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ProgressinunderstandingElNiño

ARTICLEinCOMPLEXITY·JANUARY1987

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Progress in understanding El NinoDavid B. Enfield

Prior to the work of Jacob Bjerknes the El Nino phenomenon was regarded as an aperiodic climatic eventconfined to the Pacific coast of South America. Spurred by a growing consciousness of the oceans' role inglobal climate, there has been an explosion of El Nino research in the last two decades, El Nino is nowrecognized to be an integral part of a Pacific-wide ocean relaxation, with global climatic impacts andeconomically important ecological consequences. However, we are still groping for the final prize: theultimate cause of this climate anomaly and the ability to reliably predict its onset and intensity.

In the latter half of this century scientistshave gained much greater insight intothe true nature of El Nino. It is a regionalbut important manifestat ion of i verylarge-scale interactive process involvingthe tropical ocean and global atmos-phere. With our increased knowledgewe have come to realize that El Nino isnot merely a curious but isolated pheno-menon, but rather an important link inthe physical processes that affect ourglobal climate from one year to another.By understanding the role of El Nino inthese processes we can hopefully gainthe ability to forecast short-term climaticchanges, in particular, and appreciatethe role of ocean-atmosphere interac-tion in climate changes on all time scales,in general.

The Southern Osci l lat ionResearch durins the first half of thiscentury focusedhainly on the meteoro-logical aspects of the ocean-atmospheresystem, embodied in the global-scaleatmospheric pressure fluctuation iden-tified by Sir Gilbert Walker as theSouthern Oscillation (SO) t1l. The SOwas recognized as a coherent variation ofbarometric pressures at interannual in-tervals that is related to weather anoma-lies in many different regions, particu-larly in the tropics and subtropics. The

David B. Enfield, A. 8., M. S.. Ph. D.Graduated in physics and geophysics from theUniversity of Cali fornia (Berkeley) in 1965 andgained the M.S. and Ph.D. degree in physicaloceanography at Oregon State University in1970 and 1973. He worked two years each asUNESCO Exoert in Ecuador and as Liaison Off icefor the IDOE Coastal Upwell ing EcosystemsAnalysis Program in Peru, 1973-77. From 1977to 1 987 he worked as Research Associate, Assis-tant Professor and Associate Professor at theCollege of Oceanography, Oregon State Uni-versity. His research interests include El Nino,large scale ocean-atmosphere interactions, andcoastal and equatorial dynamics. Since 1987 hehas worked as Research Oceanographer at theNOAA Atlantic Oceanographic and Meterolog ic-al Laboratory in Miami, Florida.

Endeavour, New Series, Volume 1 1, No,4,1987,0163-9327/87 $0.00 +. 50.O 1987 Pergamon Journals Ltd. Printed in Great Britain.

early research [2] has shown that thesurface atmospheric pressure in regionsdominated by tropical convection (as-cending air) and rainfall, such as Indone-sia, is inversely correlated with the pres-sure in regions typified by subsidence(descending air) and dry conditions,such as the eastern South Pacific (figure1). Air is continually transferred at lowlevels - through the zonal trade windcirculations - from the subsidence re-gions to the convective regions. The airreturns at upper tropospheric levels,completing a series of zonal cells aroundthe globe that compromise the WalkerCirculation (figure 2). The SO has beendefined as a fluctuation of the massexchange in the dominant Indo-PacificWalker cell; that is, between the eastern(Indonesian and Indian subcontinent)

lf,:.;rJrr*r (southeast Pacific) hemis-

The slate ofthe SO pressure see-saw ischaracterized by the Southern Oscilla-t ion Index, or SOI (f igure 3). The SOreaches its maximum development (highindex) when the Pacific cell of theWalker Circulation (including the tradewinds) is strong. At these times the

pressures in the Indonesian region arelowest and the convection and rainfallthere reach maximum intensity; thenalso, the South Pacific subtropical highpressure region is most intense and a dryzone extending along the equator fromSouth America toward the dateline isbest developed. The SO enters a lowindex phase when the Walker Circula-t ion weakens: at such t imes the barome-tric oressures rise in the Indonesianregion and fall in the southeast Pacific.There are periods when low pressureregions (such as Indonesia and northeastBrazil) undergo drought conditions. Atthe same time, the equatorial dry zone inthe Pacific contracts eastward and theAmazon convective regime appears toshift toward the desert region in thenorthwestern portion of the South Ame-rican continent. The El Nino (east Pa-ci{ic warming) typically occurs followinga prolonged period of high index, just asthe SO is entering a low index phase.

ElNi f ioApart from a few scientific analysesbased on oceanographic expeditions or

t50" lgo" 150" 120' 90p 60" 30"w o" -30"E 60" 90" 120" 150" 180"

Figure I The global distr ibution of the correlat ion coeff icient between the barometricpress.ure variat ions at Djakarta, Indonesia, and those elsewhere. The negativecorrelat ion between the eastern and western hemispheres, with centres over lndonesiaand the southeast subtropical Pacif ic, characterizes the pressure see-saw associatedwrth the SO, an interannualf luctuation in the strength of the Walker Circulat ion. (After H.P. Ber lage [2] ]

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regional data from South America [3, 4]knowledge of El Nino prior to the early20th century consisted largely of infor-mat ion f rom mi l i tary campaigns. missio-naries, privateers (ship logs), explorers,historical compilations, and geographicreports, as well as from the economicallyoriented activities of construction ensi-neers. hydrologists. farmers. guano ad-ministrat ion off icials. and so on. Thelimited climatic information obtainedfrom these sources. dating back nearlyf ive centuries, has been extensively re-searched and summarized bv W. H.Quinn [5] . Thc El Nino. which we reco-gnize today as an anomalous east Pacificwarming episode, originally received itsname in reference to the Christ child.because of the annual appearance ofwarm water near the Ecuador-Peru bor-der at about Christmas-t ime. The ac-companying changes in coastal marinefauna and seasonal onset of light rainsover this normally dry desert region havealways oriented the lishing and agricul-tural act ivi t ies of the local inhabitants.who take maximum advantages of them.However. most o[ lhese same act iv i t iesare adversely affected when the occasio-nal atypical E,l Nino occurs every fewyears, greatly enhancing the normal an-nual changes. The more spectacularcases have prompted often colourful ac-counts of desert floods. infestations ofinsects, disease outbreaks, exuberantblooms of desert plants (figure 4) andsouthward invasions of unusual trooicalmarine fauna. Nowadays. people asso-ciate the term'El Nino'with the extraor-

198

Figure 3 A45-yeart ime ser ies of anomal ies of the SOI der ivedfrom dif ferences between the barometric pressures atTahit i(eastern South Paci f ic subsidence region) and Darwin, Austral ia( lndonesian convect ive region). Posi t ive (negat ive) swings of theSOl, shaded when large, coincide with cool (warm)oceaniccondit ions in the eastern Tropical Pacif ic. Occurrences of El N inoare designated as'EN'.

dinary but less frequent occurrence regional manifestat ion of a large-scalerather than the annual event. oceanic counterDart to the SO. and that

The early atmospheric workers recog- the two are int imately related throughnized El Nino as one of the important large-scale ocean-atmosphere interac-climatic aberrations that occur durins tions.the low-index phase of the SO. They didnot, however, understand the cause- TheBjerknesrevolut ioneffect relationships between the SO and The Iirst maior advance in our under-El Nino; that is, that El Nino is the standing of El Nino and i ts relat ionship

Figure4 Duringthe1982-33ElNino,recordrainsfel lovercoastalEcuadorandthenorthernmost desert region of Peru. This scene depicts a port ion of the Sechura desert inNovember 1983, about f ive months af ter the rains subsided and a shal low lake had dr iedu p, pa rch ing the soi l . Ma ny vest iges rema in of the copious vegetat ion thatspontaneouslyemergedfromthe deluge. (Photo by H. Sol id i . l

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Figure5 Left , lookingnorthacrosstheequator inthePaci f ic :Schematicrepresentat ionof the equator ia l f low patterns under condi t ions of normal (posi t ive SOI) and abnormal(negat ive SOI)trade winds. Right, looking westward along the equator: Schematic v iewof the normal and abnormal tra nsverse circu lat ion patterns near the equator. At top,westward wind stress forces a surface'Ekman'transport poleward into eitherhemisphere; sea level and temperatures are low at the equator due to upwell ing and aconvergence at depth com pensates for the Ekman divergence at the su rface. At bottom,Ekman divergence and upwell ing cease as the winds relax, the convergence continuesthroughout the upper ocean, however, ra is ing sea level and depressing the thermocl ine.(From J. Bf erknes L6l, reprinted courtesY of the lnter-American Tropical TunaCommission.\

to the Southern Oscillation came withthe work ofJacob Bjerknes ( l8S7-1975)in the 1960s. The earlier SO research hadbeen phenomenological , seekingcompound periodicities that coincidedwith those of external agents such assunspot cycles. Bjerknes, on the otherhand, sought to explain the SO throughthe internal dyna'mical mechanisms ofthe atmosphere and ocean, and he ac-cepted the importance of their mutualinteractions.

Initially, Bjerknes tried to explain ElNirio as an occasional exaggeration ofthe El Nino-like state that occurs duringevery southern hemisphere summer,centered about March. As had othersbefore him, he argued that El Nino iscaused by local trade wind weakeningand north-south ocean density gradientswithin the El Nino region itself. As thenormal equatorward coastal winds de-

crease, cease, or reverse, the ubiquitousupwelling of cold, nutrient-rich waters

alons the Peru coast would be interrup-ted; i=he water would warm from the lackof a cool source and also because warm,low-salinity water of low density north of

the equator would flow southward in the

absence of opposingwinds. Both mecha-nisms would explain the collapse of thecoastal ecosystem due to deficits in thenutrient supply.

He quickly realized, however, that theregional El Nino is closely related to aPacific-wide relaxation of the equatorialocean in response to weakened tradewinds on a much larger scale [6]. Thiswas a major, new contribution that hasbeen confirmed by subsequent research.His arguments, though based on the

assumption of steady conditions, haveessentially captured the character of thetransient ocean response we have obser-ved extensively since his time (figure 5):a deceleration of the normally westwardequatorial surface currents accompaniedby cessation of equatorial upwelling;accumulation of warm upper layer waterin the equatorial zone; and consequentlarge-scale warming.

There are two principal shortcomingsin Bjerknes' contributions. As alreadynoted, his analysis assumes a steadyocean and coltrasts two quite differentbut essentially equilibrium states: ElNino and non-El Nino. This approachprecludes recognition of the wave-likenature of the transient El Nino responsethat we are now familiar with. Theoversight was probably due in large partto the second difficulty that he faced: alack of data. Because he had few obser-vations of Pacific-wide wind distribu-tions, he assumed that the trade windsweaken everywhere and that the El Ninocondi t ion can be understood eve-rywhere, including the region off SouthAmerica, as an adjustment to locallyreduced winds. This assumption ledBjerknes and other investigators at thetime [7] to erroneously conclude that thePeruvian El Nino is a locally producedwarming due to the cessation of coastalupwelling under reduced (or reversed)alongshore winds, simultaneous with thephysical ly s imi lar response occurr ingmuch farther west, along the equator.As we shall see, the Bjerknes scenariowould need modification to adequatelyexplain what happens off South Americaduring El Nino.

Evolution of modern conceptsIn the mid-1970s, as more data becameavailable and was analyzed, it becameobvious that the coastal winds alongmost of the Peru coast do not decreaseduring El Niflo episodes (figure 6), al-though the large-scale trades fartheroffshore do indeed weaken [8]. At somecoastal locat ions the upwel l ing-favourable winds actual ly intensify.Clearly, the physical process of upwel-ling is not diminished and cannot explainthe well documented warming and de-creased oroductivitv that occur in thePeru coistal wateis durine El Nino.Could i t be that the larger scale anoma-lies in the equatorial Pacific, describedand explained by Bjerknes, were insome way transmitted to the coastalregion?

Beginning in 1975, a flurry of observa-tional and theoretical studies providedcompelling evidence that E,l Nino is,indeed, remotely forced in the equato-rial Pacific [8, 9, 10]. Prior to El Nino theSO is in its high-index phase, with a welldeveloped southeast Pacific high pres-sure system and strong southeast tradewinds. Many oceanographers believethis is an important precondition to ElNino, because the strong trade circula-tion amasses a large pool of warm, upperlayer ocean water in the western tropicalPacific. Then, as the SO enters its low-index phase, the normally westward blo-wing winds in the western and west-central eouatoridl Pacific weaken or re-verse, allowing the accumulated warmwater to move eastward. However, be-cause of the impulsive nature of the windcol lapse. the ocean response is wave-like, although physically very similar tothe mechanism proposed by Bjerknes.

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Figure 7 Looking down on the equator f rom above, a numericals imulat ion of an equator ia l ly t rapped internal Kelv in wavethat impinges on the South American coast. Contours approximately represent the anomalous r ise in sea level (centimetres) anddepression of the thermocline (metres).The three pa nels depict condit ions at successive month ly intervals. (From D. B. E nf ield I1 1 Iafter O'Brien etal.)

Because the wave response involvesan eastward perturbation of existingcurrents. the earth rotation deflects theflow just off the equator - in eitherhemisphere - towards the equator. Theunleashed wave, or series of waves, isthus trapped on the equator and ad-vances eastward much like an elonsatedbubble of l iberated upper layer water(figure 7). Called Kelvin waves (afterLord Kelvin. who first described suchwaves mathematical ly), these pulsespropagate across the Pacific to the SouthAmerican coast in about two or threemonths. They are internal waves, sup-ported by the density contrast at theequatorial thermocline, a zone found50-150 metres below the sea surface

where the temperature decreases down-ward most rapidly. The behaviour ofthese waves is not altogether unlike thesloshing of an oi l layer overlying vinegar.As they pass (say, for example, an islandtide gauge or an array of submergedinstrumentation). the sea level at theequator typically rises by 10-30 centi-metres, the thermocline is depresseddownward by a comparable number ofmetres, and the normally westwardflowing equatorial currents are reversed.

As Kelvin waves reach the SouthAmerican coast, several things happen.Consistent with their behaviour alongthe equator, sea level rises and thethermal structure alons the coast is de-pressed, but due to the confining effect

of the continental boundary, the ampli-tude of these disturbances is increasedconsiderably (f igure 7, third panel). Im-mediately, part of the energy begins toreflect westward as pairs of slowly prop-agating, counter-rotat ing eddies, cal ledplanetary or Rossby waves. More impor-tantly for the coastal ecosystem, much ofthe energy continues poleward into bothhemispheres, as the Kelvin waves spl i tinto diverging coasral waves la lso wi thKelvin-l ike characterist ics).

The impact of the wave arrivals on theocean thermal structure is well illustra-ted by a two-year series of temperaturesections off Ecuador, spanning thestrongl9T2-7 3 El Nino (figure 8) [11]. InNovember-December 1971 conditions

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were normal with a shallow tropicalthermocline at 30'-50 metres and relati-vely isothermal water below. The ther-mocline deoression due to the first Kel-v in wave arr ival was detected inFebruary-March 1972. Taking the pre-El Nino condi t ion of November-December l97l as a benchmark for the'normal' temperature distribution, themaximum depression occurred one yearlater when a second rise in coastal sealevel signalled the probable arrival ofanother series of Kelvin pulses. At that

t ime, most of the isotherms at depths lessthan 400 metres were 50-150 metresdeeoer than normal. and maximum tem-oeritu.e increases of about 9-10'C wereiound 50 metres below the surface. ByMarch 1973 the system was returning tonormal, and by the end of that yearappeared to have completely recovered.

As the southern hemisphere portionof the El Nino disturbance propagatessouthward, it quickly spreads the deepe-ned thermocline and sea level rise alongthe Peru coast. In a matter of weeks the

entire coast may be affected, much soo-ner than could occur under the action ofcurrents alone. Although the coastalflow is accelerated southward. water thatstarts at the equator can travel only ashort distance in the time it takes thewaves to propagate southward. Howe-ver, since the coastal upwelling of sub-surface water from depths of 50*100metres continues unabated. and becausethe water at those depths is much war-mer than normal, the surface tempera-tures of the upwelled water becomeunusually high. Coastal surface tempe-rature anomalies of 3-4"C above normalare common for moderate El Ninos andthey reached as much as 8-10'C duringthe very intense 1982-83 episode. Moresignilicantly for the coastal ecosystem,the upwelled water is also poor in nu-trients and subsequently leads to a col-lapse of the primary productivity and ofthe heavily exploited commercial fishstocks that it normally sustains (figure 9)

[r2).Such is the conceptual model 'of El

Nino that has evolved since the work ofBjerknes, a paradigm widely acceptedby oceanographers as a framework formodern research. The rapid develop-ments in El Nino research over the lastdecade probably would not have occur-red, however, ifthe effects ofthis pheno-menon were geographically conlined tothe equatorial Pacific. Instead, we owemuch of our progress to the realizationby scientists, news media, and legislatorsthat E,l Nino has far-reaching climaticand economic repercussions around theglobe.

TeleconnectionsIt was B jerknes who, using data from theunusual 1957-58 event, advanced a ma-jor hypothesis to explain the geographicextensions of El Nino [13]. He arguedthat the equatorial warming feeds backon the lower atmosphere (troposphere)through evaporative heat transport fromthe warmed ocean surface. Thunder-storm activity then becomes frequentand unusual amounts of rainfall assaultthe normally dry equatorial zone. As theatmospheric moisture condenses, unu-sual amounts of latent heat are released.propelling additional heat and momen-tum north or south toward higher lati-tudes at the upper levels of the tropos-phere. The jet streams intensify, alteringsignificantly the extratropical atmosphe-ric circulation and weather Datterns.especial ly in the winter hemisphere.Bjerknes adapted the term 'teleconnec-t ions' to character ize the l inkagesbetween the equatorial source regionand these remote. but related weatherphenomena.

Ironically, the first strong evidence forteleconnections came from the oceanand not the atmosphere. During E,lNinos. unusual increases in mean sealevel occur simultaneouslv. not onlv off

Figure9 Schematic i l lustrat ionof upwel l ingcharacter ist icsalongthecoastof Perudur ing normal ( top) and El Nino (bottom)condi t ions. Athermocl ine and nutr ic l ineseparate the warm, nutr ient-def ic ient upper layer ( l ight colour) f rom the cool , enr ichedlower layer below (dark colou rl. lArtwork by Allen Carroll based on original drawing byR. R. Barber@ National GeographicSocietyl2l\

201

South America but also along most ofthe Pacific coast of Central and NorthAmerica as far north as Canada andAlaska (figure 10) [1a]. The data areconsistent with the continued polewardpropagation of the El Nino wave along

the coast. As sea level rises off Calilbr-nia, the normal southward drift of theCalifornia Current is decreased or rever-sed within about 500 km of the coast- andmarine organisms tend to be found northof their usual habitats [15, 16].

Public and scientific awareness of theatmospheric teleconnections proposedby Bjerknes was increased because ofsevere weather abnormalities that occur-red in North America during the 1976-77 winter. An exaggerated circulationpattern, attributed to teleconnectionsfrom a moderate El Nino that year [17],brought a paralyzing drought to the USwest coast, while the east coast sufferedthe numbing effects of repeated snowstorms and record cold. Extensive ana-lyses of historical data sets soon revealedthat the interchange between ocean andatmosphere during one of these events isa continuous process occurring in bothdirect ions, developing in phases seem-ingly locked to the march of the seasons

[18], and with major teleconnections tothe northern hemisphere occurring dur-ing the northern winter season (f igure 7).

The 1982-83 ElNif ioBy 1982, a certain complacency had setin amongst scientists, who felt that ElNino/Southern Osci l lat ion eoisodes(ENSOs) tend to fo l low a repeatablepattern and that further examination ofthe teleconnection process could soonlead to useful forecasts of maior climaticanomal ies. Hence, when the f i rst reportso[ anomalies in the tropical Pacif ic u'erereceived in mid-1982, many scientistsrefused to believe a true El Niflo was inprogress, based on the unusual time ofyear and the lack of antecedent condi-t ions thought necessary (for example., apositive SO index). In spite of its non-conformity to the canonical pattern, the1982-83 period subsequently proved tobe a disaslrous El Nino, more in lensethan any other in living memory. Thelast episode of comparable magnitudeoccurred in 1891, almost a century be-fore. Although our basic concepts ofwhat happens during a ENSO episoderemained unchanged, our faith in thefeasibility of forecasting its occurrencehad been badly shaken.

There was extensive damase to themarine environment along the SouthAmerican coast in 1983 [9]. Ironically,not al l of the f isheries impacts weredisastrous; certain commercial species,most notably shrimps, but also dol-phinfish, scallops, octopus, and manyothers appeared in unusual numbers.Others, of course, virtually disappearedor their stocks were greatly depleted.High temperatures and low concentra-tions of traditional plankton varietiesdevastated many pelagic (mid-water)fish stocks. such as anchovies and sar-dines. Many bottom dwellers like shrimpand scallops fared well because of anenriched oxygen supply, while otherdemersal (near-bottom) species such ashake were redistributed by the environ-mental changes. One can speculate, ofcourse, that the demise of some speciesfavoured the prosperity of others, direc-

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202

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Figure 1 1 An i l lustrat ion of the distr ibutions of key propert ies involved in theteleconnect ion process dur ing a typical ENSO event, (a) Upper t roposphere anomal iesof geopotent ia l heightforthe northern hemispherewinter. (b)Contoursof surfacebarometric pressure, with surface wind anomalies (arrows) in the tropics. Bottom:Distr ibut ion of anomalous sea surface temperature ("C). (From E. Rasmusson [14].Reprinted courtesy of Oceanus Magazine.l

tly or indirectly, through greatly alteredpatterns of interdependent factors suchas available food types, larval survival,predation, and so forth.

Mortality among the normally abun-dant guano birds and marine mammalsof th is coastal region was extensive.especially of their young, because theirnormal food supply of schooling fisheswas greatly reduced and/or made una-vailable to them. Since the hungry adultsabandoned their nests and broods inlarge numbers in search of scarce food,their populat ions were subsequentlyslow to recover after environmentalconditions returned to normal in 1984. Awell documented microcosm of the birdmortalities occurred near the eouator onChristmas Is land. hal l - -way aiross thePacific [20].

By far, the most extensive damage tothe South American coast came in theform of rainfall. Unheard-of amounts ofrain fell on coastal Ecuador and Peru.north of Lambayeque (6'5). Flas-h floodsroared through the desert gul l ies, r ip-ping out roads and bridges and damagingthe oleoducts that transport crude oil toports of embarkation. The importantcoastal town of Talara, built in a canyoncut through the Sechura plateau, wasinundated with mud that took more thana year to remove. Talara and otherisolated communities had to be suppliedby helicopter until road links could bere-establ ished. Vast regions of the Se-chura desert were revegetated by therainfall (figure 4), and large shallowlakes appeared, often burying majorhighways, some remaining for almosttwo years. Agriculture, highly depen-dent on a well controlled system ofirr igation, was ravaged by untimely wa-ter damage, a lack of transportationfacilities and - not the least - bv infesta-tions ofvoracious insects spawndd by therogue vegetation. Because mean sealevel had risen by as much as two feet,many low-ly ing beach communit ies werepounded by surf, streets flooded, andbeachfront buildings wiped out.

Many poignant laments of human suf-fering emerged from the 1982-83 ElNino disaster [21]:

In the center of the city of Sullana awhole block became an island of landon Alcedo street. Entire families wereimprisoned by the water which rea-ched four metres. A merchantcommitted suicide. A wall fell on ayoung woman and caused her death.The Navarro family prayed in front oftheir religious statues. La Arena, acountry town, was total ly inundated.In the television interview with asimple country woman: 'The watertook everything, our children are hun-gry. Oh dear Lord. I mean to say thatthe rain is good but our lack of prepa-rat ion to receive i t is bad'.

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broad and interactive fronts [24]: longterm (decadal) monitoring of key envi-ronmental variables; shorter term (seve-ral months to several years) field experi-ments designed to gain an understandingof the key processes that occur duringENSO; and theoretical and numericalmodelipg of ENSO with an aim to ulti-mately predicting its occurrence andtime evolut ion.

What is the greatest obstacle to fur-ther progress? This author's worst fear isthat the attention span of administratorsand politicians will be too short to keepthe effort funded for the full decade.Will another disaster be reouired toensure final success?

References

[1] Walker, G. T. Mem. Ind. Met. Dept.,24,75,1923.

[2] Berlage. H.P. Mededel. enVerhandel.,Kon. Ned. Met. Inst.. No. 69, 152 pp.,1957.

[3] Murphy, R. C. Geographic Review, 16,26.1926.

[4] Schott, G. Ann. Hydr. Marit. Meteoro-log. , 59,162,1931.

[5] Quinn, W. H., Neal V. T. and Antunezde Mayolo S. E. "/. Geophys. Res.(Oceans), in press, 1987.

[6] Bjerknes, J. Bull. Inter-Am. Trop. TunaComm., 12,1,1966.

[7] Wooster, W. S. and Guillen O. J. Mar.Res.. 32.287.1974.

[8] Wyrtki. K. l . Phys. Oceanogr., 5,572,1975.

[9] Godfrey, J. J. Phys. Oceanogr.,5,399,1975.

[10] McCreary, J. P. "r. Phys. Oceanogr.,6,632.1976.

[11] Enfield, D. B. Resource Mana7ementand Environmental IJncertainry. M. H.Glantz and J. D. Thompson, eds., WileyInterscience, Wiley and Sons, NewYork,213-254, 1981.

[12] Barber, R. T. and Chavez F. P. Science,222, 1203, 1983.

[13] Bjerknes, J. Tellus, lS, 820, 1966.[14] Enfield, D. B. and Allen J. S. l. phys.

Oceanogr., l0 557, 1980.[15] Chelton, D.B. CaICOFI Reports,Yot.

xxi l . 34-48, t981.[16] Mysak, L. A., Hsieh W. H. and parsons

T . R. Biolog. Oceanogr., 2 (1),63,1982.[17] Namias, J. Mon. Wea. Rev., 106,279,

1978.[18] Rasmusson, E. M. and Carpenter T. H.

Mon. Wea. Rey., l l0. 354, 1982.[19] Arntz, W., Landa A. and Tarazona J.

'El Nino': Su Impacto en La FaunaMarina, Bulletin, Instituto del Mar, Cal-lao, Peru, 222 pp., 1985.

[20] Schreiber, R. W. and Schreiber E. A.Science, 225,713,1984

[21] Anon. Piura'33,Charityraisingpublica-tion funded by the Catholic Archdioceseof Piura/Tumbes, Peru, 63 pp., 1983.

[22] Wooster, W. S. and Fluharty D. L., eds.El Nino Norlr, Washington Sea GrantProgram, Univ. Wash., Seatt le, WA98795,312 pp., 1985.

[23] Canby ,T.Y . National Geographic, 165,144,1984.

[24] Anon. Ocean Research for Understan-ding Climatic Variations. National Aca-demy Press, Washington, D. C.,58 pp.,1983.

The teleconnections of weather phe-nomena to more remote regions beganin June-July 1982, when central Chilesuffered the effects of record rainfall andflooding resulting from an intensifiedsouthern hemisphere winter jet stream.By December 1982 the sea surface tem-peratures along the equator had risen to4-6"C above normal and were pumpingenergy into the subtropical jet stream ofthe northern hemisphere. Frequentstorms moved south of their normaltracks and battered the California coastwith high surf. The coastal erosion wasall the more catastrophic for coastalcommunities due to the fact that meansea level had risen about a foot (30centimetres) above normal levels (figure12). The offshore circulat ion ofthe nor-mally southward California Current wasreversed, and hundreds of marine spe-cies were carried far north of theirnormal ranges [22]. Some fisheries pros-pered; others col lapsed; of these, a fewhave not yet recovered. And the list goeson: record drought in Austral ia, SouthPacific hurricanes as far east as Tahiti,

.unusual winter rainfall in the southernUSA and northern Caribbean. and soon. Certainly, the global weather distur-bances were remarkable durine 1982-83

, [23 j . conf i rming the basic hypothesis ofBjerknes. However, the pattern ofweather phenomena produced by theteleconnections turned out to be radical-ly dif ferent from that on previous occa-sions. We currently fear that the detailsof the atmospheric response to El Ninowarmings are hypersensitive to initialconditions in ways that are difficult orimpossible to predict, and we need toknow more about the teleconnectionprocess.

204

Present and future researchThe 1982-83 ENSO was a humblinsexper ience for scient ists: a comparablaEl Nino may not occur for another 100years, but this one left i ts mark. Fortuna-tely, the interest and funding generatedfrom it has caused an explosion in ENSOresearch around the world. as well aswidespread coverage in the popularpress. In 1985 a decade of ENSO re-search was inaugurated under the aegisof an international effort cal led the Tro-pical Ocean-Global Atmosphere Pro-gram, or simply, TOGA. By the end ofthe 1990s, hundreds of meteotologists,oceanographers, biologists, and engi-neers will have been involved in TOGA.try ing to unravel the many remainingmysteries of ENSO:

1. What are the mechanisms by whichthe oceanic relaxation Droduces unusualsurface warming over iarge areas of thetropical Pacific?

2. Do the tropospheric teleconnectionsoccur as Bjerknes explained, or does theatmosphere transmit anomalous variabi-lity through wavelike or other means?

3. Does the coastal wave propagation ofoceanic disturbances to high latitudesreally occur, or do the atmospheric tele-connections produce similar effects andmerely make them appear wavelike (or,more likely, do both occur)?

4. What first causes the trade winds andWalker Circulation to decelerate. initia-' t ing

an ENSO sequence, and can thisevent be predicted so as to producereliable and useful forecasts?

The TOGA research stratesv toanswer these and many other qu"i i ionsis to foster investigations along three