... Coupled ocean±atmosphere dynamics in the Indian Ocean during 1997±98

Transcription

1 optimlly doped cse (solid line). It is interesting to note tht the pek in the underdoped cse is slightly reduced in height re ecting reduction in T c. It is lso shifted to lower energies. Some experiments 9 indicte reduction in gp vlue with underdoping in YBCO while mny experiments show n importnt increse in Bi (ref. 8). Even if the gp is ssumed to sty the sme t 7 mev, the spin-polrized neutron-resonnt frequency is known to decrese with doping. Accounting for this gives lmost exctly the downwrd shift oserved in our experimentl dt of Fig.. Recent inelstic neutron scttering dt in Bi (ref. ) show resonnce pek t 3 mev in the superconducting stte nd estlish similrity with the erlier results in YBCO. We hve inverted the opticl dt of Puchkov et l. in this cse nd nd tht coupling t low tempertures to the oserved superconducting-stte spinresonnce pek is generl phenomenon in oth YBCO nd Bi. Spin excittions re thus seen in n ppropritely chosen second derivtive of the superconducting stte opticl conductivity, nd hence the strength of their coupling to the chrge crriers is determined. The coupling to the excittions including the mev resonnce is lrge enough in YBCO to ccount for superconductivity t tht temperture. At T c the spectrum otined from experiment gives vlue of the mss enhncement prmeter l which is close to the vlue used in our model clcultions to otin criticl temperture of K. Note dded in proof: While this pper ws in press, we ecme wre of relted theoreticl study y Munzr, D. et l., Physic C 3, ±3 (999). M Received 3 June; ccepted 7 July McMilln, W. L. & Rowell, J. M. Led phonon spectrum clculted from superconducting density of sttes. Phys. Rev. Lett., 8± (96).. Crotte, J. P. Properties of oson exchnge superconductors. Rev. Mod. Phys. 6, 7±7 (99). 3. Mrsiglio, F. et l. Inversion of K 3 C 6 re ectnce dt. Phys. Lett. A, 7±76 (998).. Puchkov, A. V. et l. High T c superconductors: n infrred study. J. Phys. 8, 9±8 (996).. Bourges, P. in The Gp Symmetry nd Fluctutions in High Temperture Superconductors (ed. Bok. J. et l.) 39±37 (Plenum, 998). 6. Sclpino, D. J. et l. d-wve piring ner spin-density-wve instility. Phys. Rev. B 3, 89±89 (986). 7. Monthoux, P. & Pines, D. YBCO: nerly ntiferromgnetic Fermi liquid. Phys. Rev. B 7, 669±68 (993). 8. Wollmn, D. A. et l. Experimentl determintion of the superconducting piring stte in YBCO from the phse coherence of YBCO±P dc SQUIDs. Phys. Rev. Lett. 7, 3±37 (993). 9. Tsuei, C. C. et l. Piring symmetry nd ux quntiztion in tricrystl superconducting ring of YBCO. Phys. Rev. Lett. 73, 93±96 (99).. Bourges, P. et l. Spin dynmics in high-tc superconductors. (Preprint cond-mt/9967).. Bulut, N. & Sclpino, D. J. Neutron scttering from collective spin uctution mode in CuO ilyer. Phys. Rev. B 3, 9± (996).. Demler, E. & Zhng, S. C. Theory of the resonnt neutron scttering of high-tc superconductors. Phys. Rev. Lett. 7, 6±9 (99). 3. Nuss, M. C. et l. Dynmic conductivity nd `coherence pek' in YBCu3O7 superconductors. Phys. Rev. Lett. 66, 33±338 (99).. Vrm, C. M. et l. Phenomenology of the norml stte of Cu±O high temperture superconductors. Phys. Rev. Lett. 63, 996±999 (989).. Schchinger, E. et l. Suppression of inelstic scttering on penetrtion depth nd conductivity in d x y superconductors. Phys. Rev. B 6, 738±7 (997). 6. Bsov, D. N. et l. Pseudogp nd chrge dynmics in CuO plnes in YBCO. Phys. Rev. Lett. 77, 9± 93 (996). 7. Collins, R. T. et l. Re ectivity nd conductivity of YB Cu 3 O 7. Phys. Rev. B 39, 67±67 (989). 8. DeWilde, Y. et l. Unusul strong coupling effects in the tunneling spectroscopy of optimlly doped nd overdoped Bi Sn CCu O 8+d. Phys. Rev. Lett. 8, 3±6 (998). 9. Ponomrev, Yo. G. et l. Josephson effect nd single prticle tunneling in YBCO nd YBCO single crystl rek junctions. Physic C 3, 67±76 (99).. Fong, H. F. et l. Superconducting-induced nomlies in the spin excittion spectr of underdoped YBCO. Phys. Rev. Lett. 78, 73±76 (997).. Fong, H. F. et l. Neutron scttering from mgnetic excittions in Bi Sr CCu O 8+d. Nture 398, 88±9 (999). Acknowledgements This work ws supported in prt y the Nturl Sciences nd Engineering Reserch Council of Cnd (NSERC) nd the Cndin Institute for Advnced Reserch (CIAR). Work t University of Cliforni t Sn Diego (UCSD) is supported y the NFS `Erly Creer Development' progrmme. DNB is Cottrel Scholr of the Reserch Corportion. We thnk J. E. Hirsch, P. B. Hirschfeld, P. B. Littlewood, F. Mrsiglio, E. J. Nicol, D. Sclpino, T. Timusk, nd I. Vekhter for interest. Correspondence nd requests for mterils should e ddressed to E.S. (e-mil: Schchinger@itp.tu-grz.c.t).... Coupled ocen±tmosphere dynmics in the Indin Ocen during 997±98 Peter J. Wester*, Andrew M. Moore*, Johnnes P. Loschnigg* & Roert R. Leen² * Progrm in Atmospheric nd Ocenic Sciences, Cmpus Box 3, University of Colordo, Boulder, Colordo 839, USA ² Colordo Center for Astrodynmics Reserch, Deprtment of Aerospce Engineering Sciences, University of Colordo, Boulder, Colordo 839, USA... Climte vriility in the Indin Ocen region seems to e, in some spects, independent of forcing y externl phenomen such s the El NinÄo/Southern Oscilltion ±. But the extent to which, nd how, internl coupled ocen±tmosphere dynmics determine the stte of the Indin Ocen system hve not een resolved. Here we present detiled nlysis of the strong sesonl nomlies in se surfce tempertures, se surfce heights, precipittion nd winds tht occurred in the Indin Ocen region in 997±98, nd compre the results with the record of Indin Ocen climte vriility over the pst yers. We conclude tht the 997±98 nomliesðin spite of the coincidence with the strong El NinÄo/ Southern Oscilltion eventðmy primrily e n expression of internl dynmics, rther thn direct response to externl in uences. We propose mechnism of ocen±tmosphere interction governing the 997±98 event tht my represent chrcteristic internl mode of the Indin Ocen climte system. In the Pci c Ocen, the identi ction of such mode hs led to successful predictions of El NinÄo ; if the proposed Indin Ocen internl mode proves to e roust, there my e similr potentil for predictility of climte in the Indin Ocen region. A strong, cool se surfce temperture (SST) nomly (with respect to the 9±97 men s clculted from ref. 6) developed in the estern Indin Ocen in July 997 nd reched mximum (. 8C) in Novemer 997 (Fig. ). At out the sme time, strting in June 997, wrm SSTnomly developed in the western Indin Ocen, with mximum of.+ 8C in Ferury 998. Together, these heting nd cooling nomlies produced reversed SST grdient (SST incresing est to west) etween Novemer 997 nd June 998 reltive to the climtologicl temperture grdient (SST incresing west to est). Following the ppernce of wrm SST nomlies off the est Africn cost in June 997, the usully wek climtologicl equtoril westerly winds (clculted from the NCEP/NCAR ner-surfce zonl wind component 7 ) were replced y surfce esterly winds. The wind nomlies exceeded -ms - over the centrl equtoril Indin Ocen in Decemer 997 (Fig. ). Between July 997 nd My 998, the se surfce height (SSH) ws depressed sustntilly in the estern sin, compred to the men vlue s descried in ref. 8, nd generlly higher in the west. Mximum height differences long the equtor exceeded 3 cm etween Novemer 997 nd My 998 (Fig. c). These vritions in the climte of the Indin Ocen sector hve een noted elsewhere 9. Usully, Indin Ocen SST vritions ssocited with El NinÄo re,. 8C, ut exhiit very different ptterns to tht noted in Fig. (ref. ). The sptil distriution of devitions from the long-term men SST, the outgoing longwve rdition (OLR, s proxy for rinfll), the surfce zonl wind velocity for Novemer 997, nd the SSH eld for the period Novemer ±3 997, re shown in Fig. ±d (the long-term men elds of SST, OLR nd ner-surfce wind, see Figs ±3 in Supplementry Informtion). There is cler sptil structure in the nomlous SST elds, with extreme vlues in the 999 Mcmilln Mgzines Ltd 36 NATURE VOL 3 SEPTEMBER 999

2 3 - - letters to nture estern nd western regions of the sin. Compred to the longterm verge, the ner-surfce nomlous zonl wind elds point strongly to the est (.ms - ) with the core locted just south of the Equtor etween 68 E nd 8 E (Fig. c). Usully the zonl winds in utumn (Septemer±Novemer) re wek westerlies (see Fig. 3 in Supplementry Informtion). Fields of OLR mesured y stellites provide the lck-ody rditing temperture of the tmosphere. Low OLR vlues indicte emissions from the upper troposphere nd, thus, from deep convective clouds which re minly responsile for rin in the tropics. High vlues of OLR, on the other hnd, signl emission from the low troposphere nd indicte n sence of deep convective clouds. Thus, the OLR distriution in Fig. depicts greter thn verge rinfll in the western nd northwestern Indin Ocen nd diminished rinfll in the est. Along the Equtor there is n OLR grdient of,7 Wm Ðroughly the sme mgnitude s etween the western nd estern Pci c Ocen, ut over hlf the horizontl distnce. The OLR grdient is consistent with n tmospheric circultion hving strong rising motion ove the western Indin Ocen, strong surfce ow to the west nd susidence in the est,3. Anlyses of the NCEP/NCAR dt sustntites the existence of the strong longitudinl cell. The SSH eld (Fig. d) shows lower vlues in the est, especilly ner Sumtr, nd higher vlues in the west. To the south of the Equtor nd in the centrl nd western Indin Ocen there is n elongted `ridge' some 3 cm ove djcent ocen regions. The loction nd mgnitude of the ridge is consistent N- S SST nomly ( C) N- S zonl wind nomly (m s ) c N- S SSH nomly (cm) Jul, My, 98 Mr, 98 Jn, 98.7 Nov, 97 Sep, 97 Jul, 97. My, Sep, 97 My, 97 6 Jul, Mr, 97 Mr, Mr, Jn, 97 Jn, 97 Jn, 97 E 6 E 8 E E E E 6 E 8 E E E E 6 E 8 E E Jul, 98 My, 98. Mr, 98. Jn, 98.. Nov, Jul, My, 97 My, 98 8 Mr, 98 6 Jn, 98 Nov, 97 - Sep, 97 Jul, Figure Evolution of the stte of the Indin Ocen, 997±98. Pnels nd show the nomlous SST nd the zonl wind nomly t the surfce, respectively, verged etween 8 N nd 8 S. The verge nnul cycle hs een removed. Pnel c shows the SSH eld otined from TOPEX/POSEIDON verged etween 8 N nd 8 S. The ordinte month mrker denotes the eginning of the month. 3 N N S OLR nomly DOLR, Nov 997 (W m ) 3 S 3 S 3 E E 7 E 9 E E 3 E E 7 E 9 E E - SST nomly Nov 997 ( C) N N S c 3 N N S Zonl wind nomly Du, Nov 997 Nov 997 (m s ) N N S SSH DTopex, nomly, //997 to Nov /3/ (cm) 3 S 3 S 3 E E 7 E 9 E E 3 E E 7 E 9 E E d Figure Stte of the Indin Ocen in Novemer 997. Pnels show the ltitude± longitude sections of nomlous (long-term monthly verge removed) SST (), outgoing longwve rdition (OLR) s determined from stellite, which is surrogte for - 3 precipittion (), the zonl component of the surfce wind eld (c), nd the surfce height determined from TOPEX/POSEIDON for ±3 Novemer 997 (d). 999 Mcmilln Mgzines Ltd NATURE VOL 3 SEPTEMBER

3 with Southern Hemisphere mss trnsports to the left of the surfce equtoril esterlies nd westerlies ner 8 S. The time evolution of this ridge of elevted se level south of the Equtor is shown in Fig. 3. Longitude-time sections of SSH nomlies verged etween 8 nd 68 S (Fig. 3) nd etween 8 nd 38 S (Fig. 3) re shown for the period Jnury 997 to 3 June 998. Westwrd propgtion of the ridge is evident t oth ltitudes, with lower westwrd phse speeds t higher ltitudes. The westwrd phse speed nd its decrese with incresing ltitude suggests tht ocen Rossy wves re importnt in the development nd propgtion of the ridge. Figure 3 lso shows the emergence of Rossy wves from the estern oundry round the time tht winds re oserved to increse long the cost of Sumtr, s discussed elow. The timing, formtion nd the propgtion of dynmicl fetures of the ridge hve een simulted with some ccurcy y simple one-nd--hlf lyer Gill-type model driven y oserved surfce wind elds. Anlysis of the model results shows tht the westwrd propgting modes re ocen downwelling Rossy wves. Mny of the chnges oserved in the SST nd SSH elds cn e explined y chnges in the surfce winds. Beginning in the lte summer of 997, the longshore component of the surfce winds off Sumtr were ±3 m s - stronger thn norml to the northwest nd were conducive to incresed costl, illustrted schemticlly in Fig.. Strting in lte summer, the longshore winds off the Africn cost were weker to the southwest y m s -, which resulted in reduction in south of the Equtor. Initilly, the wrmer SSTs in the western Indin Ocen strting in June 997 produced esterly wind nomlies over the centrl Indin Ocen tht would lter e y the cooler tempertures in the estern sin. The ccelerting equtoril esterlies would promote lrge-scle dynmic djustment of the SSH long the Equtor, leding to shllower estern thermocline nd deeper western thermocline. We suggest tht the nomlous stte in the Indin Ocen tht we descrie ove my not hve een n exggerted response to the strong 997±98 El NinÄo. There re numer of resons for this suggestion. First, the climte pttern round the Indin Ocen rim ws very different during 997±98 from tht usully ssocited with Pci c wrm event. The drought in Indonesi ws more intense thn nticipted, the rinfll in the 997 Indin summer monsoon nd over Austrli ws ner norml (lthough drought is usully expected in oth regions), nd the est Africn rinfll, usully only slightly higher during n El NinÄo, ws the lrgest this century. Second, the typicl El NinÄo/Southern Oscilltion (ENSO) cycle ppers more ssocited with generl wrming of the equtoril Indin Ocen thn chnge in the longitudinl SST grdient, proly due to the lighter thn verge winds tht occur over the Indin Ocen during n El NinÄo,6. The correltions, R, etween the men equtoril SST in the Indin Ocen nd ENSO is +., ut flls to non-signi cnt vlues of +.9 etween ENSO nd the equtoril SST grdient. The correltion etween Octoer± Novemer est Africn rinfll nd the long-equtor Indin Ocen SST grdient is +.6, indicting reltively strong reltionship etween these two time series from within the Indin Ocen sin. Circultion nd SST ptterns in 997±98 were similr to previous extremes in the est Africn rinfll (see Fig. in Supplementry Informtion). The correltion etween est Africn rinfll nd the centrl Pci c SST nomlies, however, is only., indicting rther wek reltionship etween est Africn rinfll nd ENSO. In support of our ndings, previous studies of Africn rinfll 7,8 hve suggested tht other fctors esides ENSO re responsile for est Africn rinfll vriility. Third, similr excursions from norml hve occurred in the Indin Ocen in the sence of ENSO extrem. During 96±6, the SST grdient cross the Indin Ocen reversed, with sustntil wrming in the western sin 9,. In ll, etween 9 nd 998 there were 6 yers in which the equtoril SST grdient reversed for t lest month. Of these yers, only three were El NinÄo yers. None coincided with L NinÄ. Spectrl nlysis of the time series of the long-equtor SST grdient (see Fig. in Supplementry Informtion) shows peks in rod nd round yers (exceeding 87% con dence limits) nd yers (exceeding S S SSH nomly (cm) S S SSH nomly (cm) Jul, 98 Jul, My, 98 Mr, My, 98 Mr, 98 Jn, 98 Nov, 97 Sep, 97 Jul, 97 My, 97 Mr, Jn, 98 Nov, 97 Sep, 97 - Jul, 97 - My, 97 - Mr, 97 Jn, 97 Jn, 97 E 6 E 8 E E E 6 E 8 E E Figure 3 The evolving se surfce height (SSH) eld. Pnels show time±longitude sections of nomlous SSH otined from TOPEX/POSEIDON verged etween 8 S nd 68 S(), nd etween 8 S nd 38 S(). Note the differentil westwrd propgtion s function of ltitude. As ltitude increses the phse speed decreses, consistent with the propgtion chrcteristics of n ocen Rossy wve. 999 Mcmilln Mgzines Ltd 38 NATURE VOL 3 SEPTEMBER 999

4 97%), indicting long-period oscilltions in the grdient etween two nomlous sttes: wrm in the west nd cool in the est, nd cool in the west nd wrm in the est. The possile sequence of events in the Indin Ocen occurring once the est±west equtoril SST grdient ws estlished is shown in Fig. ±d. With source of moist ir from the Indin Ocen, strong convection develops over the heted lnd mss of est Afric during utumn of 997, enhncing the equtoril esterly ow (Fig. ). trnsports, driven y the esterly nomlies, produce the ridge just south of the Equtor. In seeking equilirium, ocen Rossy wve dynmics cuse the ridge to propgte westwrd which, in turn, deepens the thermocline to the west of the source of the convergence (Fig. c). With deeper thermocline nd reduced, the western Indin Ocen continued to wrm, thus mintining the driving force of the nomlous esterly surfce winds. In this mnner, the ridge is sustined, llowing for continul estwrd propgtion of the equtoril ocen Rossy wves. We propose tht this selfsustining system prolonged the wrming in the western sin y numer of months. A wrming period my end, or diminish, in numer of wys. For exmple, wrm west Indin Ocen is often ssocited with strong monsoon,. This mens tht the summer monsoon following the Indin Ocen wrming should hve stronger winds in the western sin, which would induce greter mixing, greter trnsports forcing costl, nd greter evportion, ll of which would contriute to rpid cooling. Also, s the El NinÄo wekens (s it did during the spring of 998), nd the locus of convection moves ck towrds Indonesi, the equtoril wind ptterns in the Indin Ocen revert to westerlies (Fig. d). The Summer '97 F H G E nomlous longshore winds reduced t Equtor Autumn '97 convective heting t Equtor heted surfce c Autumn - Winter '97 ump downwelling t S d Summer '98 downwelling Kelvin wve incresed nd mixing t Equtor Figure Digrm of the sequence of events in 997±98., The climtologicl longshore winds off Sumtr (E) nd the est Africn cost (F). The winds oserved in the lte summer nd erly utumn re denoted y G nd H, respectively. The right-hnd pnel shows the effect t the Equtor on the upper ocen induced y incresed in the est nd decresed in the west. Wind into nd out of the plne of the pper re denoted y the ull's eye nd cross-hir symols, respectively., Distriution of the winds resulting from the nomlous SST grdient long the Equtor nd the chnges in the SSH distriution. c, Formtion of the ridge in the centrl Indin Ocen nd the forcing of westwrd-propgting downwelling equtoril Rossy wves to the west. The right-hnd pnel shows the effect on the upper ocen ner 8 S. d, Susequent cooling of the western Indin Ocen through mixing nd costl trnsports from stronger thn verge monsoon winds nd through circultion chnges ssocited with the wekening of the 997±98 El NinÄ o. 999 Mcmilln Mgzines Ltd NATURE VOL 3 SEPTEMBER

5 resulting relxtion of the SSH elds would force estwrd-propgting nd downwelling Kelvin wves which would deepen the estern mixed lyer nd return the system to norml con gurtion. These chnges my e seen in the spring nd erly summer of 998 in Fig.. The results presented ove suggest tht the Indin Ocen exhiits strong coupled ocen±tmosphere±lnd interctions tht re self-mintining, nd re cple of producing signi cnt perturtions to the nnul cycle, t lest during the 997±998 period. Arguly, the evolution of the perturtion is independent of ENSO. We note tht other hypotheses 3, hve een suggested for oserved internnul vriility in the Indin Ocen. These theories rely on the Indin Ocen responding loclly to either weker or stronger monsoon winds which, through chnges in nd mixing, introduce iennil component to the system. But the wekness of these theories is the mintennce of the upper-ocen nomlies from yer to yer. Our hypothesis dds coupled dynmicl component tht hs longer timescle thn the thermodynmics of the mixed lyer, nd which my form link from one monsoon seson to the next. Thus we suggest tht the Indin Ocen my not e pssive plyer in climte vriility on sesonl to internnul timescles, ut my enct very ctive nd independent role. M Received 6 July 998; ccepted 9 July Nicholls, N. Air-se interction nd the qusi-iennil oscilltion. Mon. Weth. Rev. 6, ±8 (983).. Nicholls, N. All-Indi summer monsoon rinfll nd se surfce temperture round northern Austrli nd Indonesi. J. Clim. 8, 63±67 (99). 3. Meehl, G. A. Coupled ocen-tmosphere-lnd processes nd south Asin monsoonvriility. Science 6, 63±67 (99).. Meehl, G. A. The south Asin monsoon nd the tropospheric iennil oscilltion. J. Clim., 9± 93 (997).. Wester, P. J. & Plmer, T. N. The pst nd future of El NinÄo. Nture 39, 6±6 (997). 6. Reynolds, R. & Mrisco, D. An improved rel-time glol se surfce temperture nlysis. J. Clim. 6, ±9 (993). 7. Klny, E. et l. The NCEP/NCAR -yer renlysis project. Bull. Am. Meteorol. Soc. 77, 37±7 (996). 8. Hendricks, J. R., Leen, R. R., Born, G. H. & Kolinsky, C. J. Empiricl orthogonl function nlysis of glol TOPEX/POSEIDON ltimeter dt nd implictions for detection of glol se level rise. J. Geophys. Res., 3± (996). 9. Yu, L. & Rienecker, M. M. Mechnisms for the Indin Ocen wrming during the 997±998 El NinÄo. Geophys. Res. Lett. 6, 73±738 (999).. Brnett, T. P. Interction of the monsoon nd Pci c Ocen trde wind systems t internnul time scles. Prt I: the equtoril zone. Mon. Weth. Rev., 76±773 (983).. Arkin, P. & Meisner, B. The reltionship etween lrge-scle convective rinfll nd cloud cover over the western hemisphere during 98±98. Mon. Weth. Rev., ±7 (987).. Wester, P. J. Response of the tropicl tmosphere to locl stedy forcing. Mon. Weth. Rev., 8± (97). 3. Gill, A. E. Some simple solutions for het-induced tropicl circultion. Q. J. R. Meteorol. Soc. 6, 7±6 (98).. Ner Rel-Time Anlysis of the Ocen nd Atmosphere Fig T 9, p. 36 (Climte Dignostics Bull. NO. 97/, Climte Dignostics Center, Ntionl Center of Environmentl Prediction, NOAA, Wshington DC, 997).. Gill, A. Atmosphere-Ocen Dynmics (Acdemic, London, 98). 6. Wester, P. J. et l. Monsoons: processes, predictility nd the prospects for prediction. J. Geophys. Res. 3, ± (998). 7. Nicholson, S. E. & Kim, J. The reltionship of the El-NinÄo Southern Oscilltion to Africn rinfll. Int. J. Climtol. 7, 7±3 (997). 8. Nicholson, S. E. An nlysis of the ENSO signl in the tropicl Atlntic nd western Indin Ocens. Int. J. Climtol. 7, 3±37 (997). 9. Reverdin, G., Cdet, D. & Gutzler, D. Internnul displcements of convection nd surfce circultion over the equtoril Indin Ocen. Q. J. R. Meteorol. Soc., 3±67 (986).. Kpl, A., Born, K. & Flohn, H. in Proc. Int. Conf. on Monsoon Vriility nd Prediction (ed. Newson, R.) 9±6 (Tech. Doc. 69, World Meteorologicl Orgniztion, Genev, Switzerlnd, 99).. Ro, K. G. & Goswmi, B. N. Internnul vritions of the se-surfce temperture over the Arin Se nd the Indin Monsoon: A new perspective. Mon. Weth. Rev. 6, 8±68 (988).. Shukl, J. & Mooley, D. A. Empiricl prediction of the summer monsoon over Indi. Mon. Weth. Rev., 69±73 (987). Supplementry informtion is ville on Nture's World-Wide We site ( or s hrd copy from the London editoril of ce of Nture. Acknowledgements This work ws supported y the Of ce of Glol Progrms, NOAA, nd the NSF (P.J.W., A.M.M., J.P.L.), nd y NASA (R.R.L.). Correspondence nd requests for mterils should e ddressed to P.J.W. (e-mil: pjw@oz.colordo.edu).... A dipole mode in the tropicl Indin Ocen N. H. Sji*, B. N. Goswmi², P. N. Vinychndrn* & T. Ymgt*³ * Institute for Glol Chnge Reserch, SEAVANS N 7F, - Shiur, Minto-ku, Tokyo 679, Jpn ² Center for Atmospheric nd Ocenic Sciences, Indin Institute of Science, Bnglore 6, Indi ³ Deprtment of Erth nd Plnetry Physics, Grdute School of Science, The University of Tokyo, Tokyo 3 33, Jpn... For the tropicl Pci c nd Atlntic ocens, internl modes of vriility tht led to climtic oscilltions hve een recognized,, ut in the Indin Ocen region similr ocen±tmosphere interction cusing internnul climte vriility hs not yet een found 3. Here we report n nlysis of oservtionl dt over the pst yers, showing dipole mode in the Indin Ocen: pttern of internl vriility with nomlously low se surfce tempertures off Sumtr nd high se surfce tempertures in the western Indin Ocen, with ccompnying wind nd precipittion nomlies. The sptio-temporl links etween se surfce tempertures nd winds revel strong coupling through the precipittion eld nd ocen dynmics. This ir±se interction process is unique nd inherent in the Indin Ocen, nd is shown to e independent of the El NinÄo/Southern Oscilltion. The discovery of this dipole mode tht ccounts for out % of the se surfce temperture vriility in the Indin OcenÐnd, in its ctive yers, lso cuses severe rinfll in estern Afric nd droughts in IndonesiÐrightens the prospects for long-term forecst of rinfll nomlies in the ffected countries. The ctstrophic rins of 96 in tropicl estern Afric nd susequent rupt dischrge of the White Nile re now known ±6 to e prt of n nomlous climte stte over the tropicl Indin Ocen. A dipole structure chrcterized the se surfce temperture (SST) nomly during this event: wrmer thn usul SSTs occurred over lrge prts of the western sin, while SSTs off Sumtr were cooler thn usul. Rinfll incresed over tropicl estern Afric nd the western Indin Ocen, while over the Indonesin rchipelgo it decresed, resulting in severe drought. Equtoril surfce winds, which in norml summer seson low towrds the est, wekened nd reversed direction. There ws no El NinÄo in the Pci c, while Indi experienced the highest summer monsoon rinfll in the pst yers (ref. ). By exmining long-term dt sets of SST Normlized nomly DMI σdmi=.3 C Nino3 σnino=.8 C Ueq σueq=.96 ms Time (yers) Figure Dipole mode nd El NinÄo events since 98. Plotted in lue, the dipole mode index (DMI) exhiits pttern of evolution distinctly different from tht of the El NinÄ o, which is represented y the Nino3 se surfce temperture (SST) nomlies (lck line). On the other hnd, equtoril zonl wind nomlies U eq (plotted in red) coevolves with the DMI. All the three time series hve een normlized y their respective stndrd devitions. We hve removed vriility with periods of 7 yers or longer, sed on hrmonic nlysis, from ll the dt sets used in this nlysis. In ddition, we hve smoothed the time series using -month running men. 999 Mcmilln Mgzines Ltd 36 NATURE VOL 3 SEPTEMBER 999