Asian monsoon simulations by Community Climate Models CAM4 and CCSM4

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1 Clim Dyn (2013) 41: DOI /s Asin monsoon simultions y Community Climte Models CAM4 nd CCSM4 Sirj ul Islm Youmin Tng Peter L. Jckson Received: 21 July 2012 / Accepted: 25 Mrch 2013 / Pulished online: 5 April 2013 Ó Springer-Verlg Berlin Heidelerg 2013 Astrct This study exmines the ility of Community Atmosphere Model (CAM) nd Community Climte System Model (CCSM) to simulte the Asin summer monsoon, focusing prticulrly on inter-model comprison nd the role of ir se interction. Two different versions of CAM, nmely CAM4 nd CAM5, re used for uncoupled simultions wheres coupled simultions re performed with CCSM4 model. Ensemle uncoupled simultions re performed for 30 yer time period wheres the coupled model is integrted for 100 yers. Emphsis is plced on the simultion of monsoon precipittion y nlyzing the internnul vriility of the tmosphere-only simultions nd se surfce temperture is in the coupled simultion. It is found tht oth CAM4 nd CAM5 dequtely simulted monsoon precipittion, nd considerly reduced systemtic errors tht occurred in predecessors of CAM4, lthough oth tend to overestimte monsoon precipittion when compred with oservtions. The onset nd cesstion of the precipittion nnul cycle, long with the men climtology, re resonly well cptured in their simultions. In terms of monsoon internnul vriility nd its teleconnection with SST over the Pcific nd Indin Ocen, oth CAM4 nd CAM5 showed modest skill. CAM5, with revised model physics, hs significntly improved the simultion of the monsoon men climtology nd showed etter skill thn CAM4. Using idelized experiments with CAM5, it is seen tht the doption of new oundry lyer schemes in CAM5 contriutes the most to reduce the monsoon overestimtion is in its simultion. In the S. u. Islm Y. Tng (&) P. L. Jckson Environmentl Science nd Engineering, University of Northern British Columi, 3333 University wy, Prince George, BC V2N 4Z9, Cnd e-mil: ytng@unc.c CCSM4 coupled simultions, severl spects of the monsoon simultion re improved y the inclusion of ir se interction, including the cross-vriility of simulted precipittion nd SST. A significnt improvement is seen in the sptil distriution of monsoon men climtology where too-hevy monsoon precipittion, which occurred in CAM4, is rectified. A detiled investigtion of this significnt precipittion reduction showed tht the lrge systemtic cold SST errors in the Northern Indin Ocen reduces monsoon precipittion nd delys onset y wekening locl evportion. Sensitivity experiments with CAM4 further confirmed these results y simulting wek monsoon in the presence of cold ises in the Northern Indin Ocen. It is found tht lthough the ir se coupling rectifies the mjor weknesses of the monsoon simultion, the SST is in coupled simultions induces significnt differences in monsoon precipittion. The overll simultion chrcteristics demonstrte tht lthough the new model versions CAM4, CAM5 nd CCSM4, re significntly improved, they still hve mjor weknesses in simulting Asin monsoon precipittion. 1 Introduction The monsoon is one of the most prominent nd dynmic phenomen of the climte system, nd hs lrge effect on wether nd climte nomlies t oth locl nd glol scles. Monsoon systems re cused y the sesonl reversl of winds due to differentil heting etween lnd nd ocen, nd result in sesonl hevy precipittion ptterns. The dominnt monsoon systems round the gloe re the Asin, Austrlin, Africn nd Americn monsoons. Among these vrious monsoon systems, the Asin summer monsoon composed of the Est Asin monsoon (EAM) nd

2 2618 S. u. Islm et l. the South Asin monsoon (SAM) (Lu nd Li 1984) receives the heviest sesonl precipittion during the summer months nd hs mjor impct on glol tmospheric circultions. The SAM region includes prts of the Arin Se, the Indin continent nd the By of Bengl (Goswmi et l. 1999); the region receives round 75 % of its nnul precipittion during the summer seson in the months of June, July, August nd Septemer (JJAS). Complex topogrphic fetures such s the Himlys (in the north nd north est) nd the Western Ghts (long the western cost of Indi) strongly influence the monsoon circultions. Any monsoon fluctutions re often ssocited with floods, droughts, nd other climtic extreme events in the region (Mlik et l. 2010). The SAM precipittion hs very strong temporl nd sptil vrition due to the interction etween regionl topogrphy nd the monsoon circultion. A detiled understnding of the different externl forcing mechnisms tht modulte the SAM precipittion is therefore necessry for the societl nd economicl needs of the South Asin region. The study of the SAM vriility nd its prediction is chllenging nd importnt issue in the scientific community. Climte models hve een significntly improved in simulting the men glol climte (Rndll et l. 2007) nd in predicting climte nomlies t the sesonl time scle (Ling et l. 2009; Lee et l. 2010; Kng et l. 2002; Kng nd Shukl 2006; Wng et l. 2004). These models re firly good t simulting the verge tmospheric stte nd lrge scle ptterns, ut poor t simulting reltively smll nd locl tmospheric systems such s the monsoon. In some of the studies such s Kng et l. (2004) nd Wng et l. (2004), it hs een seen tht even when forced with oserved se surfce tempertures (SSTs), Glol Climte Model (GCM) performnce over the SAM region is not stisfctory nd presents lrge systemtic ises. Even in coupled Atmosphere Ocen Glol Climte Models (AOGCMs), which re elieved to simulte the most relistic physicl processes, there re notle ises in simultion of the men climte nd its vriility (Covey et l. 2003; Meehl et l. 2005). These discrepncies include Pcific cold is, doule Intertropicl Convergence Zone (ITCZ), nd westwrd shift of El Nino Southern Oscilltion (ENSO) vriility (AchutRo nd Sperer 2006; Covey et l. 2000; Joseph nd Nigm 2006). Gimeno et l. (2010) hve shown the Northern Indin Ocen, prticulrly the Arin Se, to e n importnt moisture source for SAM nd ny chnges in Indin Ocen SSTs ffect monsoon precipittion y ltering the mount of moisture ville for trnsport towrds South Asi. Furthermore, due to the strong ir se coupling over the Indin Ocen, ny vrition in the strength of the SAM precipittion influences the SST vrition which significntly complictes the detection of monsoon vriility relted to other chnges in the lower oundry of the tmosphere in the coupled model. It hs een chllenging issue to correctly simulte the monsoon vriility t sesonl nd internnul time scles (Annmli et l. 2007; Di 2006; Kriplni et l. 2007; Lin 2007; Wliser et l. 2007) nd the reltionship etween SST nomlies nd SAM precipittion vriility (Annmli nd Liu 2005; Meehl nd Arlster 2002; Shukl nd Polino 1983; Rsmusson nd Crpenter 1983). An importnt issue is the simultion of the reltionship etween locl SAM nd SST vritions in the Pcific nd Indin Ocen. The link etween the SAM precipittion nd ENSO hs een well documented in oservtions nd modeling. For exmple, it hs een reported tht the wrm phse (El Nino) is ssocited with wekening of the Indin monsoon nd n overll reduction in precipittion, while the cold phse (L Nin) is ssocited with the strengthening of the Indin monsoon nd n enhncement of precipittion (Knmitsu nd Krishnmurti 1978; Krishnmurti et l. 1989; Plmer et l. 1992; Pnt nd Prthsrthy 1981; Rsmusson nd Crpenter 1983; Shukl nd Polino 1983; Shukl nd Mooley 1987; Sikk 1999). Meehl et l. (2012), descried SAM s fully coupled ir se lnd system which cn e etter reproduced y AOGCMs. Mny other studies lso reported tht AOGCMs perform etter thn tmosphere-only GCMs in simulting the SAM with moderte skill (Kumr et l. 2005; Wng et l. 2005). As discussed ove, n intensive reserch effort hs een mde to improve simultion of monsoon systems y climte models nd significnt progress hs een mde in recent yers. Among these models, Community Climte Models developed t the US Ntionl Center for Atmospheric Reserch (NCAR) hve plyed n importnt role in monsoon reserch due to their complete physicl dynmics nd esy implementtion. NCAR relesed new versions of the climte models, i.e., the Community Atmosphere Model version 4 nd version 5 (CAM4 nd CAM5) nd the Community Climte System Model version 4 (CCSM4). In the recent pper y Meehl et l. (2012) documenting the monsoon simultions in the Community Climte Model version 4 (CCSM4), the SAM nd Austrlin monsoon is thoroughly exmined for the fully coupled model. They lso compred the CAM4 tmosphere simultion with coupled run nd discussed the improvement of CCSM4 compred with the previous genertion of this model (CCSM3) in simulting the monsoon. Also in the study y Meehl et l. (2006), the monsoon in CCSM3 ws descried nd compred to previous version of the model simultion (Meehl nd Arlster 1998). In this context, it is of interest nd importnce to evlute the ility of these new model versions, prticulrly CAM5, in simulting the monsoon. The importnt fctors ffecting monsoon SST

3 Asin monsoon simultions 2619 reltionships, such s ir se coupling nd SST is from the CCSM4, need to e studied in detil to determine the strengths nd weknesses of these new models. A systemtic evlution is lso importnt if these new model versions re to e used for sesonl climte prediction or climte chnge studies. In this study, we therefore explore in detil the strengths nd limittions of CAM4, CAM5 nd CCSM4 in simulting SAM precipittion with n emphsis on the men climte, sesonl nd internnul vriility nd the reltionship etween SAM nd SST (locl nd remote) in the simultions. Our focus is (1) on the nlysis of SAM internnul vriility when simultions re forced with oserved SST nd (2) the role of ir se coupling nd SST is in simulting the SAM. In the ltter, using sensitivity experiments, we lso exmine the effect of Northern Indin SST is in coupled simultions nd its impct on SAM. This pper is orgnized s follows. Section 2 descries models, dt nd experiments. Section 3 investigtes the men climtology, nnul cycle nd evlutes simulted monsoon internnul vriility in CAM4 nd CAM5. Section 4 highlights nd compres the coupled simultions of CAM4 (CCSM4) in terms of the men climtology nd SAM SST reltionship s well s the effect nd importnce of ir se coupling over the SAM region. To ddress the effect of CCSM4 SST is on the SAM precipittion, Sect. 5 explores results of sensitivity experiments followed y summry nd conclusions in Sect Models, experiments nd vlidtions 2.1 Models Simultions re performed using the CAM4, CAM5 nd CCSM4 models. These models re the ltest in succession of GCMs nd AOGCMs tht hve een mde widely ville to the scientific community from NCAR. For the convenience of the reder we riefly introduce ech model, ut refer interested reders to the cited references for full detils of ech model. CAM4 (Nele et l. 2010) is developed from CAM3 (Collins et l. 2006, ) with modifictions to the deep convection (Nele et l. 2008), polr filtering (Anderson et l. 2009), nd the polr cloud frction in extremely cold conditions prmeteriztion schemes (Vvrus nd Wliser 2008). It uses n updted convection prmeteriztion scheme (Nele et l. 2008; Richter nd Rsch 2008). This model cn e used with three different dynmic schemes (n Eulerin spectrl scheme, semi-lgrngin scheme nd finite volume scheme) long with different resolution settings. CAM5 (Nele et l. 2010) is modified significntly compred to CAM4, with rnge of improvements in the representtion of physicl processes. It includes new shllow convection scheme (Prk nd Bretherton 2009), strtiform cloud microphysicl scheme (Morrison nd Gettelmn 2008), n updted rdition scheme (Icono et l. 2008) nd 3-mode modl erosol scheme (MAM3) (Liu et l. 2012). The CCSM4 (Gent et l. 2011) coupled model descended from its predecessors, CCSM3 (Collins et l. 2006, ) nd CCSM2 (Kiehl nd Gent 2004). It contins new coupler tht exchnges fluxes nd stte informtion mong ll the emedded models. These emedded models re the CAM4 tmospheric model, the Community Lnd Model (CLM4), the Los Almos Prllel Ocen Progrm ocen model version 2.2 (POP 2.2) (Smith nd Gent 2002) nd the Community Ice Code version 4 (CICE4) se ice model (Hunke nd Lipscom 2008). The CLM4 model opertes on the sme grids s the CAM4 model wheres CICE4 uses the sme horizontl grid s POP 2.2, which hs displced dipole grid (Smith nd Korts 1995). 2.2 Experimentl design A series of experiments re performed to chieve three gols: (1) exploring nd compring the ility of CAM4, CAM5 nd CCSM4 in simulting the SAM; (2) evluting the contriution of ir se coupling to the simultion nd (3) investigting the effect of SST is on SAM precipittion. These experiments cn e generlly ctegorized s elow: 1. Control runs: Thirty-two yers ( ) of uncoupled simultions re performed using the CAM4 nd CAM5 tmospheric models forced with oserved prescried SST (HdSST, Reynolds et l. 2002) nd se ice dt. Both models shre the sme horizontl resolution using the finite volume (FV) dynmicl core with 26 (in CAM4) nd 30 (in CAM5 1 ) verticl levels using hyrid terrin-following coordinte system. Higher resolution simultion of CAM4 nd CAM5 models re lso performed using in the horizontl for the sme time period. 2. Climtology run: CAM4 is lso run forced with the climtologicl (sed on the oservtions from 1982 to 2001) sesonl cycle of SST nd se ice for 30 yers. This is referred to s CAM4_CLIM. 3. Coupled run: In the cse of the CCSM4 coupled experiment, 100-yer coupled integrtion is performed using present dy climtologicl forcing. The 1 CAM5 is run for different set of schemes. Stndrd run (control) of CAM5 hs 30 verticl levels with ll the defult setting. The remining set of CAM5 runs will e denoted with their prticulr nme throughout the text.

4 2620 S. u. Islm et l. output of the lst 30 yers of this coupled simultion is used. 2 In this simultion, the horizontl resolution of nd finite volume grids in oth the tmospheric nd lnd model is used, wheres the ocen nd ice models shre the sme resolution with displced pole grid. To fcilitte comprison, oservtionl dtsets re interpolted to the resolution of model grids. 4. Sensitivity runs: Idelized experiments using different oundry forcings re performed with CAM4 nd CAM5. Detils of these experiments re given in the relevnt sections. Even relistic model lwys contins rndom components nd uncertinties such s those in oundry forcing or in initil conditions. To llevite the impct of these rndom components nd otin deterministic response of the model ehvior to forcing (such s SST), n ensemle strtegy is used for the ove experiments except for the coupled run. For the control run, ensemles re constructed through perturing the initil conditions, which llows us to seprte the SST-forced (or externl) response (Rowell et l. 1995). The perturtion of the initil conditions is performed y using the initil conditions lgged in time. For the climtology run, the sme method is used to construct the ensemles. A detiled summry of ll the experiments nd the ensemle runs is given in Tle 1. All simultion results from the control nd climtology runs used for vlidtion nd presented in the next sections re the ensemle men, unless otherwise indicted. 2.3 Vlidtion The following vlidtion steps re used to exmine the performnce of the models in simulting the SAM monsoon: (1) the simulted SAM precipittion nd wind y the CAM4 nd CAM5 control runs re compred ginst the oserved counterprts in terms of climtology, internnul vriility, nd the reltionship to SST. (2) The climtologicl mens from the CAM4 runs re compred ginst those from CCSM4 to explore the effects of ocen tmosphere coupling on the SAM simultion. (3) To explore the impct of SST is on monsoon simultions, CAM4 forced with modified SST climtology tht contin SST is, is compred with tht forced with the oserved SST climtology (CAM4_CLIM). The metrics nd methods used to evlute the simultions include men is, root mens squre error (RMSE), vrince, correltion nd regression nlysis. In ll the uncoupled simultions, the first yer of the integrtion 2 Suject to the computtionl conditions, the spin-up run ws crried out 70 yers, which siclly llows tmospheric sttes to rech equilirium. output is discrded s spin-up time, which is considered sufficient for tmospheric-only simultions. In the cse of coupled runs, the first 70 yers re discrded s the ocen model needs more time for equilirtion. Oserved precipittion dt from the Climte Prediction Center (CPC) Merged Anlysis of Precipittion (CMAP; Xie nd Arkin 1997), on grid, is used for vlidtion of precipittion. The All-Indi Rinfll time series (AIR; Prthsrthy et l. 1995), which is comintion of 306 uniformly distriuted sttion mesurements, is lso used in the nlysis. Ntionl Centers for Environmentl Prediction (NCEP; Kistler et l. 2001) renlysis dt, lso on grid, re used to vlidte winds. Oserved SST (HdSST) dt (Reynolds et l. 2002) re used for coupled model SST vlidtion. 3 Simultions y CAM4 nd CAM5 We first exmine ensemle men simultions of CAM4 nd CAM5 forced with prescried oserved SST nd se ice dt. Before focusing on the SAM, we evlute oth models over the tropicl region. 3.1 Climtologicl men nd sesonl cycle The distriution of precipittion is nd root mens squre error (RMSE) is shown in Figs. 1 nd 2 for the winter (Decemer Ferury, DJF) nd summer (June Septemer, JJAS) sesons. The whole tropicl region is shown to ssess overll model differences. Bis nd RMSE re clculted y compring model output dt with oservtions (CMAP) for the time period The model is for winter (DJF) precipittion simultions re shown in Fig. 1 nd for CAM4 nd CAM5 respectively nd their corresponding RMSE re shown in Fig. 1c, d. Significnt lrge-scle sptil ises (Fig. 1, ) over the equtoril Indin Ocen nd Western Pcific Ocen, long with mny regionl ises, re found in the CAM4 nd CAM5 simultions. The mgnitude of these ises is higher in CAM4 wheres CAM5, to lrge extent, significntly rectifies them in its simultion. Mjor improvements in the CAM5 simultion occur over the South Africn lnd res where the precipittion overestimtion seen in the CAM4 simultion is diminished. The RMSE ptterns (Fig. 1c, d) further highlight the improved winter precipittion in the CAM5 simultion. In summer, when the precipittion ctivity over the northern hemisphere is enhnced, oth models show ises in the form of excessive precipittion over the western Indin Ocen, centrl Chin, Himlys, nd in the sutropicl Pcific Ocen (Fig. 2, ). Over the estern Indin Ocen, Chin Se, centrl prts of Afric nd in the west nd est Pcific

5 Asin monsoon simultions 2621 Tle 1 Summry of the experimentl setups Experiment nme Model used Time period Description/oundry conditions (BC) No. of runs CAM4 CAM horizontl resolution nd 26 verticl levels. Prescried oserved SST dt s BC CAM5 CAM horizontl resolution nd 30 verticl levels. Prescried oserved SST dt s BC CAM4_CLIM CAM4 30 yers horizontl resolution nd 26 verticl levels. Climtology SST dt repeted ech yer CCSM4 CCSM4 100 yers horizontl resolution nd 26 verticl levels for CAM4 nd horizontl resolution with 60 verticl levels for POP2.2. Present dy climtology forcing CAM5_BAM CAM horizontl resolution nd 30 verticl levels with ulk erosol model (BAM) scheme. Prescried oserved SST dt s BC CAM5_BAM_CAMRT CAM horizontl resolution nd 30 verticl levels with BAM nd CAMRT (rdition) schemes. Prescried oserved SST dt s BC CAM5_BAM_HB CAM horizontl resolution nd 30 verticl levels with BAM nd HB (Holtslg Boville, oundry lyer turulence) schemes. Prescried oserved SST dt s BC CAM5_BAM_CAMRT_HB CAM horizontl resolution nd 30 verticl levels with BAM, HB, nd 01 CAMRT schemes. Prescried oserved SST dt s BC CAM4_POP CAM Sme s CAM4 ut with predicted SST from CCSM4 climtology run 03 CAM4_AS_BoB CAM Sme s CAM4_CLIM ut with climtology SST dt modified in the AS nd 03 BoB region CAM4_AS CAM Sme s CAM4_CLIM ut with climtology SST dt modified in the AS region only 03 CLIM climtology, BC oundry conditions, AS Arin Se, BoB By of Bengl Ocen, the models underestimte precipittion. The sptil ptterns of is from oth models re similr, ut the mgnitude of ises is higher in the CAM4 simultion wheres CAM5 is le to reduce mny regionl ises. Compred to the lnd, ises re significntly higher over the ocen. High mgnitudes of RMSE (Fig. 2c, d) re seen over the northern Indin Ocen (Arin Se nd By of Bengl) nd the complex topogrphy of the Himlys. Similr to the winter, the summer RMSE of the CAM5 simultion is less thn for the CAM4 simultion. In generl it is seen tht, while oth CAM4 nd CAM5 re le to cpture mny oserved fetures, they hve regionl ises somewht similr to those in previous versions of these models (i.e., CAM3, reported in Meehl et l. 2006). In fct, these precipittion ises especilly over the Indin nd Pcific Ocens re proly n intrinsic error of the tmospheric model itself, s seen in Lin (2007) nd our simultions. Compred to CAM4, the CAM5 simultion is improved with less regionl is. The mgnitudes of tropicl two meter ir temperture ises (not shown) in oth CAM4 nd CAM5 re smll except in res with complex topogrphy such s the Himlyn region which is true for mny climte models (IPCC 2007). Both models showed wrm ises over most of the tropicl domin. Lrger errors re in regions of shrp elevtion chnges which my result simply from the mismtches etween the models smoothed topogrphy nd the ctul topogrphy. As this pper minly focuses on the Asin region, the rest of the nlyses for CAM4 nd CAM5 include only the Asin domin, nd prticulrly discuss the SAM region (JJAS only). Figure 3 shows sesonl men summer (JJAS) precipittion nd 850 m winds for () CAM4, () CAM5 nd (c) oservtions (CMAP/NCEP). In the oservtions, there re two precipittion mxim, with hevier precipittion round the northern Indin Ocen nd weker precipittion mximum long the equtoril Indin Ocen. This is n importnt chrcteristic of the SAM precipittion. Although oth of these mxim re cptured in the models, significnt lrge-scle ises such s excessive precipittion over the Arin By nd diminished precipittion in the centrl nd the estern Indin Ocen extending into the By of Bengl is seen. Simultions lso show reduced precipittion long the cost of Bngldesh nd excessive rin over the Western Ghts of Indi. CAM4 simultes excessive precipittion in the estern Arin Se nd in the By of Bengl, with the mximum center round the By of Bengl shifted to the west of the oserved mximum center. This is lso true for the CAM5 simultion ut the sptil mgnitude of the precipittion is reduced ringing its climtology close to the oservtion. This sme conclusion regrding the CAM4 simultion is

6 2622 S. u. Islm et l. c d Fig. 1 Sesonl men (Decemer Ferury; DJF) precipittion differences (ises) nd root mens squre error (RMSE) from oservtion (CMAP) for, c CAM4 nd, d CAM5. Units re in mm/dy found in the recent study y Meehl et l. (2012). Aprt from the SAM region, the EAM system, covering oth sutropics nd mid-ltitudes, is well cptured in oth models. CAM5 shows sptil ptterns similr to those oserved wheres the CAM4 simultion is drier thn oservtions over the South Chin Se. All simultions show very good correltion nd RMSE skill for the EAM, compred with the SAM. In the oserved 850 m wind pttern (Fig. 3c), the most importnt fetures re the monsoon westerlies, the northwrd movement of the low pressure re from the By of Bengl nd the low level jet strem pssing cross the equtor onto the Indin su-continent. The strengthening of westerly 850 m winds during the summer monsoon sesons cn e seen in oth models (Fig. 3, ) over the 10 N 25 N ltitude elt extending estwrd from the western Arin Se through Indi nd By of Bengl. The By of Bengl is considered s the moisture source of hevy precipittion events over the centrl South Asin region (Mlik et l. 2010) nd precipittion over this centrl region is minly cused y the northwrd movement of low pressure res from the By of Bengl (Ll et l. 1995). This interprettion is seen in oth CAM4 nd CAM5 simultions showing strong winds flowing from the By of Bengl to the north over centrl South Asi. In generl, CAM4 nd CAM5 re le to simulte the wind circultion t 850 m (such s the equtoril monsoon flow nd lower level jet strem) relisticlly, even though there re ises in the strength of monsoon westerlies over the Indin region. The simultion of the sesonl migrtion of the ITCZ is chllenging issue in GCMs. Mny studies (such s Hck et l nd Wu et l. 2003) reported tht most GCMs re unle to reproduce the sesonl migrtion of the ITCZ

7 Asin monsoon simultions 2623 c d Fig. 2 Sme s Fig. 1 ut for the summer seson (June Septemer; JJAS) precipittion. Gdgil nd Sjni (1998) found tht the tmospheric models which cn simulte the northwrd migrtion of the ITCZ cn lso simulte the internnul vrition of the Indin monsoon resonly well, wheres in models with poor SAM simultion, the ITCZ remins over the equtoril ocens in ll sesons. Over the SAM region, the sesonl migrtion of the ITCZ from the equtoril region in winter to the heted continent in summer is the most importnt feture of the sesonl vrition. We therefore riefly nlyzed the sesonl migrtion of ITCZ in our model simultions y nlyzing the men Jnury nd July surfce winds (not shown). It is found tht this plnetry scle feture of the generl circultion is well cptured y oth CAM4 nd CAM5. Also the loction nd strength of oth the westerly jets over the northern Indin region during Jnury nd the tropicl jets during July (erly Monsoon) re firly well reproduced in simultions which indicte tht oth models relisticlly cpture the lrge shift of the ITCZ from Jnury to July. We hve lso performed simultions of CAM4 nd CAM5 t higher resolution ( ) to nlyze the effect of etter resolved topogrphy (which is n importnt spect for the simultion of precipittion). We found (not shown here) tht incresing the resolution improved the simultion over res of complex terrin such s the Western Ghts nd Himlys in the SAM region. The Western Ghts cpture much of the rin on the Arin Se-fcing side, while the other side of these mountins (to the est in southestern Indi) remins dry in the summer seson. This is loclized effect nd cn only e seen in the higher resolution simultion. Also in the higher resolution run, hevy precipittion on the costl mountin slopes of Mynmr, cross the By of Bengl, is well simulted ut with the sme overestimtion in the mount

8 2624 S. u. Islm et l. c Fig. 3 Sesonl men (June Septemer; JJAS) precipittion nd 850 m winds from: CAM4, CAM5 nd c oservtions (CMAP/NCEP). Precipittion (shded) in mm/dy nd 850 m wind (vectors) in m/s s seen in the lower resolution run. Also the excessive precipittion over the Tietn Plteu in CAM4 (lso the cse in CAM5) is reduced in its higher resolution simultion. Both higher resolution simultions re somewht closer to oservtions for the complex terrin regions of the SAM. The sesonl evolution of SAM precipittion is exmined in time-ltitude digrms verged over the SAM longitudes ( E) for oservtions, CAM4 nd CAM5 (Fig. 4). In the oservtions (Fig. 4c), welldefined sesonl precipittion pttern tht vries with ltitude nd moves significntly northwrd from 5 N in winter to 20 N in summer is visile. In models (Fig. 4, ), the northwrd shift of precipittion strting in winter nd reching mximum in summer is well cptured ut there re considerle systemtic errors such s the simulted summer precipittion northwrd extent reching too fr north with second mximum round 30 N. In the CAM4 nd CAM5 simultions, the precipittion reches mximum t 15 N nd exhiits n essentilly relistic sesonl migrtion, ut the simultions hve hevier thn oserved precipittion over the Western Ghts. The overestimtion is higher in CAM4 thn in CAM5 long with more penetrtion of precipittion towrd the north. In CAM5, the mximum contour of precipittion is somewht reduced nd is comprle to oservtions. The nnul cycle of precipittion verged over the ltitude nd longitude of the SAM region is simulted quite well in models, with well-defined sesonl cycle s shown in Fig. 4d. Severl chrcteristics of the nnul cycle of SAM precipittion, such s the rpid onset etween My nd June, the sustined high precipittion from June to August nd the slow withdrwl during Septemer Octoer, re well simulted y oth models. As noted erlier in the discussion of sptil ptterns, oth models produce relistic sesonl vrition of precipittion in the SAM region, ut with considerle overestimtion. The nnul cycle highlights this overestimtion more clerly y showing excessive precipittion in the months July, August nd Septemer s well s in Decemer, Jnury nd Ferury. From the ove discussion of men climtology simultions, it is seen tht CAM5 hs significntly improved simultions compred to CAM4. We further investigted this improvement y performing different sets of CAM5 simultions using the sme rdition (CAMRT), erosol (BAM) nd oundry lyer (HB, Holtslg Boville 1993)

9 Asin monsoon simultions 2625 Fig. 4 Time-ltitude evolution of precipittion verged over ( E) for: CAM4, CAM5, c oservtions (CMAP) nd d represents nnul cycle of precipittion re verged over SAM region. Units re in mm/dy c d schemes which re used in the CAM4 defult configurtion. This switching of new schemes (in CAM5) with old ones (in CAM4) llowed us to explore the effect of ech individul scheme in improving CAM5 simultions over the SAM. We nmed ech individul experiment of CAM5 s CAM5_BAM, CAM5_BAM_CAMRT, CAM5_ BAM_HB nd CAM5_BAM_CAMRT_HB (see Tle 1 for detils). Figure 5 represents the precipittion difference (JJAS) of CAM5_BAM, CAM5_BAM_CAMRT, CAM5_BAM_HB nd CAM5_BAM_CAMRT_HB from oservtion. Compring these differences, with the defult CAM4 nd CAM5 runs (Fig. 2, ), revels tht the implementtion of new oundry lyer schemes (UW moist turulence) in CAM5 hs the gretest effect (in our cse) on decresing the overestimtion seen in the CAM4 simultion. As reported y Prk nd Bretherton (2009), the new UW moist turulence scheme improved the cloud top oundry lyers in the CAM model. They used the CAM3.5 version to test this scheme nd found significnt reduction in model is. In our cse, lthough the new RRTMG rdition scheme nd the full representtion of erosol indirect effects do not contriute s much to the improved SAM simultion, virtully every tmospheric process (revised/replced) nd its physicl representtion in the new version mkes n improvement in the simultion (the individul discussion of ll these new fetures of CAM5 is eyond the scope of this study). 3.2 Monsoon internnul vriility In this section, the monsoon vriility is exmined y focusing on the simultion of monsoon indices (precipittion nd circultion) for oth CAM4 nd CAM5 models. The strong nd wek monsoon composite nlysis is lso discussed to further explore the simultion s internnul vriility. It hs een well recognized tht the internnul vriility of mny climtologicl vriles on erth cn stem from ENSO, which is the strongest internnul vriility of the erth s climte system. Thus, the link etween simulted SAM precipittion nd ENSO is lso explored using lg correltion. This is lso performed to explore the reltionship etween the Indin Ocen Dipole (IOD, Sji et l. 1999) nd SAM precipittion Asin monsoon indices We exmined the models simultions of monsoon internnul vriility with severl commonly used monsoon

10 2626 S. u. Islm et l. c d Fig. 5 Sesonl men (June Septemer; JJAS) precipittion differences from oservtions (CMAP) for: CAM5_BAM (CAM5 with ulk erosol model (BAM) scheme), CAM5_BAM_CAMRT (CAM5 with BAM nd CAMRT (rdition) schemes), c CAM5_BAM_HB (CAM5 with BAM nd HB (Holtslg Boville, oundry lyer turulence) schemes) nd d CAM5_BAM_CAM- RT_HB (CAM5 with BAM, HB nd CAMRT schemes). Units re in mm/dy indices, including Indin Summer Rinfll (ISR), the Wester Yng monsoon index (WY index) (Wester nd Yng 1992), the Southest Asin monsoon (SEAM) index or Western North Pcific monsoon (WNPM) index (Wng nd Fn 1999), Indin monsoon (IM) index (Wng et l. 2001) nd South Asin monsoon index (SAM i, here the suscript i denotes index to differentite it from SAM) (Goswmi et l. 1999). The definitions of these indices re given in Tle 2. Figures 6 nd 7 show the simulted nd oserved internnul vriility of ISR, WY, IMI, WNPM, nd SAM i monsoon indices normlized with their respective stndrd devition. All these indices re circultion indices except ISR which represents SAM precipittion. To nlyze model simultions for individul strong nd wek monsoons, the ISR index is seprted from the circultion indices (Fig. 7) nd is presented in Fig. 6. In Fig. 6, the oserved ISR index representing the strength (strong nd wek monsoon) nd internnul vrition of SAM precipittion, is shown. Strong monsoon yers such s 1980, 1988 nd 2007 re differentile in the oservtions, wheres in CAM4 nd CAM5 (Fig. 6, c) only the yer 1988 hs the sme sign. Both 1980 nd 2007 re chrcterized s wek monsoon yers in oth models opposite to the oservtions. Similrly, the oserved wek monsoon yers 1984, 1986 nd 2002 re simulted s strong monsoon yers in model. This mens tht oth CAM4 nd CAM5 filed to cpture the internnul vriility of the SAM, except in some yers. We find tht there re lrge errors in the simultion of some extreme sesons which leds to the overll poor skill. Considering SAM extreme precipittion, for those ssocited with ENSO, oth models simulte t lest the sign of SAM ccurtely. For exmple, the L Nin of yer 1988 ws successfully simulted y oth models while CAM4 filed to sptilly cpture the El Nino yer of This nlysis suggests tht the low skill in simultion

11 Asin monsoon simultions 2627 Tle 2 Detils concerning frequently used Asin monsoon indices Nme of the index Type of index Definition References Correltion CAM4 CAM5 Indin summer rinfll (ISR) Precipittion PREC (5 40 N, E) Averged JJAS precipittion over the domin Wester Yng monsoon (WY) Circultion U850 U200 (0 20 N, E) Verticl sher of zonl winds etween 850 nd 200 m levels Western North Pcific monsoon (WNPM) Circultion U850 (5 15 N, E) - U850 ( N, E) Difference of 850 m zonl winds Indin monsoon (IM) Circultion U850 (5 15 N, E) - U850 (20 30 N, E) Difference of 850 m zonl winds South Asin monsoon index (SAM i ) Circultion V850 - V200 (10 30 N, E) Verticl sher of meridonl winds etween 850 nd 200 m levels Wester nd Yng (1992) Wng nd Fn (1999) Wng et l. (2001) Goswmi et l. (1999) The lst two columns give the correltion of simulted indices with oservtions for oth CAM4 nd CAM5 of monsoon internnul vrition rises minly from poor simultion ENSO monsoon teleconnections rther thn the lck of ir se interction (coupling). Since the SAM hs remote nd locl SST teleconnections, the poor simultion of SAM internnul vriility in these tmospheric models (forced with oserved SST) cn e explin y nlyzing the ENSO monsoon teleconnection nd regression of Nino SST on SAM precipittion. We will further discuss this issue in the next section. In cse of circultion indices (Fig. 7), it is seen tht oth CAM4 nd CAM5 show considerle skill in simulting the internnul vrition of the WY nd WNPM indices with significnt correltion coefficients (see Tle 2). For the IM nd SAMi indices, correltion coefficients re insignificnt, mening tht these indices re not well simulted in these models. As the WNPM index represents the Est Asin summer monsoon, the significnt higher correltion of this index mens tht Est Asin monsoon circultions re etter simulted in oth models compred to the SAM. Anlyses of monsoon sptil ptterns lso support this result. This my e due to the fct tht the Est Asin monsoon hs stronger response to ENSO thn the Indin monsoon. To get etter insight into the simulted circultion in CAM4 nd CAM5 simultions, JJAS men velocity potentil nd divergent wind nomlies re clculted for oth models nd oservtions (NCEP). These show upperlevel convergence nd lower-level divergence over the equtoril centrl Pcific, nd upper-level divergence nd lower-level convergence over the SAM region (not shown here). In Fig. 8, we nlyze the difference etween model nd oservtion (NCEP) of the JJAS men velocity potentil nd corresponding divergent winds t 850 m nd 200 m. Although some of the circultion indices hve higher correltion for CAM4 thn CAM5, the overll sptil ptterns from the CAM5 simultions re etter. At oth tmospheric levels, over the Pcific nd Indin regions, CAM5 hve much etter skill (less difference) compred to CAM Composite nlysis Modeling extreme events is one of the most chllenging issues nd vlidting model extreme event simultions is therefore importnt to ssess their performnce. In this susection, we will focus on severl prticulr yers which were recorded s strong nd wek monsoon yers over SAM region (s seen in Fig. 6). Strong nd wek monsoon yers re chrcterized on the sis of significnt wek or strong summer precipittion over the monsoon region. Yers with nomlies of summer men precipittion greter thn 0.5 stndrd devition ove the men re ctegorized s strong monsoon yers (1980, 1981, 1983, 1988, 1994, 1996, 1998 nd 2007) nd those with men precipittion less thn -0.5 stndrd devition elow the men re ctegorized s wek monsoons (1982, 1984, 1986, 1987, 1989 nd 2002). Composites of strong nd wek monsoon yers from oservtions (CMAP), CAM4 nd CAM5 re shown in Fig. 9. The oserved strong nd wek monsoon composites hve lrge-scle structure with nomlies of the sme sign over mny prts within the SAM region (Fig. 9, ). The oserved wek monsoon composite hs negtive nomlies over the whole Indin region wheres in the strong monsoon composite there re positive nomlies over the Indin lnd re, By of Bengl, nd the mritime continent, while there re

12 2628 S. u. Islm et l. c Fig. 6 Time series of Indin summer rinfll (ISR) index for CMAP, CAM4 nd c CAM5. Detils of this index re given in Tle 1. The time series re normlized nd thus unitless negtive nomlies over the equtoril Indin Ocen. The incresed precipittion during strong monsoon over the Indin monsoon region is ccompnied y decrese in precipittion over the equtoril estern Indin Ocen. The models composites (Fig. 9c f) show tht the simultions filed to reproduce the oserved nomly ptterns with some res hving significnt differences. For its wek monsoon composite, CAM4 nd CAM5 hve much different pttern with more intense nd lrge scle positive nomlies (which is opposite from the oserved composite ptterns) over the Western Ghts nd By of Bengl. CAM4 nd CAM5 to some extent, reproduce the strong monsoon composite over southern Indi. In short, oth models show poor skill in differentiting the strong nd wek monsoon yers. This is lso seen in the models ISR index (Fig. 6) s oth showed poor skill in simulting the internnul vriility of the SAM region. This is proly due to the simulted overestimtion of precipittion, s well s strong internl dynmics (noise) in the models. An exmintion of the internnul vriility of CAM4 nd CAM5 monsoon simultions showed tht, lthough the oserved SST tends to enhnce the vriility, the internl dynmics lso produce considerle internnul vriility in these simultions. It is interesting to note tht the lrge vrince in control runs is dominted y few events such s 1983, 1988 nd For mny yers, the internnul vriility produced y internl dynmics is often lrger thn tht in the control runs, suggesting tht the internnul vriility generted purely y internl dynmics is comprle to tht forced y the slowly vrying SST oundry forcing in mny cses. This is proly the reson why these models perform poorly in differentiting strong nd wek monsoon sesons. 3.3 Teleconnection of SAM with ENSO nd IOD in CAM simultions In this section, we explore how well CAM4 nd CAM5 cpture SAM ENSO nd SAM IOD reltionships. We perform regression nlysis to nlyze sptil ptterns of these reltionships wheres lg-led correltion is used for temporl nlysis. Figure 10 shows the liner regression of Nino3.4 (-5 S 5 N, W) nd IOD [(-10 S 10 N, E) - (-10 S 0, E)] SST indices

13 Asin monsoon simultions 2629 c d Fig. 7 Time series of Wester Yng (WY) index, Indin monsoon index (IMI), c Western North Pcific monsoon (WNPM) index, nd d South Asin monsoon (SAM i ) index of oservtion (CMAP/NCEP, lck line), CAM4 (lue line) nd CAM5 (red line). Detils of ll the indices re given in Tle 1. The time series re normlized nd thus unitless c d Fig. 8 Difference etween model (CAM4 nd CAM5) nd oserved (NCEP) JJAS men velocity potentil (10-6 /s) nd corresponding divergent winds (m/s) t, 850 m nd c, d 200 m

14 2630 S. u. Islm et l. Fig. 9 June Septemer (JJAS) nomly precipittion composites of wek (1982, 1984, 1986, 1987, 1989 nd 2002) nd strong (1980, 1981, 1983, 1988, 1994, 1996, 1998 nd 2007) monsoon yers for, oservtion (CMAP), c, d CAM4 nd e, f CAM5. Units re in mm/dy with JJAS precipittion nd 850 m winds for oservtions nd oth models. Unless stted otherwise, ll regression mps show the covrince of the normlized Nino3.4 nd IOD indices. The regression of precipittion nd 850 m wind onto the Nino3.4 index is n importnt key to understnding the ehvior of CAM4 nd CAM5 in simulting relistic ENSO properties s these fields re direct indictors of the connection etween the ocen nd the tmosphere. In Fig. 10, the regression of the oserved Nino3.4 index with precipittion nd 850 m winds is shown. The oservtions revel enhnced precipittion over the By of Bengl, ccompnied y westerly wind nomly nd decresed precipittion over most of the Indin region. In the models results (Fig. 10c, e) quite relistic ptterns re seen over the equtoril Indin Ocen wheres over the Western Ghts nd centrl Indi, oth CAM4 nd CAM5 hve opposite response compred to oservtions. In the IOD regression pttern (Fig. 10d, f), sptil modes re well reproduced over the Indin Ocen (enhnced precipittion over the Western equtoril Indin

15 Asin monsoon simultions 2631 c d e f Fig. 10 The liner regression of oserved June Septemer (JJAS) Nino3.4 (-5 S 5 N, W) nd IOD [(-10 S 10 N, E) - (-10 S 0, E)] SST indices with June Septemer Ocen nd decresed precipittion over the Estern Indin Ocen) wheres over the Indin continent the models hve the opposite sign similr to the ENSO regression. The nlysis ove showed tht precipittion response to locl nd remote SST in the models simultions is not preserved. To further investigte this issue, the lgged (JJAS) oserved nd simulted precipittion nd 850 m winds., Oservtion (CMAP/NCEP), c, d CAM4 nd e, f CAM5 correltion of Nino3.4 nd IOD indices with the simulted ISR index is shown in Fig. 11. The re verged (0 40 N, E) time series of oserved CMAP precipittion (ISR, solid lck line), oserved All-Indi Precipittion (AIR) index (dshed lck line) nd simulted (CAM4, solid lue line nd CAM5, solid red line) time

16 2632 S. u. Islm et l. series re correlted with oserved Nino3.4 nd IOD SST indices. The months with negtive (positive) sign indicte tht SST leds (lgs) the ISR with mximum led of 12 months (1 yer). Months 0 nd 12 indicte June wheres months 4 nd 8 correspond to Ferury nd Octoer (minus sign for previous months). Correltions re clculted using 5 month sliding window. The oserved positive correltions occur when the SST leds the SAM precipittion nd negtive correltions occur when SST lgs the SAM precipittion. A negtive correltion is seen for SST from the sme summer to the following winter, showing wek (strong) SAM in El Nino (L Nin) developing yers. The highest negtive correltions re noted if the monsoon lgs Nino 3.4 SSTs which suggests, s reported in Kirtmn nd Shukl (2000), tht monsoons cn provide fvorle conditions for triggering or enhncing El Nino or L Nin events in the Pcific during the following winter. The oserved ISR index hs significnt lgged nd led correltions with SST, with the highest vlue of round 0.5 when monsoon precipittion is slightly led y SST. This suggests cross-interction etween monsoon nd ENSO, nmely, ENSO impcts on SAM precipittion nd menwhile the monsoon vriility my ffect the ENSO evolution, intensity, nd periodicity. The mutul influence etween ENSO nd SAM hs een widely reported in other studies s well (Chung nd Nigm 1999; Kitoh nd Arkw 1999; Meehl nd Arlster 1998; Wng et l. 2004). It is noted tht the difference in correltion mgnitude etween Fig. 11 nd those in other studies (Kirtmn nd Shukl 2000; Ysunri 1990) my e ccounted for y drmtic chnge of the ENSO SAM reltionship in the lte 1970s, since Fig. 11 is otined only using the dt fter In the CAM4 nd CAM5 simultions, this reltionship is poorly cptured, prticulrly when SST lgs monsoon. CAM4 shows somewht comprle result when SST leds monsoon. Both of these models filed to mintin the monsoon nd ENSO reltionship ecuse, s discussed in the composite nlysis, the internl dynmics of these models cn overwhelm the Pcific SST influence on monsoon precipittion. In Fig. 11, the lg-led correltion of SAM with IOD is computed. A positive correltion during lte spring nd the simultneous summer is seen. The correltion chnges to negtive in the following fll, suggesting negtive feedck of SAM on the IO. For the models, the simultneous response is not cler ut when IOD leds monsoon, oth models show comprle response to oservtion. Overll the ove nlysis shows tht the internnul skill of CAM4 nd CAM5 in simulting SAM is poor. Both models filed to differentite strong nd wek monsoon which is indirectly linked to their poor reproduction of the ENSO monsoon reltionship. The correct representtion of the ENSO SAM reltionship in models is crucil, since it is the sis for sesonl forecsts of SAM using climte models. The poor skill of CAM4/CAM5 internnul vriility, even when forced y oserved SST, is ttriuted to the models poor skill in simulting the SST precipittion reltionship over the Indin nd Pcific ocens nd lck of tmosphere ocen coupling tht hs een reported s criticl for skillful simultion of the monsoon (Wng et l. 2005). Since this lck of tmosphere ocen coupling in GCMs is one of the possiilities for their poor skill, we focus our nlysis on the coupled CCSM4 model in the next section nd will compre its simultion with CAM4 nd oservtions. 4 Coupled simultion using CCSM4 We now investigte the men climtology nd reltionship etween SAM precipittion nd SST in the CCSM4 fully coupled model, which will llow insight into the role of coupling on the simultion of SAM precipittion. As previously discussed, CCSM4 uses CAM4 s its tmospheric model nd POP2.2 s its ocen model. Along with oservtions, we will lso contrst CCSM4 with the CAM4 results presented in previous sections. Here we will minly use the CCSM4 climtology run wheres for some sensitivity experiments, dt from CCSM4 trnsient run (CCSM4_TR, downloded from NCAR) forced with oserved forcing of green house gses) is lso used. 4.1 Men climtology of CCSM4 We first nlyze, s for the tmospheric simultions, the coupled model over the tropicl region including oth Indin nd Pcific Ocens. Figure 12 shows the oserved (CMAP) JJAS men precipittion differences (in mm/dy). The difference etween the CAM4 climtology run (CAM4_CLIM) nd oservtions is lso shown in Fig. 12. CCSM4 shows significnt differences, prticulrly over the Indin nd Pcific Ocen. Specificlly, the coupled simultion underestimtes precipittion over the western nd estern equtoril Pcific nd over the estern Indin Ocen, nd it overestimtes precipittion over the western Indin Ocen nd centrl equtoril Pcific Ocen. These differences, shown in Fig. 12, result from the ocen component of CCSM4. The SST is from the ocen model influences the precipittion directly, mking it different from the oserved precipittion climtology. Compring CCSM4 with CAM4_CLIM shows tht CCSM4 precipittion ises re t rod scle (especilly over ocens). In CCSM4, the negtive precipittion is increses over the equtoril re in the Pcific Ocen. This is proly due to the feedcks of ocen-ir coupling in the coupled model tht mplifies the is in the tmospheric nd

17 Asin monsoon simultions 2633 Fig. 11 Lg-led correltion of monthly men precipittion with Nino3.4 (-5 S 5 N, W) nd IOD [(-10 S 10 N, E) - (-10 S 0, E)] indices. Are verged (0 40 N, E) time series (ISR) of oserved CMAP (solid lck line) precipittion, oserved All-Indi Precipittion (AIR dshed lck line) time series nd simulted (CAM4 solid lue line nd CAM5 ocenic components. CCSM4 hs the doule ITCZ is, chrcterized y hevy simulted precipittion over much of the tropicl Pcific (centrl Pcific ner 5 south) nd the equtoril Indin Ocen, nd light precipittion in the west nd centrl Pcific etween 15 nd 30 south. The doule ITCZ prolem ws lso present in previous versions of the CCSM model. Lin (2007) found tht most of the current coupled models nd uncoupled models hve this doule ITCZ prolem to some extent. Focusing on the SAM region revels tht while the CAM4 simultion overestimtes precipittion over much of this region, the coupled simultion is more relistic nd comprle to oservtions lthough the mounts re still overestimted somewht. This is similr to results from the CCSM3 model reported in Meehl et l. (2006). The CCSM4 simultion hs reduced is over the Arin Peninsul nd the western cost of Indi (Fig. 12), which is direct consequence of the thermodynmic ir se interctions in the Arin Se, By of Bengl, nd South Chin Se, which re sent in uncoupled simultions. Meehl et l. (2012) lso reported tht the CCSM4 simultion over the SAM region is much etter thn the CAM4 simultion. It is lso reported in Wng et l. (2004) tht the implementtion of ir se coupling could improve the model simultion of monsoon precipittion nd circultion in the Asin monsoon. The sence of ir se coupling in CAM4 results in continuous heting of the tmosphere y the prescried SST (thus keeping the SST wrm) which increses the evportion, resulting in incresed precipittion in those models. This positive feedck on precipittion mplifies the SAM vriility nd therefore CAM4 simultions overestimte solid red line) time series re correlted with oserved Nino3.4 nd IOD SST indices. The month with negtive (positive) sign indicte tht SST leds (lgs) the ISR with mximum led of 12 months (1 yer). Month 0 nd 12 indictes June wheres month 4 nd 8 correspond to Ferury nd Octoer (minus sign for previous month). Correltions re clculted using 5 month sliding window precipittion in the SAM region. We will further discuss ir se coupling nd role of SST is in the next section. In generl, CCSM4 shows lrge reduction in precipittion nd less is over the SAM region including the Arin Se, By of Bengl nd equtoril Indin Ocen s compred with the CAM4 simultion. The reduction of SAM precipittion in the coupled model cn lso e seen in the sesonl cycle (not shown) verged over the region. Sesonl men JJAS precipittion nd 850m wind from CCSM4 shown in Fig. 13 is more relile nd comprle to the oservtions thn results from CAM4 nd CAM5 (compre with Fig. 3). The extreme high precipittion re over the Northern West Indin Ocen is diminished in the coupled simultion mking it more comprle to oservtions. However, CCSM4 lso removed the hevy oserved precipittion over the By of Bengl mking its simultion different thn oservtion. This is significnt shortcoming of the coupled simultion. Also, in CCSM4 runs, the precipittion is more concentrted in the western Indin Ocen, which ws lso seen in CCSM3 (Meehl et l. 2006). Considering these sptil ptterns only, the overll men climtology of CCSM4 seems to e more relistic nd much etter thn from the uncoupled simultion, providing evidence tht coupled ir se interction is necessry for climte models. 4.2 Teleconnection of SAM with ENSO in coupled simultion A correct ENSO monsoon reltionship is one of the prerequisites needed for coupled model to produce relile