SEASONAL VARIATI0,NS OF THE RELATIONSHIP BETWEEN SOME ENSO PARAMETERS AND INDIAN RAINFALL

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1 INTERNATIONAL JOURNAL OF CLIMATOLOGY, VOL. 16, (1996) l(4) SEASONAL VARIATI0,NS OF THE RELATIONSHIP BETWEEN SOME ENSO PARAMETERS AND INDIAN RAINFALL K. D. PRASAD nd S. V SINGH' Indin Institute of Tropicl Meteomlogv, Pune-4llOO8, Indi emil: Received I June 1994 Accepted 16 November 1995 Cnonicl correltion nlysis is used to exmine the sesonl reltionship between ENSO nd Indin rinfll by nlysing their 12 monthly vlues for n 80-yer period. Three ENSO indices re considered. These ENSO indices re the Drwin surfce pressure, the se-surfce temperture of the centrl nd estern equtoril Pcific, nd rinfll of centrl equtoril Pcific islnds (herefter denoted s DSP, SST, nd RAIN respectively). The ENSO indices re lso nlysed for reltionships between themselves. The nlysis revels tht the sesonl vritions of these ENSO indices re. highly intercoupled with no lg. These indices show the minimum ssocition during April nd the mximum fter the monsoon seson. Further, the sesonl vrition of the Indin rinfll is found to be better ssocited with the sesonl vritions of SST s compred with tht of DSP or RAIN. This ssocition is t its strongest during the period August-October. An pprent reversl in the reltionship between ENSO nd Indin rinfll is lso observed from summer to winter. The wrm ENSO yers re ssocited with wek summer monsoon rinfll nd t the sme time high winter monsoon rinfll. KEY WORDS; ENSO; Indin rinfll; cnonicl correltion nlysis. INTRODUCTION The se-surfce temperture exerts significnt control over the tmospheric circultion, the remrkble exmple of which is the El Niiio - Southern Oscilltion (ENSO) phenomenon. The ENSO is complex phenomenon involving the ocen-tmospheric interction on short-term climtic scle. The most well-known tmospheric component of the phenomenon is the Southern Oscilltion, representing the se-sw of the pressure vrition between the Indonesin-Austrlin nd the equtoril South Pcific regions. When the pressure is high over the south-est Pcific it is low over the Indonesin region. The trde winds in this cse re stronger nd n upwelling is observed off the west cost of South Americ. In n opposite sitution, wrming of the se-surfce temperture over the centrl through estern equtoril Pcific is noted. This nomlous wrming is known s the El Niiio nd the combined tmospheric-ocenic phenomenon is known s ENSO. Although the reltionship between the Southern Oscilltion nd Indin rinfll ws noted during the erly prt of the present century, considerble revivl of the interest in the reltionship is found during recent yers. The occurrence of deficient monsoon rinfll during the yers of incresed Line Islnds (centrl Pcific) rinfll ws noted by Sikk (1980). Pnt nd Prthsrthy (1981) nd Bhlme et l. (1983) found significnt correltion between the Indin monsoon rinfll nd the Southern Oscilltion index. Rsmusson nd Crpenter (1983) nd Ropelewski nd Hlpert (1987) found less rinfll over the Indin region during the extreme negtive phse of the Southern Oscilltion, i.e. El Niiio yers. Lter, Ropelewski nd Hlpert (1989) noted n increse in the Indin rinfll during the yers of extreme positive phse of the Southern Oscilltion. Angel1 (1981) found reltionship between the Indin monsoon rinfll nd the following December-Jnury equtoril se-surfce temperture from the dte line to the South Americn cost. Bmett (1983, 1984;b) hs shown tht the wrm episodes in the equtoril Pcific re ccompnied by wind nd pressure fluctutions over the tropicl Indin Ocen s well. *Current ffilition: Ntionl Centre for Medium Rnge Wether Forecsting, Musm Bhvn Complex, Lodi Rod, New Delhi, , Indi CCC /96/ by the Royl Meteorologicl Society

2 924 K. D. PRASAD AND S. V SINGH Shukl nd Polino (1 983) found chnge of sign in the reltionship between the Drwin surfce pressure nd the Indin rinfll from Jnury to April. Hence, he suggested the Drwin surfce pressure tendency from Jnury to April s better predictor of the Indin monsoon rinfll thn the mgnitude of the pressure in n individul month. Subsequently, Shukl nd Mooley (1987), Hstenrth (1988) nd h d nd Singh (1992) used this tendency s one of the predictors in their prediction schemes for the Indin monsoon rinfll. The bove studies hve, generlly, relted the sesonl (June to September) Indin monsoon rinfll with the monthly or sesonl ENSO indices. The temporl (nnul) evolution of the reltionship between the monthly Indin rinfll nd the ENSO indices seems to hve received little ttention. In the present pper, we study the temporl evolution of reltionship between the three different ENSO indices nd of these ENSO indices with the Indin rinfll by nlysing their monthly vribles. Although such n nlysis my lso be crried out by the use of simple correltion technique, we use here n dvnced multivrite technique, nmely cnonicl correltion, which effectively nd optimlly summrizes the reltionship contined in lrge smple of the correltions nd lg correltion coefficients. The technique hs been used previously nd discussed by Glhn (1 968), Brnett (1 98 1, 1983), Nicholls (1 987) nd Bretherton et l. (1992) mong others. We provide brief discussion of the technique in section 2. The dt utilized s well s the design of the present study is lso described in tht section. The results re discussed in section 3 nd the importnt conclusions re presented in section 4. w b- 2-0 > -2 YEAR Figure 1. Time series of the first cnonicl vrites for the pir of prmeten nlysed: (A) DSP-SST nd (B) DSP-RAIN.

3 2. I. Dt ENSO AND INDIAN RAINFALL 925 DATA AND METHODOLOGY The monthly rinflls of 306 sttions uniformly distributed over the contiguous re of Indi for the period (Prthsrthy et l., 1987) form the bsic rinfll dt used in this study. The re-weighted verge for the whole of contiguous Indi (TRN) hs been formed by weighting ech rinfll sttion by the re of the dministrtive district in which the rinfll sttion lies. Some of the ENSO indices considered in the study re Drwin se-level pressure (DSP), se-surfce temperture (SST) of the centrl nd estern equtoril Pcific, nd rinfll (RAIN) of the centrl equtoril Pcific islnds. Although the difference of the surfce pressure between Thiti nd Drwin is commonly used index in the study of the internnul vribility nd the teleconnection of the ENSO, we use here the pressure t Drwin s n index of the chnges in the tmospheric circultion relted with the ENSO. This is becuse we hve consistent ccurte record of the surfce pressure t Drwin for sufficiently long period. The pressure t Drwin shows stronger reltionship with the Indin monsoon rinfll compred with the pressure t Thiti. The significnt reltionship between the pressure t Drwin nd the Indin monsoon rinfll hs been demonstrted by Shukl nd Polino (1983) nd Shukl nd Mooley (1 987). Hence, we feel tht the considertion of the Drwin surfce pressure insted of the pressure difference between Thiti nd Drwin will not ffect the results of the present study. The dt of DSP re obtined from the Climtic Anlysis Centre, USA nd tht of SST nd RAIN re obtined from Wright (1989). Wright s SST index is defined s the -SST --IRN - = 2 -DSP 4 > --- I R N A 0 U V - z -2 0 z -RAIN ---I RN. 1 I I N 0 \ t t 0! m m Q, n n c - m T - m -? Y E A R Figure 2. Sme s in Figure 1 but for the prmeters (A) SST-IRN, (B) DSP-IRN, nd (C) RAIN-IRN. :

4 Tble I. The first () nd second (b) cnonicl correltions nd the first cnonicl vectors for different pk of prmeters nlysed. Cnonicl correltion significnt t (*) 1 per cent nd (**) 0.1 per cent levels determined using the Brtlett (1946) test. The 5 per cent levels of significnce, for Monte Crlo procedure, re 0.71 nd 0.68 for the first nd second cnonicl correltions (respectively). F No Thepr- Cnonicl Cnonicl Vectors for Months; meters correltions P consid- April June JdY August September October November December Jnury Februry Mrch 3 ered 1 DSP SST 2 DSP RAIN 3 SST IRN 4 DSP IRN 5 R A I N IRN b b b b b 0.92** 0.72* 0.91** ** * * k 0.16 $ 0.22 u 0-13 F 0.27 S:

5 Tble II. Individul nd verge vrinces explined (in per cent) by the first cnonicl mode for ech pir of the prmeters nlysed. Number Individul Months Averge vrince vrince April My June July August September October November December Jnury Februry Mrch 1 SST IRN DSP IRN R A I N IRN m 3 E W N 4

6 928 K. D. PRASAD AND S. V SINGH men se-surfce temperture over the region 6-2"N nd " W, 2-6" S nd 180"-90" W, nd 6-1 0"s nd O"W, nd RAIN is defined s the men rinfll of six sttions close to the Equtor in the longitudinl rnge, 160"W-150 W (for more detils, see Wright, 1989). These dt re normlized by subtrcting the monthly longterm men nd dividing by the monthly stndrd devition to remove the nnul cycle. The 12 monthly stndrdized vlues for period of 80 yers ( ) re considered in the cnonicl correltion nlysis. 2.2 Principle nd method of cnonicl correltion nlysis Nicholls (1987) hs provided the bsic detils of the principle nd method of computtions involved in the cnonicl correltion nlysis relevnt to the present study. Recently, Yglom (1 990) nd Bretherton et l. (1 992) hve given more extensive review of the technique. We provide here only the limited detils, sufficient for understnding the nlysis procedure nd the terminology used in the following. \ I'\ If \I \ v I., L -. I Figure 3. Correltion coefficients between the first cnonicl vrites nd their corresponding originl monthly vrites for yer -1, yer 0, nd yer + 1 for the pir of prmeters nlysed: (A) DSP-SST nd (B) DSP-RAIN. Horizontl dshed lines show correltion coefficient significnt t the 5 per cent level.

7 ENS0 AND INDIAN RAINFALL 929 Consider two vector vribles x nd y of dimension p nd q, with q I p. The cnonicl correltion nlysis derives the new vribles s liner combintion of the originl vector vribles, such tht the correltion between them (the new vribles) is mximized. These new vribles re known s the cnonicl vrites. Let U nd W be the first cnonicl vrites, such tht U=',X nd W=b',Y,.AM OCT APR OCT I " I" A.4 - -*4 - 'I fp 1-0 Y Figure 4. Sme s in Figure 3 but for the prmeters (A) SST-IRN, (B) DSP-IRN, nd (C) RAIN-IRN.

8 930 K. D. PRASAD AND S. V: SINGH The objective is to determine the vectors 1 nd bl such tht the correltion between U nd W given by is mximized Where N is the number of observtions. This leds to eigenvlue solutions of complicted chrcteristic eqution given by; (R;;R2lR,'Rlz - AjZ)bj = 0 (4) with the normliztion constrint of bjr22bj = 1 where A, re the eigenvlues, Rlz is the cross-correltion mtrix betweenxnd I: nd RZ1 is its trnspose. RI nd R22 re the correltion mtrices of X nd Y respectively. The first cnonicl correltion is obtined s the squre root of the mximum eigenvlue of the chrcteristic eqution (4) nd the ssocited eigenvector (bl) is the first cnonicl vector of Y; The corresponding cnonicl vector of X is obtined from the reltion (Anderson, 1958): 1 = (R;;RI2bl)/fi (6) The other cnonicl correltions nd cnonicl vectors corresponding to the other eigenvlues nd eigenvectors of eqution (4) re similrly determined. Thej cnonicl vrites V, nd W, re obtined by substituting the cnonicl vectors j nd bj in eqution (1). The significnce of the cnonicl correltions is tested by pplying Brtlett's (1 946) clssicl confirmtory test. The trditionl significnce test sometimes cn give rise to misleding results of the cnonicl correltions. Hence, the significnce is further confirmed using the Monte Cr10 procedure described in section 3.1. The cnonicl structure ptterns (Nicholls, 1987) re determined by correlting the cnonicl vrites with their corresponding originl vribles. These ptterns provide the insight into the temporl evolutions of the reltionships between the vribles nlysed Design of nlysis procedure As mentioned bove, in the cnonicl correltion nlysis set ofp vribles x is optimlly relted with set of nother q vribles y. The nlysis design followed here, for investigting the evolution of the temporl reltionship mongst two sets of vribles, is similr to tht the Nicholls (1987) who considered 11 bimonthly periods s 11 vribles. The 12 monthly vlues of the ENSO indices or the Indin rinflls here mke the 12 vribles nd therefore, the number of vriblesp nd q, mentioned bove, re ech equl to 12. These 12 vribles correspond to the period April through to Mrch, rther thn the period Jnury through to December, llowing the month of the strongest utocorreltion in ENSO indices to fll in the middle of the period. The cnonicl correltions nd cnonicl vectors corresponding to the eigenvlues nd eigenvectors of eqution (4) re computed. The cnonicl vrites (U,, 6) re obtined by the liner trnsformtion of the originl vector of observtions on the cnonicl vectors (see eqution 1). The cnonicl correltions, representing the strength of the reltionship between the corresponding cnonicl vrites, re tested for their significnce s mentioned in section 2.2. The cnonicl structure ptterns re derived by correlting the cnonicl vrites with their corresponding originl monthly vribles Cnonicl correltions RESULTS In order to quntify the overll strength of the reltionships between the monthly vrying indices of the ENSO nd the Indin rinfll, cnonicl correltion nlyses re performed with these prmeters using their 12 monthly

9 ENSO AND INDIAN RAINFALL 93 1 vlues. The different sets of cnonicl correltions computed re: two sets for the reltionships between the ENSO indices (nmely, (i) DSP-SST nd (ii) DSP-RAIN) nd three sets for the reltionships of ech of these ENSO indices with the Indin rinfll. The significnce of the cnonicl correltions is tested by the clssicl confirmtory test nd lso by Monte Cr10 procedure. In the ltter method, the cnonicl correltion nlyses re performed repetedly (over 100 times) ech time using the resmpled dt. The level of significnce of the cnonicl correltion for ech cnonicl mode is determined bsed on these trils nd is compred with the cnonicl correltion of the ctul dt. The significnt vlues t the 5 per cent level in the present cse re found to be nd 0.68 for the first nd second cnonicl correltions respectively. The sesonl vritions of these ENSO indices seem to be highly intercoupled, becuse the first cnonicl correltion in ech cse is found to be highly significnt (Tble I). Between the ENSO indices the highest intercoupling is found between DSP nd SST s compred with DSP nd RAIN. The second cnonicl mode is lso found to be significnt in cse of DSP-SST. This shows tht generlly two cnonicl modes my be sufficient to describe the sesonlly evolving reltionship between the ENSO indices exmined. The second cnonicl mode, however, could not be found significnt in the cse of DSP-RAIN. This shows tht RAIN is contminted with more rndom noise nd cn be considered less relible indictor of n ENSO phenomenon. The overll sesonl vritions of these ENSO indices lso pper to be intercoupled with tht of the Indin rinfll (s the first cnonicl mode is found significnt in ech of these cses, Tble I). It lso ppers from the tble tht the best ssocition of the Indin rinfll is with SST. We will see lter on tht the verge nd the individul (monthly) vrinces explined re lso found to be high in this cse. Tble I lso lists the first cnonicl vectors (i.e. 1 nd bl of eqution 1, section 2.2). The vribles nlysed were for the months April through to Mrch, s such these cnonicl vectors lso correspond to these months. The cnonicl vectors indicte (Bretherton et l., 1992) which vlues (here months) re dominnt in forming the first cnonicl vrites (which hve the mximum correltion coefficient but explin smller frction of the covrince between the two sets of vribles s compred with the other similr method) Cnonicl vrites The cnonicl vrites, which re the liner combintions of their originl vector vribles, re mximlly correlted with the ech other. The first set of cnonicl vrites hve the highest correltion mong ll the other pirs of cnonicl vrites. Ech vlue of the cnonicl vrites corresponds to the observtion time (here the yers) of the originl vribles. Accordingly, the cnonicl vrites cn be plotted s the time series corresponding to the observtionl time of the originl dt. These cnonicl vrites, portrying the reltionship mongst the different ENSO indices nd mong the ENSO-Indin rinfll nlysed, re presented in Figures 1 nd 2, respectively. The greement in the internnul vritions mong these ENSO indices nd mong the ENSO-Indin rinfll cn be seen from these figures. The better intercoupling between the Indin rinfll nd SST, s indicted in the mgnitude of their cnonicl correltion (Tble I), is lso seen from these figures. The correltions of the originl vribles with their corresponding first cnonicl vrites re lso determined. The squred correltions indicte how much of the vrince in the vribles cn be ccounted for by prticulr cnonicl vrite. Summing the squred correltions for prticulr cnonicl vrite nd dividing by the number of vribles provides the verge vrince in the vrible set ccounted for by prticulr cnonicl vrite. The individul nd verge vrinces explined by ech of the cnonicl vrites for the three cses of the ENSO- Indin rinfll nlyses re presented in Tble 11. The individul vrinces explined re generlly lrge during August nd September in ech cse. The first cnonicl vrite of SST-Indin rinfll explins lrger verge or individul vrince compred with the other two cses. The verge vrince explined by the first cnonicl vrites of the Indin rinfll in ll these cses is found to be of the order of 10 per cent. Apprently, the three ENSO indices exmined shre common informtion. It my be desirble to investigte whether they contin significnt uncommon informtion, prticulrly with respect to Indin rinfll. It is noted from Tble I tht the reltionship of IRN is highest with SST. We hve investigted whether DSP contins ny dditionl informtion with respect to IRN. For this, first we find the residul Indin rinfll series by subtrcting the rinfll series, reconstructed from the first cnonicl mode of SST-IRN, from the originl IRN. This moves the influence of SST from IRN. Next, we conduct cnonicl correltion nlysis between this residul rinfll

10 932 K. D. PRASAD AND S. V: SINGH series nd DSP. The mgnitude of the first cnonicl correltion from this nlysis is found to be 0.63, which is lower thn the vlue of significnce ( = 0.68) t the 5 per cent significnce level. Therefore, we conclude tht DSP does not provide ny dditionl informtion, over nd bove SST, regrding I N. 3.3 Cnonicl structure ptterns The cnonicl structure ptterns re nlysed in this section in order to understnd the phse reltionships between different ENSO indices nd between ENSO nd Indin rinfll. The cnonicl structure ptterns re obtined following Nicholls (1 987) by correlting the first cnonicl vrites with their corresponding originl monthly vrites for the preceding (yer -l), concurrent (yer 0), nd the subsequent (yer + 1) yers. This wy we obtin 36 correltions for ech pir of the vribles exmined nd thus provide n effective nd simple summry of the interreltionship between the sets of vribles, otherwise portryed by 36 x 36 mtrix of simple correltions. The plots of the correltion coefficients (Figure 3) for different pirs of the ENSO indices (i.e. DSP-SST nd DSP-RAIN) indicte well-mrked nnul cycle of the reltionship between these ENSO indices, s found by the other investigtors. The reltionship between these vribles strts building up from April of yer 0, ttining pek vlue during October-December. The reltionship flls therefter but it remins significnt until April of yer + 1. The plots lso revel tht the sesonl vritions between these ENSO indices re in phse nd tht there is no evidence of ny lg reltionship between them, consistent with the findings of Rsmusson nd Crpenter (1982) nd Nicholls (1987). The similr plots of the cnonicl correltion structures for different ENSO indices nd Indin rinfll (Figure 4) indicte tht Indin rinfll is generlly significntly correlted with ech of these ENSO indices during yer 0 nd tht the nnul ptterns of the reltionships remins more or less similr in ll three cses. The reltionship increses from April of the yer 0 through October nd chnges its phse during winter (November to Februry). Exmintion of Figure 4 further revels tht the wrm ENSO yers re relted with wek Indin summer monsoon rinfll nd winter Indin rinfll is incresed during wrm ENSO yers. The reltionship between the ENSO nd the Indin rinfll is found to be strongest during August to October. For exmining the reversl of the reltionship in winter, we further prepred sctter plots between ENSO indices nd IRN. These sctter plots (not presented) support the reltionship noted bove. It my be noted tht Rsmussen nd Crpenter (1983) reported similr reltionship but for Sri Lnk rinfll. They reported tht rinfll over Sri Lnk during the post-monsoon period increses during wrm ENSO episode. The wind (north-est monsoon) system which ffects Sri Lnk during this period lso ffects the south-est peninsul of Indi. Therefore, we hve lso prepred sctter plots between rinfll over the south-est peninsul nd ENSO for the winter period. These sctter plots show mrginl improvement of the reltionship over those of the ll Indi rinfll. However, ll these results support n opposite sense of the reltionship between ENSO nd IRN during the winter. CONCLUSIONS We used cnonicl correltion nlysis for exmining the sesonl vritions of the reltionship between ENSO nd Indin rinfll. Three ENSO indices, nmely, DSP, SST nd RAIN re nlysed, ech seprtely, with the Indin rinfll using their 12 monthly dt for n 80-yer period. The ENSO indices re lso exmined by similr method for reltionships between themselves. Monte Cr10 procedure s well s the clssicl confirmtory tests re pplied to test the significznce of the cnonicl correltions. The sesonl vritions of the reltionships re exmined by correlting the significnt cnonicl vrites with their corresponding originl monthly vrites for three consecutive (yer -1, yer 0, nd yer + 1) yers. From this nlysis the following importnt conclusions re drwn. The sesonl vrition of these ENSO indices is found to be in phse nd there seems to be no lg reltionship between them. The mximum coupling between them occurs fter the monsoon seson (October-December) nd the minimum during April. The sesonl vritions of the Indin rinfll re found to be better linked with the sesonl vritions of SST, s compred with DSP or RAIN. Hence, it ppers tht the vritions of the Indin rinfll cn be best inferred from the vrition of SST if the ltter is forecst well in dvnce by ny suitble technique.

11 ENSO AND INDIAN RAINFALL 933 There seems to be chnge in the reltionship between the ENSO nd the Indin rinfll during the course of the yer. The reltionship between the ENSO indices exmined nd Indin rinfll is negtive during summer, wheres during winter it ppers to be positive. Hence, summer monsoon rinfll is likely to be deficient nd winter monsoon rinfll is likely to be incresed during wrm ENSO yers. The reltionship between ENSO nd the Indin rinfll, however, reches pek vlue only fter the monsoon seson, tht is during August to October. ACKNOWLEDGEMENTS We would like to thnk Professor R. N. Keshvnurty, Director nd Dr S. S. Singh, Hed, Forecsting Reserch Division, Indin Institute of Tropicl Meteorology, Pune, for the encourgement nd providing the fcilities. Thnks re due to Dr H. N. Bhlme for going through the mnuscript, the nonymous referees nd Dr Brin D. Giles for their helpful suggestions. REFERENCES Anderson, T. W An Inhoduction to Multivrite Sttisticl Anlysis, Wiley, New York. Angell, J. K Comprison of vritions in tmospheric quntities with se surfce temperture vritions in the equtoril estern Pcific, Mon. We. Rev., 109, Bmett, T. P Sttisticl prediction of Northem Americn ir tempertures from Pcific predictors, Mon. We. Rev., 109, Bmett, T. P Interction of the monsoon nd Pcific trde wind systems t internnul time scles, Prt I. The equtoril zone, Mon. We. Rev., 111, Bmett. T. P Interction of the monsoon nd Pcific trde wind systems t internnul time scles. Prt 11. The tropicl bnd, Mon. We. Rev., 113, Bmett, T. F! 1984b. Interction of the monsoon nd Pcific trde wind systems t internnul time scles. Prt 111. The ntomy of the Southern Oscilltion, Mon. We. Rev., 113, Brtlett, M. S The sttisticl significnce of cnonicl correltions, Biometric, 32, Bhlme, H. N., Mooley, D. A. nd Jdhv, S. K Fluctutions in the drought/flood re over Indi nd reltionship with the Southem Oscilltion, Mon. We. Rev., 111, Bretherton, C. S., Smith, C. nd Wllce, J. M An intercomprison of methods for finding coupled ptterns in climte dt, 1 Climte, 5, Glhn, H. R Cnonicl correltion nlysis nd its reltionship to discriminnt nlysis nd multiple regression, 1 Amos. Sci., 25, Hstenrth, S Prediction of Indin monsoon rinfll. Further explortion, 1 Climte, 1, Nicholls, N The use of cnonicl correltion to study teleconnections, Mon. We. Rev., 115, Pnt, G. B. nd Prthsrthy, B Some spects of n ssocition between the Southern Oscilltion nd Indin Summer Monsoon, Arch. Meteoml. Geophys. Bioklimtol Se,: B,, 29, Prthsrthy B., Sontkke, N. A., Munot, A. A. nd Kothwle, D. R Droughts/Floods in the summer monsoon seson over different meteorologicl subdivisions of Indi for the period Climtol,, 7, Prsd, K. D. nd Singh S. V Possibility of predicting Indin monsoon rinfll on reduced sptil nd temporl scles, 1 Climte, 5, Rsmusson, E. M. nd Crpenter, T. H Vritions in tropicl se surfce tempertures nd surfce wind fields ssocited with the Southem Oscilltion/El Nino, Mon. We. Rev., 110, Rsmusson, E. N. nd Crpenter, T. H The reltionship between estern Pcific se surfce tempertures nd rinfll over Indi nd Sri Lnk, Mon. We. Rev., 111, Ropelewski, C. F. nd Hlpert, M. S Globl nd regionl scle precipittion ptterns ssocited with the El Nino/Southem Oscilltion, Mon. We. Rev., 115, Ropelewski, C. F. nd Hlpe, M. S Precipittion ptterns ssocited with high index phse of the Southern Oscilltion, 1 Climte, 2, Shukl, J. nd Mooley, D. A Empiricl prediction of the summer monsoon rinfll over Indin, Mon. We. Rev., 115, Shukl, J. nd Polino, D. A The Southem Oscilltion nd long-rnge forecsting of the summer monsoon rinfll over Indi, Mon. We. Rev., 111, Sikk, D. R Some spects of the lrge-scle fluctutions of summer monsoon rinfll over Indi in reltion to fluctutions in the plnetry nd regionl scle circultion ptterns, Proc. Ind. Acd. Sci. (Erth-Plnet, Sci.), 89, Wright, F! B Homogenized long-period Southern Oscilltion indices, 1 Climtol, 9, Yglom, A. M Method of cnonicl correltions nd its pplictions in meteorology, Imestiy, Amos. Ocen Phys., 26,