Journal of the Meteorological Society of Japan, Vol. 75, No. 6, pp ,

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1 Journl of the Meteorologicl Society of Jpn, Vol. 75, No. 6, pp , Internnul nd Interdecdl Vritions of the Western North Pcific Monsoon nd Biu Rinfll nd their Reltionship to the ENSO Cycles By Minoru Tnk Meteorologicl Reserch Institute, 1-1 Ngmine, Tsukub, Ibrki 305, Jpn (Mnuscript received 17 December 1996, in revised form 13 October 1997) Abstrct Internnul nd interdecdl vritions of the Western North Pcific Monsoon (WNPM) nd the Est Asin Biu rinfll re nlyzed using 30-yer ( ) dily se-level pressure, 30-yer monthly nd dily rinfll dt for the Est Asi, nd 15-yer (April 1978-December 1992) 5-dy men 1-degree ltitudelongitude gridded GMS high-cloud-mount dt. The reltionship to the El Nino-L Nin cycles re investigted by using monthly se surfce tempertures (SST) dt over region defined by 4N-4S, 150W-90W in the equtoril Estern Pcific (NINO3 SST) for 30-yer period from 1963 to An nlysis of Internnul vritions in intensity of the WNPM in lte July shows high negtive correltions with the NINO3 SST during recent 15-yer period from 1978 to In the L Nin (El Nino) yers, strong (wek) WNPM re observed. For the Biu rinfll, high positive correltions with the NINO3 SST re observed in July during this period. In the L Nin (El Nino) yers, light (hevy) Biu rinfll is observed. These correltions re the results of dely of bout 20 dys in the onset dtes of the WNPM nd the pek dtes of the Biu rinfll between the L Nin nd El Nino yers. In the L Nin yers, the erly onset of the WNPM nd erly pek of the Biu rinfll re observed. In the El Nino yers, the lte onset of the WNPM nd lte pek of the Biu rinfll re observed. The Biu rinfll lso shows high positive correltion with the NINO3 SST during the erly 15-yer period from 1963 to However, difference in the dtes of pek of the Biu rinfll between the L Nin nd El Nino yers re smller. An nlysis of the lg-correltions mong the WNPM, the Biu rinfll nd the NINO3 SST shows tht the SST leds the WNPM by 1 to 2 months. On the other hnd, the Biu rinfll leds (lgs) the SST by 3 (2) months during recent (erly) 15-yer period from 1978 to 1992 (1963 to 1977). An utocorreltion nlysis of the NINO3 SST shows tht the biennil oscilltion mode ws pronounced during 15-yer period from 1963 to In recent 15-yer period, the low-frequency mode ws dominnt. These chnges in the periods of the SST oscilltions my be relted to the recent increse in the correltions mong the NINO3 SST nd the WNPM nd Biu rinfll. 1. Introduction In recent yers, the Western North Pcific monsoon (WNPM) in summer hs come to be recognized s mjor tropicl monsoon system. Tnk (1992), using GMS high-cloud-mount dt, determined the onset (retret) dtes of the northern summer monsoon over Est, Southest Asi nd the western North Pcific. Mtsumoto(1992) nd Nkzw (1992), using ECMWF wind nd OLR dt, showed tht the WNPM extends up to 150E. Wng (1994), using OLR dt, determined the onset (retret) dtes of the globl summer monsoon. Collectively, these studies showed tht the onset of the WNPM is sudden nd is observed round July. (c) 1997, Meteorologicl Society of Jpn Murkmi nd Mtsumoto (1994) nlyzed the structure of the WNPM nd concluded tht the WNPM is driven by the combined effects of tonl symmetry in the SST t 10 to 20N nd estwest het contrsts between pproximtely 20N nd 30N. Ued et l. (1995) nd Ued nd Ysunri (1996) nlyzed the brupt sesonl chnge of lrgescle convective ctivity nd the circultion pttern during the onset phse of the WNPM. They showed tht the lrgest pressure chnges during the onset re observed round 25N, 140E. The Est Asin Summer monsoon is chrcterized by the erly summer riny seson clled Biu nd Mei-yu in Jpn nd Chin respectively. The lrge-scle spects of the Biu front re nlyzed by Akiym (1973), who showed tht the Biu front over Est Asi forms belt-shped zone of mxi-

2 1110 Journl of the Meteorologicl Society of Jpn Vol. 75, No. 6 mum cloudiness nd coincides with the zone of mximum rinfll mount. The wter vpor convergence within the frontl zone is shown to be minly due to the southwesterly wind long the northwestern rim of the North Pcific Subtropicl High. The reltionship between the monsoon nd El Nino hve been investigted in mny studies: Ysunri nd Seki (1992) showed tht the occurrence of the El Nino is closely ssocited with the preceding wek Indin summer monsoon. For the Est Asin monsoon, Tnk (1987), Ytgi nd Ysunri (1995) showed tht the June-August rinfll in the North Chin region is positively correlted to the Indin monsoon rinfll. Shen nd Lu (1995) found negtive correltion between the April-September rinfll ner 30N, E in Chin nd the equtoril Centrl Pcific SST during the following utumn nd winter. In this study, we investigte internnul nd interdecdl vritions of the WNPM nd the Biu rinfll nd their reltionship to the equtoril Estern Pcific SST during 30-yer period from 1963 to We first nlyze the WNPM nd its reltionship to the El Nino event. Then we explore the Est Asin Biu rinfll nd its reltionship to the sme events. The investigtion of possible correltions mong the WNPM, the Biu rinfll nd the tropicl Pcific SST is the primry im of this pper. 2. Dt Sources Four dtsets (se-level pressure, rinfll, highcloud mount, nd SST) re used in this study: The se-level pressure dt re extrcted from the "Dily Wether Mps" published by the Jpn Meteorlogicl Agency (JMA) for 30-yer period from 1963 to The pressure vlues re red from the dily 12 Z GMT se-level mps t sptil resolution of 5 ltitude-by-10 longitude for region bounded by 40N to 10N, 110E to 180. The dily nd monthly rinfll dt for Jpn for 30-yer period from 1963 to 1992 re obtined from the "Monthly Report of the Jpn Meteorologicl Agency". The monthly rinfll dt for Chin contining 160 sttions from 1951 to 1994 re provided by the Chin Meteorologicl Administrtion. The monthly rinfll dt for the Republic of Kore re extrcted from the "Monthly Climtic Dt for the World" published by NOAA. The high-cloud-mount dt re the 5-dy men 1-degree ltitude-longitude grid dt of the Geosttionry Meteorologicl Stellite (GMS) creted by the Meteorologicl Stellite Center of the JMA. The dt from April 1978 to December 1992, which re sme s Tnk (1992,1994), re used in this study. The monthly SST dt for region bounded by 4N to 4S, 150W to 90W for period from 1956 to 1994 re obtined from the JMA to monitor the equtoril Estern Pcific SST. This region (defined s NINO3 SST by the JMA) is min region used by the JMA for monitoring the El Nino events. In the following discussion, the nme "NINO3 SST" is used for equtoril Estern Pcific SST. For ll dily dt, 5-dy verged vlues re obtined. The filter is then pplied to ll 5-dy men dt to remove high frequency noise less thn bout 7.5 dys. For the monthly NINO3 SST, the filter is lso pplied to remove short-period fluctutions. For brevity, the term "smoothed" is not used in the subsequent discussions. These dt re divided into two 15-yer periods from 1963 to 1977 nd from 1978 to 1992 for n investigtion of the interdecdl vritions. The yer of this division ws bsed on the initition of GMS observtions in 1978 nd n brupt interdecdl wrming of the tropicl SST round 1977 (1979) reported by Nitt nd Ymd (1989) (Wng, 1995). 3. Internnul vritions of the WNPM nd its reltionship to the NIN03 SST Ued et l. (1995) nd Ued nd Ysunri (1996) showed tht the lrgest low-level pressure chnge during the onset of the WNPM is observed ner 25N, 140E nd the consequent rpid increse in the westerly winds is observed ner 15N, 140E. In this study, the 5-dy men se-level pressure difference between 10N nd 20N t 140E is used to monitor the strength of the westerly winds of the WNPM ner 15N (defined here s WNPM Index). The yer-to-yer chnge in the strength of the WNPM Index for 15-yer period from 1978 to 1992 is shown in Fig. 1. The onset of the WNPM ws erly (lte) in 1978, 84, 85, 88 (1983, 87, 91, 92). Figure 2 shows the internnul vritions of the monthly NINO3 SST from 1956 to Using 0.6C deprture for threshold vlue, defined here to include most of the erly nd lte onset of the WNPM, there re 4 L Nin yers (1978, 84, 85, 88), nd 5 El Nino yers (1982, 83, 87, 91, 92) in June of the clendr yers during 15-yer period from 1978 to With the sole exception of 1982, ll the L Nin (El Nino) yers coincide with the erly (lte) onset of the WNPM (Fig. 1). Figure 3 shows the utocorreltions of the NINO3 SST with respect to June. During recent 15-yer period from 1978 to 1992 (solid line), the low frequency mode of bout 4 yers is dominnt. The biennil mode is nerly bsent with ner zero correltion t led (lg) of bout one yer from reference month. During the 15-yer period from 1963 to 1977, the biennil mode ws pronounced. There exists strong persistence (utocorreltions over 0.9) in the SST from My to the following Jnury. Hence lrge interdecdl differences in the internnul oscilltion of the NINO3 SST re observed.

3 December 1997 M. Tnk 1111 Fig. 1. Internnul vritions of the WNPM monitored by the 5-dy men se-level pressure difference between 10N nd 20N t 140E. Thin horizontl lines show the vlue of zero in the left-hnd scle for ech yer. The intervl between the djcent yer corresponds to vlue of 10hP. The drk shded res show the periods of ctive WNPM defined s the periods of the westerly geostrophic winds ner 15N. The horizontl scle shows the dtes of the lst dy of ech pentd nd the months. Figure 4 shows the reltionship between the WNPM Index during July nd NINO3 SST in June for 15-yer period from 1978 to There is cler tendency (correltion=-0.87; 1% level=0.68) for strong (wek) WNPM in the L Nin (El Nino) yers. The significnce level of the correltions is obtined by the method used by Chen (1982) nd Dvis (1976), where the utocorreltions between two continuous time series (smples) were used to estimte the reduction in the degree of freedom. All the significnce levels of the correltions in this pper re tested by this method. Figure 5 shows the lg-correltions between the WNPM Index during July 20 to 29 nd the NINO3 SST. During recent 15-yer period, the mximum vlues of the correltions re observed for the My nd June SST. Hence the SST leds the WNPM by bout 1 to 2 months. During the 15-yer period from 1963 to 1977, no significnt correltion t the 5 % level is observed. Therefore significnt correltions between the WNPM Index nd the NINO3 SST re restricted to the recent 15-yers from 1978 to Figure 6 shows sptil ptterns of the se-level pressure for the 10-dy period from 20 to 29 July. In the L Nin yers (1978, 84, 85, 88) the WNPM is strong nd the westerly wind extends est to 155E between the ltitudes from 10N to 20N. In the El Nino yers (1982, 83, 87, 91, 92) the subtropicl high extends west to 125E nd the trde wind is observed. A strong, moist southerly wind is observed ner 25N, 130E. This synoptic pttern is observed during the Biu riny seson in Jpn. The difference between the El Nino nd the L Nin yers shows the lrgest pressure difference ner 25N, 140E. The dots shown in Figs. 6 nd 6b re the gridpoints where the difference is significnt t the 95% confidence by the ppliction of onetiled student t-test. Herefter ll the differences in the se-level pressure in this pper re tested by this method. Figure 7 shows sptil ptterns of the GMS highcloud mounts for the 10-dy period from July. In the L Nin yers (1978, 84, 85, 88) the ctive phse of the convection ner 20N, 150E indictes the mture phse of the WNPM. In the El Nino yers (1982, 83, 87, 91, 92) the ctive phse of the convection is observed south of 12N nd ner the Biu front locted ner 35N. Figure 8 shows the correltion between the GMS high-cloud mounts nd June NINO3 SST for the 10-dy period from 20 to 29 July. The regions with significnt correltion t 5% level re shded. A region with significnt negtive correltion ner 20N, 150E with mximum correltion of t 19N, 149E shows tht the WNPM is under the strong influence of internnul vritions of the NINO3 SST. In ddition, severl regions with significnt positive correltion exist ner the Equtor nd north of 30N. 4. Intrsesonl vritions of the WNPM nd their reltionship to the NINO3 SST In the preceding discussion, it ws shown tht the onset of the WNPM is erly (lte) in the L Nin (El Nino) yers during the recent 15-yer period from 1978 to 1992 (Fig. 1). In this section, Intrsesonl vritions of the WNPM nd its reltionship to the NINO3 SST re nlyzed. Figure 9 shows the ltitude-time cross-section of the GMS high-cloud mounts for the longitude 145E to 154E. Comprision of the L Nin (Fig. 9) nd

4 1112 Journl of the Meteorologicl Society of Jpn Vol. 75, No. 6 Fig. 2. Time series of the monthly NINO3 SST from 1956 to The short lines on the horizontl scle indictes December of the clendr yer. The numbers on the horizontl scle re the clendr yer. Fig. 3. The utocorreltions of the NIN03 SST for June. The solid line (dots) shows the vlue for the 15-yer period from 1978 to 1992 (1963 to 1977). Y(0) denotes the yer contining zero-lg month. Y(-1) (Y(-2)) denotes the yer before (two yers before) Y(0). Y(+1) (Y(+2)) denotes the yer fter (two yers fter) Y(0). El Nino (Fig. 9b) yers shows tht there is bout 20-dy difference in the dtes of the mximum vlues of the high-cloud mounts ner 15-25N. Figure 10 shows the correltion between the GMS high-cloud mounts nd June NINO3 SST for the ltitude-time cross-section shown in Fig. 9. The lrgest vlues of the significnt negtive correltions re observed ner 15-25N during lte July which coincides with the erly onset of the WNPM in the L Nin yers. In ddition, significnt negtive correltions re observed during lte My-erly June nd lte June. These negtive correltions suggests tht the climtologicl intrsesonl oscilltions with period of bout 25 to 30-dys re enhnced in the L Nin yers. The lg-correltions between the WNPM nd the NINO3 SST (Fig. 5) suggest tht significnt correltions between these two systems re restricted to recent 15-yer period from 1978 to The ltitude-time cross-sections of the se level pressure t 140E from 25 June to 13 August (23 August for the El Nino yers) re shown in Fig. 11 for 15- yer period from 1978 to The dots shown in Fig. 11 re the ltitude-time periods where the difference is significnt t the 95% confidence by the ppliction of one-tiled student t-test. A strong westerly wind is observed between 10N nd 20N in the L Nin yers. The climtologicl onset dte of the WNPM ner 15N is round July. As discussed erlier, there re lrge differences of bout 20-dys in the onset dtes of the WNPM between the El Nino nd the L Nin yers. In the 15-yer period from 1963 to 1977, the climtologicl onset dte of the WNPM ner 15N is round July. The onset dtes of the WNPM were erly (lte) in 1967, 68 nd 72 (1966, 74). With the sole exception of 1972, the NINO3 SST were ner norml in these yers. Therefore, the reltionship between the onset of the WNPM nd NINO3 SST is not significnt in this period. 5. Internnul vritions of the Est Asin Biu rinfll nd its reltionship to the NINO3 SST The rinfll dt over Est Asi re used to investigte the long-term reltionship between the Biu rinfll nd the NINO3 SST. The lg-correltion nlysis between the monthly rinfll dt over Est Asi nd the NINO3 SST showed tht the lrgest

5 December 1997 M. Tnk 1113 Fig. 4. Reltionship between the June NINO3 SST nd the WNPM during July for the 15-yer period from 1978 to The verticl scle on left shows the NINO3 SST nomlies in C. The horizontl scle shows intensity of the WNPM t 140E. Fig. 5. Sme s Fig. 3, but for the lg-correltions between the WNPM during July 20 to 29 nd the NINO3 SST. numbers of the sttions with significnt correltions re observed between July rinfll nd the NINO3 SST in September (My) for the 15-yer period from 1978 to 1992 (1963 to 1977). Figures 12 nd 12b shows the lg-correltion between the July rinfll in Est Asi nd September (My) NINO3 SST for the 15-yer period from 1978 to 1992 (1963 to 1977). In both periods the highest correltion is observed in the vicinity of Kyushu Islnd nd significnt correltions (5% level re round 0.6 using the method discussed erlier) re observed in estern Chin. These res of high correltions coincide with the regions of hevy Biu rinfll in July. These lg-correltion nlyses show tht the rinfll ws leding the SST by bout 3 to 4 (1 to 2) months in Jpn (Chin) during recent 15-yer period. 6. Internnul vritions of the Biu rinfll in Northwest Kyushu nd its reltionship to the NINO3 SST In the foregoing discussion, it ws shown tht the July rinfll over Kyushu Islnd hs high lgcorreltion to the NINO3 SST. Since the onset of the WNPM is observed during July, it is necessry to investigte the intrsesonl time-scle rinfll to obtin the reltionship between the Biu rinfll nd the synoptic pttern in Est Asi. Therefore, 5-dy men rinfll dt for 13 sttions on Kyushu re used for the lg-correltion nlysis with the NIN03 SST. Among these sttions, the verge of 4 sttions (Sg 33.27N, E, Ngski 32.73N, E, Kummoto 32.82N, E, Fukue 32.70N, E), herefter clled Northwest Kyushu, showed the highest lg-correltion with the NINO3 SST. Figure 13 shows the time series of the composite 5-dy men Northwest Kyushu rinfll during June nd July for the L Nin (solid line) nd El Nino (dots) yers. During 1978 to 1992 (1963 to 1977), the L Nin yers re the composite of 1978, 84,

6 1114 Journl of the Meteorologicl Society of Jpn Vol. 75, No. 6 Fig. 6. Sptil ptterns of the se-level pressure for July ) the L Nin yers (1978, 84, 85, 88). b) the El Nino yers (1982, 83, 87, 91, 92). c) the difference between the El Nino nd L Nin yers. The contour intervl is 2hP for ) nd b), nd 1hP for c). The units re deprture from 1000hP for ) nd b). The dots in ) nd b) show the gridpoints where the difference between the El Nino nd L Nin yers is significnt t the 95% confidence by the students t-test. b 85, 88 (1964, 71, 75), nd the El Nino yers re the composite of 1982, 86, 87, 91 (1965, 69, 72). The yers selected for these composites re slightly different from tht for the WNPM becuse October (My) NINO3 SST re used for selection to tke dvntge of mximum lg-correltions between the NINO3 SST nd Northwest Kyushu rinfll during the 15-yer period from 1978 to 1992 (1963 to 1977). Following Section 3, 0.6C SST deprtures ws used for the threshold vlue. In the L Nin yers for the recent 15-yers, the negtive October NINO3 SST deprtures were smll (Fig. 2). Hence, 4 yers with coldest SST were selected s the L Nin yers which were found to be sme s tht for the June SST used in the WNPM. The lrgest difference in the rinfll between the L Nin nd El Nino yers ws observed round July 5-24 (June 30-July 14) during 1978 to 1992 (1963 to 1977). The ppliction of the students t-test showed tht the rinfll difference for 20-dy (15-dy) totl rinfll for July 5-24 (June 30-July 14) is significnt t the 95% confidence during 1978 to 1992 (1963 to 1977). Figure 14 shows the reltionship between the Northwest Kyushu rinfll nd the NINO3 SST. Except for 1991, ll the El Nino yers experienced hevy Biu rinfll (Figs. 14 nd 14b). Figure 14c shows the lg-correltions between the Northwest Kyushu rinfll nd the NINO3 SST. The highest lg-correltion (+0.75) ws observed between the October-November (My with secondry mximum in lte utumn) NINO3 SST for the 15-yer period from 1978 to 1992 (1963 to 1977). The 5 % level of significnce for this correltion is 0.64 (0.59) for the 15-yer period from 1978 to 1992 ( ). Hence the Biu rinfll leds (lgs) the SST by 3 (2) months during the recent (erly) 15-yer period from 1978 to 1992 (1963 to 1977). The sptil ptterns of the se-level pressure during the period of high rinfll correltion (July 5-24) between the Biu rinfll nd the NINO3 SST for the recent 15-yer period from 1978 to 1992 re shown in Fig. 15. A L Nin composite (Fig. 15) shows trnsition between the Biu pttern (Fig. 6b) nd the mture phse of the WNPM (Fig. 6). Compred to the Biu pttern, the flow of the moist southerly winds is wek ner 25N, 130E. A lrge re of wek winds is observed in the tropicl nd subtropicl ltitudes between the longitudes from 130E to 150E. This reduction in the winds is fvorble for rise in the SST nd subsequent increse in the convection nd the onset of the WNPM. An El Nino composite (Fig. 15b) shows the ctive phse of the Biu seson. The southwesterly winds ner 30N, 125E nd southesterly winds ner 25N, 130E re the importnt sources of the moisture for the very ctive Biu front (Akiym, 1975). The difference between the El Nino nd L Nin

7 December 1997 M. Tnk 1115 b Fig. 7. Sptil ptterns of the GMS high-cloud mounts for July ) the L Nin yers (1978, 84, 85, 88). b) the El Nino yers (1982, 83, 87, 91, 92). The scle for the cloud mount (percent) is shown on the left. Fig. 8. Correltions (x 10) between July GMS high-cloud mounts nd June NINO3 SST for the 15-yer period from 1978 to Shded res show the gridpoints which re significnt t the 5% level.

8 1116 Journl of the Meteorologicl Society of Jpn Vol. 75, No. 6 b Fig. 9. Ltitude-time cross-section of the GMS high-cloud mounts t the longitudes from 145 to 154E. The verticl scle shows the ltitudes. The horizontl scle shows the months in the clender yer. ) the L Nin yers (1978, 84, 85, 88). b) the El Nino yers (1982, 83, 87, 91, 92). The scle for the cloud mount (percent) is shown on the left. Fig. 10. Sme s Fig. 9, but for the correltions (x10) between the GMS high-cloud mounts nd June NINO3 SST. Shded res show the significnt correltions t the 5% level.

9 December 1997 M. Tnk 1117 Fig. 11. Ltitude-time cross-sections of the se-level pressure t 140E. ) the verge for the 15-yer period from 1978 to b) the L Nin yers. c) the El Nino yers. The units re deprture from 1000hP. The dtes shown in the horizontl scle re the lst dy of ech pentd. The dots in b) nd c) show the ltitude nd middle dy of the pentd where the difference between the El Nino nd L Nin yers re significnt t the 95% confidence by the students t-test. yers (Fig. 15c) shows significnt positive (negtive) se-level pressure nomly ner 25-30N, 140E (40N, E). The sptil ptterns of the GMS high-cloud mounts during the sme period s Fig. 15 re shown in Fig. 16. In the L Nin yers (1978, 84, 85, 88), the ctive phse of the convection ner 20N, 150E indictes trnsition towrd the onset of the WNPM. The convection ssocited with the Biu front is wek ner 33N, 125E nd 35N, 175E. In the El Nino yers (1982, 86, 87, 91), the ctive phse of the convection is observed long the Biu front locted north of 30N nd ner the Philippines. Figure 17 shows the correltion between the GMS high-cloud mount nd August NINO3 SST for 20-dy period from 5 to 24 July. The regions with significnt correltion t the 5% level re shded. A region ner 33N, 125E with mximum positive correltion of 0.92 shows tht the Biu convection is under the strong influence of internnul vritions of the NINO3 SST. The lrge-scle pttern of the correltion is similr to Fig. 8. However, the correltions ner the Biu front (20N, 150E) re higher (lower). The se-level pressure pttern (Fig. 15c) nd GMS high-cloud mount correltion pttern (Fig. 17) suggest the propgtion of nomlous Rossby wves, emnting from the convective region ner the Philippines in the El Nino yers. A wve trin of nticyclonic cells ner 25N, 140E nd cyclonic cells ner 40N, 170E in Fig. 15c is roughly ligned long gret circle tht intersects the convective center ner the Philippines where the positive correltions between the GMS high-cloud mount nd the NINO3 SST re observed (Fig. 17). This pttern is similr to the nlysis by Kwmur et l. (1996). A similr nlysis of the sptil pttern of the se-level pressure during 15-yer period from 1963 to 1977 ws crried out. The results indicte n intensifiction of the flow of the moist southerly winds ner 25N, 130E in the El Nino yers. However, the difference between the El Nino nd L Nin yers ws sttisticlly not significnt. The sptil ptterns of the se-level pressure suggest tht the internnul vritions in the NINO3 SST my be relted to the ending dtes of the Biu seson in the recent 15-yer period from 1978 to We investigted this reltionship using subjectively determined ending dtes of the Biu seson obtined from the JMA for 11 regions in Jpn for 30-yer period from 1963 to The lg-correltions between these dtes nd the NIN03 SST show tht 3 regions (South Kyushu locted ner Kgoshim 31.55N, E (+0.78), North Kyushu locted ner Fukuok 33.58N, E (+0.85), South Touhoku locted ner Sendi 38.27N, E (+0.68)) hve significnt

10 1118 Journl of the Meteorologicl Society of Jpn Vol. 75, No. 6 b Fig. 12. Correltions (x10) between July rinfll in Est Asi nd NINO3 SST. ) the correltions between the rinfll during nd September NINO3 SST. b) the correltions between the rinfll during nd My NINO3 SST. The dots show the loctions of the sttions. correltions to the August or September NINO3 SST in the recent 15-yer period from 1978 to During the erlier 15-yer period, significnt positive correltion ws not found for time lg of less thn one yer. 7. The reltionship between the mture phse of the WNPM nd the end of the Biu seson in Jpn Ued nd Ysunri (1996) suggested tht the onset nd subsequent intensifiction of the WNPM coincides with the end of the Biu seson round Jpn. However, this suggestion ws mde without the benifit of sttisticl nlysis between these two events. We investigte this reltionship using ending dtes of the Biu seson determined by the JMA nd the dtes of the mture phse of the WNPM which re obtined from the se-level pressure difference between 10N nd 20N t 140E. For the recent 15-yer period, these dt re shown in Fig. 1. The first mximum in the westerly winds during the July-September summer seson ws defined s the dtes of the mture phse of the WNPM for ech of the yers. Significnt correltions re found between ending dtes of the Biu seson nd the dtes of the mture phse of the WNPM during the recent 15-yer period from 1978 to 1992 t 5 regions (South Kyushu (+0.58), North Kyushu (+0.62), Hokuriku locted ner Niigt 37.92N, E (+0.64),

11 December 1997 M. Tnk 1119 b b Fig. 13. Sesonl chnge in the composite 5-dy men rinfll during the Biu seson over Northwest Kyushu (verge of Sg (32.25N, E), Ngski (32.73N, E), Kummoto (32.82N, E), nd Fukue (32.70N, E)). Solid line (dots) shows the L Nin (El Nino) yers. ) the composite for the L Nin (El Nino) yers of 1978, 84, 85, 88 (1982, 86, 87, 91). b) the composite for the L Nin (El Nino) yers of 1964, 71, 75 (1965, 69, 72). The numbers shown on the horizontl scle re the lst dy of ech pentd. South Tohoku (+0.58), North Tohoku locted ner Aomori 40.82N, E (+0.60)) in Jpn. During the 15-yer period from 1963 to 1977, no significnt correltion ws found between these two dtes. The dtes of the mture phse of the WNPM hve siginficnt correltion of with July NINO3 SST during the recent 15-yer period. No signficnt correltion ws found in the erlier 15-yer period. Therefore, the dtes of the mture phse of the WNPM nd the end of the Biu seson in Jpn re correlted only when both of these events show high correltion to the NINO3 SST. 8. Summry The nlysis of internnul nd interdecdl vritions of the WNPM nd the Biu rinfll for 30-yer period from 1963 to 1992 showed high positive correltions between the Biu rinfll nd the Fig. 14. Time series of the internnul vritions of the Northwest Kyushu rinfll (solid line) nd the NINO3 SST (dots), nd the lg-correltions between the Northwest Kyushu rinfll nd the NINO3 SST c). ) (b) shows the 15-yer period from ( ). The rinfll mount is from July 5 to 24 (June 30 to July 14) for ( ). The scle on the left shows the rinfll in mm. The scle on the right shows the SST nomlies in C. c) is sme s Fig. 3, but for the lg-correltions between the Northwest Kyushu rinfll nd the NINO3 SST.

12 1120 Journl of the Meteorologicl Society of Jpn Vol. 75, No. 6 Fig. 15. Sme s Fig. 6, but for July The El Nino yers re 1982, 86, 87, nd 91. b The NINO3 SST show biennil oscilltion during the 15-yer period from 1963 to In the recent 15-yer period, the low frequency mode of bout 4 yers is dominnt. These chnges in the period of the oscilltions of the NINO3 SST my be relted to the recent increse in the correltions between the SST nd the WNPM. Ysunri nd Seki (1992), using the Indin monsoon rinfll nd Pcific SST dt for the 21-yer period from 1966 to 1986, suggested tht these dt show biennil oscilltions. Shen nd Lu (1995), using the rinfll dt in Chin nd tropicl SST dt for the 30-yer period from 1956 to 1985, found similr biennil oscilltions in these dt. In both of these studies, more thn hlf of their nlysed periods re observtions prior to Therefore, the low frequency mode in the recent 15-yer period from 1978 to 1992 is not documented in these studies. Lrge interdecdl vritions in the reltionship mong the WNPM, the Biu rinfll nd the NINO3 SST show tht it is importnt to use long-term dt nd perform continuous updte of these dt for investigtion of the stbility of these reltionships. Ued nd Ysunri (1986), using the dt from 1980 to 1994, suggested tht the onset of the WNPM is closely relted to erly July SST in locl region in the western North Pcific. However, Lu nd Sui (1997), using the dt from November 1992 to Februry 1993, showed tht the fluctutions of locl SST on timescles of weeks to months re response of the SST to the tmospheric low-frequency vribility. Figure 10 shows tht the first significnt negetive correltions between the GMS highcloud mounts nd June NINO3 SST re observed in lte My-erly June. Therefore the ENSO influence on the WNPM becme significnt well before July. These fcts suggest tht during the recent 15- yer period from 1978 to 1992, the reltionship to the ENSO cycles is the most importnt influence in internnul vritions of the onset dtes of the WNPM. NINO3 SST. On the other hnd, significnt correltion between the WNPM nd the NINO3 SST is restricted to the recent 15-yer period from 1978 to During this period, erly (lte) onset of the WNPM nd erly (lte) end of the Biu rinfll re observed in the L Nin (El Nino) yers. These dtes re bout 20 dys lter in the El Nino yers. The lg-correltions mong the WNPM, the Biu rinfll nd the NINO3 SST show tht the SST leds the WNPM by 1 to 2 months during the recent 15- yer period from 1978 to On the other hnd, the Biu rinfll leds (lgs) the SST by bout 3 (2) months during the recent (erly) 15-yer period from 1978 to 1992 (1963 to 1977). Acknowledgments The uthor is indebted to Dr. Tketo Mruym for his helpful comments on this study. The uthor lso cknowledges two nonymous reviewers for their constructive comments. Thnks re lso due to the Meteorologicl Stellite Center for providing the GMS dt. The rinfll dt for the Chin re provided by the Chin Meteorologicl Administrtion. Prt of this work ws supported by the Jpnese Experiment on Asin Monsoon nd the Globl Reserch Network, both supported by the Science nd Technology Agency of Jpn.

13 December 1997 M. Tnk 1121 Fig. 16 Sme s Fig. 7, but for July The El Nino yers re 1982, 86, 87, nd 91. Fig. 17. Sme s Fig. 8, but for the correltions (x10) between July 5-24 GMS high-cloud mounts nd August NINO3 SST.

14 1122 Journl of the Meteorologicl Society of Jpn Vol. 75, No. 6 References Akiym, T., 1973: The lrge-scle spects of the chrcteristic fetures of the Biu front. Pp. in Meteor. nd Geophys., 24, Akiym, T., 1975: Southerly trnsversl moisture flux in to the extremely hevy rinfll zone in the Biu seson. J. Meteor. Soc. Jpn, 53, Chen, W.T., 1982: Fluctutions in northern hemisphere 700mb height field ssocited with the Southern oscilltion. Mon. We. Rev., 110, Dvis, RE., 1976: Predictbility of se surfce temperture nd se level pressure nomlies over the North Pcific ocen. J. Phys. Ocen., 6, Kwmur, R., T. Murkmi nd B. Wng, 1996: Tropicl nd mid-ltitude 45-dy perturbtions over the Western Pcific during the northern summer. J. Meteor. Soc. Jpn, 74, Lu, K.M., nd C.H. Sui, 1997: Mechnisms of shortterm se surfce temperture regultion: observtion during TOGA COARE. J. Climte, 10, Mtsumoto, T., 1992: The sesonl chnges in Asin nd Austrlin monsoon regions. J. Meteor. Soc. Jpn, 70, Murkmi, T., nd J. Mtsumoto, 1994: Summer monsoon over the Asin continent nd Western North Pcific. J. Meteor. Soc. Jpn, 72, Nkzw, T., 1992: Sesonl phse lock of intrsesonl vritions during Asin monsoon. J. Meteor. Soc. Jpn, 70, Nitt, T., nd S. Ymd, 1989: Recent wrming of tropicl se surfce temperture nd its reltionship to the northern hemisphere circultion. J. Meteor. Soc. Jpn, 67, Shen, S. nd K.-M. Lu, 1995: Biennil oscilltion ssocited with the Est Asin summer monsoon nd tropicl se surfce tempertures. J. Meteor. Soc. Jpn, 73, Tnk, M., 1987: Internnul chnge in the rinfll in North Chin region nd its reltionship to the Asin monsoon. Tenki, 34, (in Jpnese). Tnk, M., 1992: Intrsesonl oscilltion nd the onset nd retret dtes of the summer monsoon over Est, Southest Asi nd the Western Pcific region using GMS high cloud mount dt. J. Meteor. Soc. Jpn, 70, Tnk, M., 1994: The onset nd retret dtes of the Austrl summer monsoon over Indonesi, Austrli nd New Guine. J. Meteor. Soc. Jpn, 72, Ued, H., T. Ysunri nd R. Kwmur, 1995: Abrupt sesonl chnge of lrge-scle convective ctivity over the Western Pcific in the northern summer. J. Meteor. Soc. Jpn, 73, Ued, H. nd T. Ysunri, 1996: Mturing process of the summer monsoon over the Western North Pcific -A coupled ocen/tmosphere system. J. Meteor. Soc. Jpn, 74, Wng, B., 1994: Climtic regiemes of tropicl convection nd rinfll. J. Climte, 7, Wng, B., 1995: Interdecdl chnges in El Nino onset in the lst four decdes. J. Climte, 8, Ysunri, T. nd Y. Seki, 1992: Role of the Asin monsoon on the internnul vribility of the Globl climte system. J. Meteor. Soc. Jpn, 70, Ytgi, A. nd T. Ysunri, 1995: Internnul vritions of summer precipittion in the rid/semi-rid regions in Chin nd Mongoli: their regionlity nd reltionship to the Asin monsoon. J. Meteor. Soc. Jpn, 73,

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