Tropospheric biennial oscillation of summer monsoon rainfall over East Asia and its association with ENSO

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1 Clim Dyn (2015) 45: DOI /s Tropospheri iennil osilltion of summer monsoon rinfll over Est Asi n its ssoition with ENSO Yunyun Liu Zeng Zhen Hu Arun Kumr Peito Peng Dn C. Collins Bhskr Jh Reeive: 25 Mrh 2014 / Aepte: 21 Novemer 2014 / Pulishe online: 9 Deemer 2014 The Author(s) This rtile is pulishe with open ess t Springerlink.om Astrt Bse on oservtions n set of Atmospheri Moel Interomprison Projet (AMIP)-type simultions, the limti hrteristis n ominnt sptil ptterns of summer rinfll on tropospheri iennil osilltion (TBO) time sles over the Est Asin summer monsoon (EASM) region were exmine, n the ssoition with se surfe temperture nomlies (SSTAs) n El Niño-Southern Osilltion were nlyze. It ws note tht to some extent, the AMIP run suessfully simulte the sptil istriution n mplitue of the oserve TBO omponent. Furthermore, the AMIP ensemle men inrese the frtion of totl vrine of the TBO omponent, suggesting tht SSTAs my hve rinfll response over the EASM region on TBO time sles. The nlysis lso inite tht sptil pttern of rinfll on TBO time sles with opposite vritions etween northern n southern Chin showe onsistent n roust reltionship with SSTAs in the tropil Pifi Oen in oth the AMIP simultions n oservtions. Sttistilly,when n El Niño (L Niñ) evelops, northern Chin fvors ry (wet) onitions n southern Chin fvors wet (ry) onitions t TBO time sles. Y. Liu (*) Ntionl Climte Center, Chinese Meteorologil Aministrtion, No. 46 Zhong Gun Cun South Avenue, Beijing , People s Repuli of Chin e-mil: liuyuny@m.gov.n Z. Z. Hu A. Kumr P. Peng D. C. Collins B. Jh Climte Preition Center, NCEP/NOAA, College Prk, MD 20740, USA B. Jh Innovim, LLC, Greenelt, MD, USA Keywors Tropospheri iennil osilltion (TBO) Summer monsoon rinfll Est Asi ENSO Atmospheri Moel Interomprison Projet (AMIP) 1 Introution There is signifint frtion of vriility t iennil osilltion sles in tropospheri irultion n preipittion in the monsoon region. This phenomenon ws terme the tropospheri iennil osilltion (TBO) (Meehl 1994, 1997) to istinguish it from the qusi-iennil osilltion in the strtosphere (Ree et l. 1961). The stuies of TBO over the Asin monsoon region egn in the 1980s. For exmple, Mooley n Prthsrthy (1984) note tht ll Ini rinfll verge from June to Septemer hs signifint omponent of TBO. Lter, some TBO signls were lso foun in Est Asin summer monsoon (EASM) rinfll (Tin n Ysunri 1992; Chng n Li 2000; Chng et l. 2000; Ding 2007), inluing some regions of the minln of Chin (Hung 1988; Wng et l. 1995; Nitt n Hu 1996; Zhn et l. 2013). In ft, TBO ommonly exists in tmospheri irultions, tropil se surfe temperture (SST) n other surfe meteorologil elements in the Inin n Pifi Oens s well s whole Asin Austrlin monsoon region (Rsmusson et l. 1990; Ysunri 1990; Shen n Lu 1995; Meehl n Arlster 2002; Meehl et l. 2003; Qio et l. 2005; Li et l. 2011; Liu n Ding 2012; Liu et l. 2013). Moreover, the TBO omponent in the EASM region lso experiene interel vritions. It ws intensifie fter the mile of the 1970s in the vritions of summer rinfll n temperture nomlies long the mile n lower rehes of the Yngtze River (Nitt n Hu 1996; Hu 1999).

2 1748 Y. Liu et l. The physil mehnism of the TBO over the Asin monsoon region is still not well unerstoo. One hypothesis propose is tht TBO is result of oen tmospheri intertion. For exmple, Shen n Lu (1995) n Lu n Yng (1997) rgue tht ir se oupling over the region from the South Chin Se to the western Pifi wrm pool is ssoite with TBO of EASM vritions. Alterntively, Rsmusson et l. (1990) n Ysunri (1990) suggeste the ontriution y the entrl n estern tropil Pifi Oen ssoite with El Niño-Southern Osilltion (ENSO) to TBO of the Asin summer monsoon vritions. Meehl n Arlster (2002) emphsize tht the TBO is funmentl feture of the ouple limte system over the entire Inin Pifi region, n the oen tmospheri intertion over the region ws the ominnt ontriutor to Asin Austrlin monsoon vritions t TBO time sles. They note n intrinsi oupling of the nomlous strength of the onvetive mximum in the sesonl yle over Austrlsi to surfe win foring, oen ynmil response, n ssoite SST nomlies tht fee k to the strength of the onvetive mximum, n so on. The onsequent feek to the monsoon irultion y the SSTAs in the tropil Pifi n Inin Oens les to the TBO in the Asin Austrlin monsoon region. Chng n Li (2000) n Li et l. (2001), however, rgue tht the TBO my e lol ir se ouple phenomenon. Supporting eviene is tht the TBO signl over the Inin Oen is still present without the role of remote foring, suh s equtoril Pifi SSTAs. In the lol oupling prigm, the feeks etween the Inin Oen SSTA n the Asin monsoon, etween the surfe win n evportion, n etween the win stress n oen thermoline ll ontriute to TBO like vritions. Nevertheless, oth Meehl n Arlster (2002) n Meehl et l. (2003) note tht without the influene of ENSO, TBO in the Asin Austrlin monsoon region woul e wekene. Thus, these results suggest tht the vritions of Est Asin summer limte t TBO time sles re lrgely ssoite with lol ir se intertion n remote foring, suh s ENSO, my ffet the intensity of the TBO signl to some extent. In this stuy, the limti fetures of summer rinfll n its TBO omponent over the Asin summer monsoon regions were first nlyze in the oservtions n in the Atmospheri Moel Interomprison Projet (AMIP) simultions to ssess the fielity of the AMIP simultions. Then, the leing ptterns of rinfll in the EASM region t TBO time sles, isolte using the empiril orthogonl funtion (EOF) tehnique, re ompre etween the oservtions n AMIP simultions to ientify roust n onsistent sptil ptterns. Lstly, the ssoition of roust EOF pttern evolution with glol SST is exmine to further ientify the role of SSTAs, prtiulrly the ssoition with ENSO, in the TBO of summer rinfll in Est Asi. The pper is orgnize s follows. Setion AMIP run, t, n pprohes provies esription of t, AMIP simultions, n pprohes use in the stuy. Setion Anlysis of men stte shows the limtologil hrteristis of the JJA rinfll n its TBO omponent in the oservtions n AMIP simultion. Setion Leing moes of TBO in summer rinfll ientifies the roust n onsistent EOF moes of rinfll in the EASM region. Setion Assoition of ENSO with leing EOF moes exmines their onnetion with glol SST, prtiulrly with ENSO evolution. Summry n isussions re given in the setion Summry n isussion. 2 AMIP run, t, n pprohes The AMIP-type simultions re from the tmospheri omponent (Glol Forest System: GFS) of the Climte Forest System version 2 moel of the Ntionl Centers for Environmentl Preition (Kumr et l. 2012; Sh et l. 2014). In GFS, the su-gri physil proess inlues simplifie Arkw Shuert eep onvetion n shllow onvetion with n upte mss flux sheme. The moel hs horizontl resolution of T126 n 64 vertil levels extening from the surfe to 0.26 hp. For eh simultion, the sme oserve evolution of SST, se ie extent, n oserve time-evolving greenhouse gs onentrtions were speifie s externl foring. AMIP simultions onsist of 18 integrtions (ensemle memers) with slightly ifferent tmospheri initil onitions from 1 Jn Monthly men preipittion rte (Prte) from the moel is nlyze in this work. The orresponing oservtionl t inlues the Climte Preition Center monthly preipittion nlysis over the glol ln on resolution (Chen et l. 2002), n the Extene Reonstrute SST tset (ERSSTv3) t 2 2 resolution (Smith et l. 2008). The nlysis perio is from 1957 to In the ontext of Asin monsoon rinfll, the TBO is efine s the teneny for reltively strong monsoon to e followe y reltively wek one, n vie vers (Meehl n Arlster 2002). The TBO omponent is isolte from the June July August (JJA) totl rinfll y removing the 3-y running men. In ition, monthly SST t re filtere with month n-pss filter prior to lulting the le-lg orreltions with the priniple omponent (PC) of the EOF pttern of filtere rinfll in JJA. To ientify the leing sptil ptterns of TBO rinfll in the EASM region n their evolution, the EOF tehnique is pplie y using the o-vrine mtrix of JJA rinfll t TBO time sles with re (ltitue) weighting (North et l.

3 Tropospheri iennil osilltion of summer monsoon rinfll 1749 Fig. 1 Climtologil summer (JJA) rinfll (mm/y) of the oservtions n the AMIP run, n the ifferene of () (). The gry line represents Tietn Plteu 1982). In ition, liner regression is lso use to exmine the onnetion etween SSTAs n TBO rinfll nomlies in the EASM region. 3 Anlysis of men stte In moel simultions, n urte epition of the men limte my e essentil to prouing relisti TBO signl. In this setion, the limte men, n the vrine of JJA rinfll n its TBO omponent re ompre for the oservtions n the AMIP simultions to ssess fielity of the moel. 3.1 Climtologil men istriution Over ro Asin summer monsoon region, Fig. 1 shows the 55-y men ( ) limtologil summer (JJA) rinfll from the oservtions (Fig. 1), AMIP simultions verge over ll 18 memers (Fig. 1), n moel ises reltive to the oservtions (Fig. 1). The oserve sesonl men rinfll is generlly ove 5 mm/y over most of the region exept for North Chin. Climtologilly, rinfll over the region is hrterize y hevy rinfll enters minly lote ner the western ost n northest of Ini, the northern By of Bengl, the Inohin Peninsul, n the Philippines (Fig. 1). The sptil istriution of the oserve limtologil rinfll is well pture y the moel (Fig. 1). The overll pttern of ifferenes (ises) etween moel simulte n oserve limtology (Fig. 1) is ominte y wet is to the south n est of the Tietn Plteu, inluing the north of Ini n the Inohin Peninsul, n entrl n western prts of Est Chin. Menwhile, ry ises re present over the southern Inohin Peninsul n most of the Philippines, s well s over Kore, southern Jpn, n the southestern ost of Chin (Fig. 1). The resons for these ises re still unler, prtiulrly for

4 1750 Y. Liu et l. Fig. 2 Vrine of JJA rinfll nomlies in the oservtion, AMIP iniviul memers, the AMIP ensemle men, n the vrine rtio etween () n (). The units for shing n ontours re mm 2 /y 2 in ( ) n % in (). The gry line represents Tietn Plteu the pronoune wet is roun the Tietn Plteu. The is is lrger in low ltitues thn in high ltitues, whih is expete s the totl rinfll mount ereses from low to high ltitues. Over the EASM region, the moel is is smller thn in the tropis. 3.2 Vrine of the monsoon rinfll Vrines of the monsoon rinfll from oservtions n moel simultions re shown in Fig. 2. The results for the oserve rinfll show tht lrger vriility in JJA rinfll generlly ollotes with mjor enters of men rinfll mounts (Figs. 1, 2), mening tht lrge (smll) vriility orrespons to lrge (smll) men rinfll mount. Figure 2, show the orresponing vrine of summer monsoon rinfll from the AMIP runs, lulte for eh AMIP memer iniviully first (Fig. 2) n then verge for ll 18 memers (Fig. 2). The sptil fetures of the oserve vriility re well pture y the moel. However, the vriility is generlly overestimte over northern Ini, the northestern By of Bengl, the entrl Inohin Peninsul, southern Chin n the northern Philippines (Fig. 2, ). Compre with Fig. 2,, the mplitue of vrine ereses signifintly, when rinfll from the 18 memers is verge first n then vrine is lulte se on the ensemle men (Fig. 2). The vrine of the AMIP ensemle men (Fig. 2) is lrgely the result of internnul vriility fore y SST vritions tht re ommon for ll AMIP simultions, euse the ontriution from rnom internl vriility is mostly nele out y verging of ensemle memers (Kumr et l. 2001). Nevertheless, it is note tht the ensemle men (Fig. 2) reproues the enters of lrge vriility lthough with smller mplitues in the tropis n sutropis, suh s for the western ost n north of Ini, the northwestern prt of the Inohin Peninsul, n southern Chin, implying possile ssoition of SSTAs with rinfll vriility over these regions.

5 Tropospheri iennil osilltion of summer monsoon rinfll Figure 2 is the rtio of rinfll vrine etween the AMIP 18-memer verge n the ensemle men (rtio of Fig. 2, ). Usully, from pure sttistil rgument, the vrine of the ensemle men of n 18-memer ensemle shoul e out 1/18 (5 6 %) tht of single memer if ll the vriility is ue to rnom internl vriility (Wu et l. 2004). It is note tht the vrine rtio etween the ensemle men of the TBO omponent n iniviul AMIP simultions is lrger thn 10 % over most of the monsoon region in Fig. 2. This is n inition tht the internnul vrition (inluing TBO omponent) of Asin summer monsoon rinfll my prtilly e ssoite with externl foring, suh s y SST. 3.3 Vrine n frtion of TBO omponent of monsoon rinfll Consiering tht the Asin summer monsoon rinfll hs signifint frtion of vriility t TBO time sles (Lu n Wu 2000; Meehl n Arlster 2002; Li et l. 2006), we ompute the vrine of the TBO omponent n its vrine frtion reltive to rw (unfiltere) t for the oservtions, for the iniviul AMIP simultions verge, n for the ensemle men, respetively (Fig. 3). The oservtions show tht the vrine istriution pttern of the TBO omponent is similr to tht of the totl monsoon rinfll, espeilly for its primry enters, whih re nerly ollote (Figs. 2, 3), suggesting tht lrge (smll) totl rinfll mount orrespons to lrge (smll) mount of rinfll t TBO time sles. On verge, the vrine frtion of TBO reltive to tht of rw t usully exees 35 %, n rehes 45 % over some regions, suh s northest Ini, the entrl Philippines, the southern Inohin Peninsul, the entrl Yngtze River sin, n most of North Chin (Fig. 3, ). This implies tht TBO is n importnt omponent of summer monsoon internnul vritions in these regions. The AMIP results well reproue the sptil istriution of TBO vrine (Fig. 3, ), while the frtion of vrine of the TBO omponent to the totl rinfll is unerestimte ompre with the oservtions (Fig. 3, e). The istriution of the frtion in the oservtions (Fig. 3) shows ler sptil inhomogeneity with lrge vlues in the entrl prt of Est Chin, while the AMIP simultions isply n lmost sptilly homogeneous istriution (Fig. 3e). The sptilly homogeneous istriution my imply tht internl ynmil proesses ply ominnt role in TBO time sle vritions in eh iniviul ensemle memer. Interestingly, lthough the ensemle men (Fig. 3) hs muh smller mplitues for the vrine of TBO rinfll, the mplitue n sptil istriution of the vrine rtio of TBO to rw t is muh loser to the oservtions ompre with tht of the AMIP 18-memer verge 1751 (Fig. 3 f). This result suggests tht the TBO signl in the EASM region is enhne y eliminting the tmospheri internl vriility through the ensemle men. This inites the ontriution of SST to the TBO time sle vritions n is onsistent with Meehl et l. (2003). The onnetion of TBO vrition n SST will e further exmine in the next two setions. 4 Leing moes of TBO in summer rinfll The omprison in the previous setion suggests tht the AMIP run ptures the sptil istriution of the si limte sttes of summer rinfll n its TBO omponent over the Asin monsoon region. This provies justifition for further nlyzing the leing moes of the TBO omponent of rinfll in the EASM region n their onnetion with SSTA. The sptil omin for the EOF nlysis is hosen to e from 22.5 to 54 N n from 105 to 145 E, so s to fous on the EASM region (Wng et l. 2001). As in the previous setion, the AMIP simultions re hnle in two wys. One is to ontente 18 memers of the AMIP run s one long time series, n then o the EOF nlysis. Suh n nlysis is equivlent for oservtions n is influene y oth the externl foring (suh s SST) n internl tmospheri vriility. The other is to o EOF nlysis for the ensemle men of 18 memers of the AMIP run. The results from suh EOF nlysis mplify the response of the tmosphere to SSTA, sine the internl vriility is suppresse y the ensemle verging. Pttern orreltions etween the EOF ptterns from moel simultions n those from the oservtions re ompute to ompre their sptil similrity. 4.1 EOF for oservtions n iniviul AMIP simultions Figure 4 shows the first four EOF moes of the TBO omponent of JJA rinfll in the oservtions, whih explin 19.8, 14.6, 11.7 n 9.9 % of the totl vrine of the TBO omponent, respetively. The umultive expline vrine y the first four moes is 56 %, whih re inepenent from eh other oring to North s et l. (1982) signifine test. EOF1 (Fig. 4) is tripole pttern ( +,, + ), whih is lso typil pttern of totl summer rinfll (Nitt n Hu 1996; Ding et l. 2008) with opposite vritions of rinfll long the Yngtze River sin n most of Kore n Jpn to rinfll to the north n south. EOF2 (Fig. 4) represents opposite vritions of rinfll over the regions from the mile of the Yngtze River to Northest Chin vi the HuiHe River sin to rinfll in southestern Chin, southern Kore n Jpn. EOF3 (Fig. 4) hs some similrities with EOF1 in estern Chin. EOF4 is ipole

6 1752 Y. Liu et l. e f Fig. 3 Vrine of the TBO omponent of JJA rinfll n its rtio to the vrine of rw t in, oservtion,, e the AMIP 18-memer verge n, f the AMIP ensemle men. The units for shing n ontours re mm 2 /y 2 in ( ) n % in ( f). The gry line represents Tietn Plteu pttern ( +, ), whih is nother typil pttern for totl summer rinfll tht usully ours uring n ENSO yer (Wng et l. 2000; Ding et l. 2008), with opposite vritions etween northern n southern Chin. Figure 5 shows the orresponing EOF moes of the AMIP simultions se on iniviul memers. Compre with the oservtions (Fig. 4), it is note tht the first three EOF moes from the AMIP run hve orresponing

7 Tropospheri iennil osilltion of summer monsoon rinfll 1753 Fig. 4 The first four EOF moes of JJA rinfll in the oservtions. EOFs 1 4 explin 19.8, 14.6, 11.7 n 9.9 % of the totl vrine of TBO omponent, respetively moes in the oservtions, though the sequene n mplitues of the EOF moes re somewht ifferent. For exmple, EOF1 from the AMIP run (Fig. 5) is similr to EOF4 from the oservtions (Fig. 4) with pttern orreltion etween the two EOFs of 0.55 (Tle 1). Similrly, EOF2 of the AMIP run hs signifint pttern orreltion oeffiient of 0.68 with EOF1 of the oservtions, while EOF3 of the AMIP run orrespons to EOF2 of the oservtions with pttern orreltion oeffiient of 0.57 (Tle 1). All of these pttern orreltion oeffiients exee the 0.01 signifine level. 4.2 EOF nlysis for ensemle men To isolte the impt of SSTA on TBO, we first mke the ensemle men of the 18 memers of the AMIP run, n then ompute the EOF moes (Fig. 6). Compre with the results of the 18 iniviul memers of the AMIP run (Fig. 5) n the oservtions (Fig. 4), we get notiele ifferenes. For exmple, the mplitues of these EOF moes (Fig. 6) re generlly smller thn tht from the oservtions n from the AMIP 18 iniviul memers, n the sequene of the EOF moes is not the sme either. The ifferenes in sequene of these EOF moes shown in Figs. 4, 5 n 6 my inite tht either the oserve smple size is not ig enough or there is n impt of moel ises. EOF1, EOF2, n EOF4 for the AMIP ensemle men orrespon to EOF4, EOF1 n EOF2, respetively, from the oservtions. The orreltion oeffiients of these three ominnt moes with their ounterprts from the oservtions re 0.52, 0.55 n 0.60, respetively, n ll of them exee the 0.01 signifine level (Tle 2). However, similr moe s EOF3 in the iniviul memers n ensemle men (Figs. 5, 6) oesn t pper in the first four moes of the oservtions (Fig. 4). Tht my suggest n impt of moel ises n lk of roustness of this moe. Bse on pttern orreltion, it is note tht three out of the first four EOF ptterns in the oservtions n e reproue y the AMIP run either for the nlysis se on iniviul memers or on the ensemle men. Also, it

8 1754 Y. Liu et l. Fig. 5 Sme s Fig. 4, ut for the AMIP 18 iniviul memers. EOFs 1 4 explin 25.6, 9.7, 5.8 n 5.0 % of the totl vrine of TBO omponent, respetively Tle 1 Pttern orreltions of leing EOF moes etween the oservtions (Os) n the AMIP iniviul 18 memers (In) ACC Os_EOF1 Os_EOF2 Os_EOF3 Os_EOF4 In_EOF * In_EOF2 0.68* In_EOF * In_EOF The ol numer with * represents signifine t 0.01 signifint level using T test is evient tht eh pir of EOF ptterns from the 18 iniviul memers n the ensemle men of the AMIP run is similr with pttern orreltion oeffiients of 0.98, 0.69, 0.55 n 0.61 (Tle 3), respetively. These signifint pttern orreltions inite the fielity of the AMIP simultions. However, it is ler tht the mplitues of EOF2 4 erese sustntilly in the AMIP ensemle men (Fig. 6), ompre with those in the AMIP iniviul memers (Fig. 5). These resemlnes s well s the ifferenes my further suggest tht the TBO of rinfll in the EASM region is ffete y oth internl tmospheri proesses n externl foring, suh s SST. Following this, the onnetion of these EOF moes with SSTA is investigte further. 5 Assoition of ENSO with leing EOF moes As mentione in setion Introution, existene of TBO in the Asin monsoon region is minly the result of the oen ln tmosphere intertion; however, the ontriutions of the tropil oens, espeilly ENSO, re lso notiele (Meehl n Arlster 2002; Rsmusson et l. 1990; Ysunri 1990; Meehl et l. 2003). To further verify the onnetion of SSTA with TBO in the EASM region, we exmine the le-lg orreltion of SSTA with the prinipl omponents (PCs) of TBO rinfll from the ensemle men of the AMIP simultions n from the oservtions.

9 Tropospheri iennil osilltion of summer monsoon rinfll 1755 Fig. 6 Sme s Fig. 4, ut for the AMIP ensemle men. EOFs 1 4 explin 29.6, 9.8, 7.4 n 6.5 % of the totl vrine of TBO omponent, respetively Tle 2 Pttern orreltions of the EOF moes etween the oservtions (Os) n the AMIP ensemle men (Ens) Os_EOF1 Os_EOF2 Os_EOF3 Os_EOF4 Ens_EOF * Ens_EOF2 0.55* Ens_EOF Ens_EOF * The ol numer with * represents signifine t 0.01 signifint level using T test Tle 3 Pttern orreltions of the EOF moes etween two types of AMIP simultion (iniviul 18 memers n ensemle men) In_EOF1 In_EOF2 In_EOF3 In_EOF4 Ens_EOF1 0.98* Ens_EOF * Ens_EOF * Ens_EOF * 0.13 The ol numer with * represents signifine t 0.01 signifint level using T test SSTAs verge in three key regions, Niño3: 5 S 5 N, W (Meehl n Arlster 2002; Meehl et l. 2003), Pifi wrm pool re (WP): 5 S 5 N, E (Hung et l. 2006) n tropil western Inin Oen (WIO): 5 S 5 N, E (Chng n Li 2000; Li et l. 2001), re use to represent the tropil oen vriility. Bsing on the le-lg orreltions of these SST inies with PCs, we first ientify the moes hving signifint orreltion with ENSO. Then, relevnt SSTA sptil ptterns re isolte y the le-lg liner regression of glol SSTA onto the PCs of TBO rinfll from the ensemle men of the AMIP run n from the oservtions. By ompring the similrity of the le-lg orreltion n the le-lg regression of glol SSTA in the AMIP run n in the oservtions, the roust n onsistent reltion of SST n the TBO in the EASM n e estlishe.

10 1756 Y. Liu et l. e f Fig. 7 Time-lg orreltion of the PCs in the oservtions ( PC4; PC1; e PC2) n their ounterprts in the AMIP ensemle men results ( PC1; PC2; f PC4) with SSTAs in three key regions, respetively ( N (N) uner the x-xis enote the SSTA leing (lgging) N months to PCs). The purple she lines re the 0.05 signifine level using T test Bse on the reproution of the leing EOF moes in the AMIP run (Tle 2), the nlysis fouses on EOF1, EOF2 n EOF4 from the ensemle men of the AMIP simultions n the oservtions. Figure 7 shows the lelg orreltions of three PCs (PC1, PC2 n PC4) from the oservtions (left olumn) n from the ensemle men of the AMIP run (right olumn) with the SST inies in the three tropil regions: Niño3, WP n WIO. As for the first pir of TBO PCs (PC4 from the oservtions n PC1 from the ensemle men of the AMIP run), signifint n onsistent le-lg orreltions (Fig. 7, ) re foun in oth the oservtions n the AMIP run,

11 Tropospheri iennil osilltion of summer monsoon rinfll 1757 f g h i e j Fig. 8 Le-lg liner regression of PC4 of the oservtions (left olumn) n PC1 of the AMIP (right olumn) with tropil Pifi SSTA. The regression oeffiients re enote y ontours, n 0.1, 0.05 n 0.01 signifine levels using T test re she from light to rk olors, respetively suggesting roust onnetion of this EOF pttern of TBO rinfll with the tropil SSTA inies. The mximum orreltion ours when Niño3 n the PC re simultneous (Fig. 7, ). Sine ENSO normlly peks in orel winter, the simultneous mximum orreltion suggests tht TBO rinfll in the EASM region is ssoite with ENSO evelopment. Aoring to the le-lg orreltions (Fig. 7, ) n the EOF pttern (Figs. 4, 6), we note tht when El Niño (L Niñ) evelops, northern Chin fvors ry (wet) n southern Chin fvors wet (ry) onitions t TBO time sles. The onnetion of the TBO PC (PC4 from the oservtions n PC1 from the ensemle men of the AMIP run) with ENSO is further ssesse through liner regression of glol SSTA onto the PC, oth for the oservtions (left olumn, Fig. 8) n for the ensemle men of the AMIP run (right olumn, Fig. 8). For 6-months le of SST, orreltions re smll, exept for some negtive vlues in the western Pifi n Inin Oens. For 3-months le, the ENSO-like pttern is visile, with positive orreltions in the tropil entrl n estern Pifi, n negtive vlues in the western Pifi. The orreltion rehes its pek in the tropil entrl n estern Pifi for 0-month le. The orreltions then erese when SST lgs the PC. Although the regressions with the PC from the oservtions n from the AMIP simultions (left n right olumns of Fig. 8) re in goo greement, there re mny ifferenes in the etils of lotions n mplitues of the regressions. For exmple, the positive regressions in the Inin Oen n the tropil entrl n estern Pifi

12 1758 Y. Liu et l. re generlly lrger in the AMIP run thn in the oservtions. This is proly euse using the ensemle men in the EOF omputtion for the AMIP run suppresses the SST-unrelte vriility (i.e., the tmospheri internl vriility) n enhnes the vriility ssoite with SST foring. Furthermore, the regression ptterns for the oservtions seem more like ENSO Mooki, n those for the ensemle men of the AMIP run re more nlogous to the onventionl ENSO (Ashok et l. 2007; Hu et l. 2012). Nevertheless, it is unler if the sptil pttern ifferene is ue to moel is or smpling, prtiulrly for the oservtionl nlysis. For the other two pirs of TBO PCs, the reltionships with tropil SST nomlies re not well reproue y the AMIP ensemle men (Fig. 7 f), lthough the sptil istriutions of the EOF moes resemle the orresponing oservtions. The filure of the AMIP simultions in reprouing the possile link of these EOF moes of TBO rinfll in the EASM region with tropil SST my e use y the influene of moel ises. It my e lso euse the feek of the tmosphere to the oen is not inlue in the AMIP-type experiment. The ir se intertion is prtiulrly importnt for the summer limte in the western North Pifi (Wng et l. 2005, 2009; Zhu n Shukl 2013). Further, it is lso possile tht the onnetion in the oservtions might e osure ue to smll smple size. 6 Summry n isussion Bse on n ensemle AMIP run n oservtions, this stuy exmine the hrteristis n ominnt sptil ptterns of rinfll t TBO time sles over the EASM region n its ssoition with ENSO. To some extent, the AMIP run well simultes the sptil istriution n mplitue of the TBO omponent. The verge of the ensemle men of the AMIP run inreses the rtio of the TBO omponent to the rw t, whih suggests tht SSTA my enhne rinfll vritions in the EASM region t TBO time sles. Further investigtion of the onnetion with SST inite tht only the EOF pttern t TBO time sles with opposite vritions etween northern n southern Chin h roust reltion with SSTAs in the tropil Pifi Oen. Sttistilly, for evelopment of El Niño (L Niñ) events, northern Chin fvors ry (wet) onitions while southern Chin fvors wet (ry) onitions in TBO time sles. There re other EOF moes in the oservtions hving signifint orreltions with the SSTA in the tropil oens, ut the AMIP run ws unle to reproue the possile link. This my e prtilly use y moel ises, n lso my e ue to the ft tht the feek of the tmosphere to the oen is prohiite in the AMIP-type experiment, while the feek my e importnt for the summer limte vriility in the EASM region (Wng et l. 2005; 2009; Zhu n Shukl 2013). The onnetions of SSTA with these EOF moes on TBO time sles in the oservtions nee to e further verifie either using longer t reor or se on ifferent moels to onfirm the roustness of the onnetions s well s the roustness of the EOF moes (Dommenget n Ltif 2002). Lstly, we note tht the summer rinfll in the EASM region seems lrgely riven y internl ynmil proesses. Thus, it is still hllenge to inorporte the reltionship etween ENSO n TBO time sle rinfll vrition into opertionl forests of the estern Asin summer rinfll, sine the overll sttistil reltionship etween ENSO n the summer rinfll vritions over Est Asi re wek (Wu et l. 2003). Aknowlegments This work ws supporte jointly y the Ntionl Speil Siene Reserh Projet (2012CB955203), the Ntionl Bsi Reserh Progrm of Chin (2012CB417205), the Ntionl Nturl Siene Fountion of Chin ( ), n Chin Meteorologil Aministrtion R&D Speil Fun for Puli Welfre (Meteorology) (GYHY , GYHY ). The le uthor (Y. Liu) lso thnks the support of the Ntionl Innovtion Tem of Climte Preition of Chin Meteorologil Aministrtion. Most of this work ws onute when Liu visite the NOAA/NCEP Climte Preition Center. The uthors ppreite the onstrutive omments n suggestions from two reviewers, whih signifintly improve the qulity of this pper. Open Aess This rtile is istriute uner the terms of the Cretive Commons Attriution Liense whih permits ny use, istriution, n reproution in ny meium, provie the originl uthor(s) n the soure re reite. Referenes Ashok K, Beher SK, Ro SA, Weng H, Ymgt T (2007) El Niño Mooki n its possile teleonnetion. J Geophys Res 112:C oi: /2006jc Chng CP, Li T (2000) A theory of the tropil tropospheri iennil osilltion. J Atmos Si 57: Chng CP, Zhng YS, Li T (2000) Internnul n interel vritions of the Est Asin summer monsoon n tropil Pifi SSTs. prt I: roles of the sutropil rige. J Clim 13: Chen MY, Xie PP, Jnowik EJ, Arkin AP (2002) Glol ln preipittion: 50-y monthly nlysis se on guge oservtions. J Hyrometeor 3: Ding YH (2007) The vriility of the Asin summer monsoon. J Meteor So Jpn 85:21 54 Ding YH, Wng ZY, Sun Y (2008) Interel vrition of the summer preipittion in Est Chin n its ssoition with eresing Asin summer monsoon. Prt I: oserve evienes. Int J Climtol 28: Dommenget D, Ltif M (2002) A utionry note on the interprettion of EOFs. J Clim 15: Hu Z-Z (1999) Internnul vriility of summer limte moe over Est Asi n its mehnism. At Oen Sin 21:26 39

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