mrch nd April etween 60 nd 40 W, coinciding with the riny seson there (Fig. 11, c). During this period precipittion in the AMIP ensemle men is underestimted y pproximtely 50%, indictive of systemtic model deficiencies in representing convection in the region. The sesonlity of the South Americn precipittion ises is further illustrted y ltitude-time sections (Fig. 12). While the oserved South Americn ITCZ migrtes northwrd from Jnury to July following the sesonl mrch of insoltion (Fig. 12), the model rinfll ises re sttionry nd confined to the equtor nd slightly north of it (Fig. 12c). The center of the deficient precipittion in MAM is locted just inlnd of the equtoril South Americn cost (Fig. 4), which coincides with the oserved precipittion mximum (not shown). The origin of this precipittion mximum hs not received much ttention ut the few

9 I. Richter, S.-P. Xie: On the origin of equtoril Atlntic ises in coupled generl circultion models 595 c Fig. 11 Longitude-time sections of continentl precipittion (mm/ dy) over equtoril South Americ (, c) nd Afric (, d). The upper pnels show CMAP oservtions, the lower pnels the is of the AMIP ensemle men. Fields re verged etween 5 S nd 5 N. Positive vlues in c nd d re shded studies ville point to the importnce of mesoscle processes (Kousky 1980; Cohen et l. 1995; Rickench 2004). Specificlly the interction of the locl se reeze system nd orogrphy with the northest trde winds during MAM is elieved to generte mesoscle convective systems tht propgte further inlnd nd produce precipittion over wide re. Typicl AGCM resolutions t 2 3 re too corse to cpture such processes nd this d might e one of the resons for the deficient precipittion over equtoril South Americ. Another feture of interest in Fig. 11c is the excessive precipittion over the Andes, which persists throughout the yer. Such ises ssocited with steep orogrphy re well known nd likely relted to the formultion of the pressure grdient force in sigm coordintes (e.g., Mesinger et l. 1988). The spurious convection generted over the mountins excites grvity wves tht cn potentilly ffect precipittion in the surrounding region ut whether this contriutes to the is in equtoril South Americ is not cler. Over equtoril Afric the oserved sesonl cycle of precipittion (Fig. 11) fetures two mxim during orel spring nd fll consistent with insoltion. The AMIP simultions exggerte this sesonl cycle, producing too much precipittion during spring nd fll, nd too little during summer (Fig. 11d). The mgnitude of the is, however, does not exceed 3 mm/dy nd is thus sustntilly smller thn over South Americ. The ltitude-time sections (Fig. 12, d) show similr exggertion of the sesonl cycle in precipittion. While the AMIP models trck the sesonl migrtion of the ITCZ quite well, they overestimte its strength y 1 3 mm/dy. While prt of the continentl ises might e ttriutle to the models shortcomings in simulting convection, the misrepresenttion of lnd surfce processes my lso contriute to the errors. For exmple, comprison with the NCEP renlysis suggests tht the AMIP runs underestimte ltent het flux everywhere in South Americ, with differences exceeding 20 Wm -2 in equtoril South Americ, where the mximum rinfll deficit exists (Fig. 13). The sitution is more complex in tropicl Afric where there re positive nd negtive ltent het flux differences in the Shel nd equtoril Afric, respectively, while precipittion differences re positive in oth regions. Since there re few oservtions of ltent het flux in these regions of interest, we do not hve solid reference to judge the models performnce. At lest for South Americ, however, the comprison with NCEP does hint t potentil prolem in the simultion of lnd surfce processes. This egs the question whether there re ny common deficiencies in the lnd surfce prmeteriztions of the models exmined here. While the lnd surfce models employed vry sustntilly in their degree of sophistiction, their sic formultions shre mny common fetures: clcultion of surfce turulent fluxes through the erodynmic ulk formul, surfce energy lnce tht includes ground het flux, nd surfce wter lnce tht includes the contriutions from precipittion, runoff, infiltrtion, nd evportion. The IPCC model rchive, however, does not provide sufficient dt to evlute lnd surfce prmeters, nd neither is there sufficient oservtionl dt to vlidte

10 596 I. Richter, S.-P. Xie: On the origin of equtoril Atlntic ises in coupled generl circultion models c d c Fig. 12 Ltitude time sections of continentl precipittion (mm/dy) over equtoril South Americ (, c) nd Afric (, d). Averging longitudes re W (South Americ) nd E (Afric). The upper pnels show CMAP oservtions, the lower pnels the is of the AMIP ensemle men. Positive vlues in c nd d re shded them ginst (see Dirmeyer et l. 2006). Potentil deficiencies in the lnd surfce prmeteriztions re therefore left s topic of future studies. In view of severl previous studies, it might not come s surprise tht continentl convection exerts strong influence on the climte of smll ocen sin like the Atlntic. The rpid SST cooling in the Gulf of Guine from My to July hs een ttriuted to the rupt onset of West Africn monsoon nd southerly cross-equtoril winds (Mitchell nd Wllce 1992; Okumur nd Xie 2004). Recent results suggest tht precipittion over lnd ffects the ltitudinl position of the ITCZ (Wng nd Fu 2007; Hgos nd Cook 2005). Richter et l. (2008) show tht Fig. 13 Ltent het flux (W m -2, contours) nd precipittion (mm/ dy, shding) for the AMIP ensemle men, NCEP renlysis (ltent het flux) nd CMAP (precipittion), nd c the difference minus. In c, precipittion vlues ove +1 mm/dy nd elow -1 mm/dy re indicted y solid nd cross-htched shding, respectively. Negtive ltent het flux contours re dshed improving precipittion over tropicl South Americ nd Afric hs the potentil to llevite tmospheric circultion ises over the South Atlntic. 6 Summry nd discussion We hve evluted simultions y the IPCC AR4 coupled GCMs ginst oservtions. Our nlysis shows tht these models suffer mjor ises over the tropicl Atlntic nd djcent continents. During MAM when the equtoril Atlntic is wrm, the simulted ITCZ is erroneously

11 I. Richter, S.-P. Xie: On the origin of equtoril Atlntic ises in coupled generl circultion models 597 displced south of the equtor, with wrm SST ises in the tropicl Southest Atlntic (meridionl ises). Along the equtor, models fil to develop the estern cold tongue, with SST grdients opposite to oservtions in the centrl equtoril Atlntic (zonl ises). The lrgest SST errors long the equtor re found in the estern sin during JJA, the seson when the cold tongue develops in oservtions. The lrgest surfce wind errors, on the other hnd, occur one seson erlier in MAM nd ct to decelerte the previling esterlies on the equtor. Without exception oth surfce wind errors nd the southwrd shift of the ITCZ during MAM re lredy present in the tmospheric component of the CMIP models. These tmospheric model errors re mplified in CMIP runs y ocen tmosphere interctions such s the Bjerknes nd WES feedcks. Specificlly, westerly wind nomlies in MAM deepen the thermocline in the estern equtoril Atlntic. Even though the wind ises decrese in JJA nd the totl winds re generlly fvorle for ocenic upwelling, the excessively deep thermocline does not llow n estern cold tongue to develop in coupled GCMs. As result of the filure to develop the cold tongue, the equtoril SST is peks during JJA. Focusing on MAM, we further exmine the AMIP simultions nd trce the source of the equtoril westerly wind errors to zonl dipole of precipittion errors on the djcent continents: deficient precipittion over equtoril South Americ nd excessive precipittion over tropicl Afric. These rinfll errors drive surfce pressure grdient tht decelertes the esterly winds. An importnt result useful for model developers is tht errors of tmospheric GCMs during MAM re mplified in coupled simultions nd led to errors in the ocenic component tht persist into the following seson. Therefore, relistic simultion of the MAM climte in tmospheric GCMs my hold the key to improving coupled model simultions. Specificlly, our nlysis indictes tht the continentl precipittion ises strongly ffect, ut re not significntly influenced y, climte ises over the Atlntic Ocen, which underlines the importnce of exmining ises over lnd. While the present study points to errors in tmospheric GCMs nd in representing terrestril precipittion in prticulr, it does not rule out other possile sources of error in coupled models, such s the representtion of ocenic upwelling nd mixing (Hzeledger nd Hrsm 2005; Lrge nd Dnsoglu 2006). These processes likely lso ply role in the pronounced wrm is of the southestern Atlntic (Fig. 1), which, in turn, might contriute to the southwrd shift of the ITCZ in MAM s proposed y Hung et l. (2007). In our study, the southwrd ITCZ shift is limited to the western nd centrl Atlntic for AMIP models (Fig. 4) ut extends ll the wy to the Africn cost for CMIP models (see Sect. 4). This suggests locl effect of the southestern SST is on the ITCZ ut the sic doule ITCZ prolem origintes in the uncoupled AGCMs s illustrted y Fig. 4. In ny event, it is evident tht mny fctors my contriute to coupled GCM ises in the tropics. Clerly much work is necessry to understnd nd reduce the errors tht hve long plgued coupled simultions in the tropicl Atlntic. Acknowledgments This study ws supported y the NOAA CLI- VAR Progrm nd the Jpn Agency for Mrine-Erth Science nd Technology through its sponsorship of the Interntionl Pcific Reserch Center. All the model output ws downloded from The IPCC Dt Archive t Lwrence Livermore Ntionl Lortory, which is supported y the Office of Science, U.S. Deprtment of Energy. The uthors would like to thnk Justin Smll nd two nonymous reviewers for their helpful suggestions. IPRC puliction #498. References Bisutti M, Soel AH, Kushnir Y (2006) AGCM precipittion ises in the tropicl Atlntic. 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