S. George Philander

EI Nino

A Predictable Climate Fluctuation

El Nino is so versatile and ubiquitous - he causes torrential rains in Peruand Ecuador, droughts and fires in Indonesia, and abnormal weather glob~(llly - that the term is now part of everyone's vocabulary; it designates amischievous gremlin. Hence, if the stock market in New York is erratic, orthe traffic jams in London are exceptionally bad, it must be El Nino. This isconsistent with our practice of using meteorological phenomena as metaphorsin our daily speech: the president is under a cloud, the examination was abreeze. We know exactly what these statements mean because we have alife~long familiarity with clouds and breezes.

Despite all the publicity itreceives, EI Nino, Spanish for"Child Jesus," remains a puzzle.Why is the rascal named afterChild Jesus? How can scientistsclaim that they are able to predictit months in advance when theyare unable to predict the weathermore than a few days ahead?

O rigina~lY El Nifi~ was the

apposite name given tothe warm, southward, seasonal cur-rent that appears along the barrencoasts of Peru and Ecuador aroundChristmas when it provides arespite from the very cold, north-ward current that otherwise pre-vails. Every few years the south-ward current is exceptionally warmand intense, penetrates far south,and bears gifts. A visitor to Perudescribed one such occasion, in theyear 1891, as follows: "'. . . the sea

is full of wonders, the land evenmore so. First of all the desertbecomes a garden. . . . The soil issoaked by heavy downpour, andwithin a few weeks the wholecountry is covered by abundantpasture. The natural seems impossi-ble." The "wonders" in the sea caninclude long yellow and blackwater snakes, alligators, bananas,and coconuts (Philander S. G. H.Is the Temperature Rising? TheUncertain Science of GlobalWarming. Princeton UniversityPress, 1998).

With time, we reserved the useof the term El Nino, not for theannual, coastal current, but for themore spectacular, interannualoccurrences that affect much of theglobe. Not only our terminology,but also olir perceptions changed;we now have a pejorative view ofEl Nino, not because its characterhas changed, but because we havechanged. Heavy rains still trans-

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form the desert into a garden, butthey also wash away homes,bridges, and roads, products of eco;'nomic development and of a hugeincrease in population. As oureconomy arid numbers continue togrow, the damage inflicted by nat-ural phenomena such as EI Nino,hurricanes, and severe storms islikely to continue increasing, evenin the absence of global climatechanges.

motion, winds in the lower atmos-phere converge onto the regions ofmaximum temperature. Thosewinds, the trades over the Pacific,harvest moisture over the oceansand deposit it in the cumulusclouds. During La N ifia, the warm

"Atmospheric Teleconnectionsfrom the Equatorial Pacific." Mon,Weather Rev. 97, 163-172. 1969) torealize that El Nino is one phase ofthe Southern Oscillation, as is evi~dent in Figure 1. El Nino is a peri~od of high sea~surface temperaturesand heavy rain~fall in the easternPacific, ofdroughts in the"maritime" conti~nent of south~eastern Asia andnorthernAustralia. DuringEl Nino of 1997 ~98, the droughtsin the west con~tributed to devas~tating forest fires,while in the east,tropical stormsspawned offMexico, and. ...heavy downpours Figure 1: The Southern Oscillation, the Interannuald h d Ch'l fluctuations in pressure that are out of phase at Darwin

renc e 1 e (Australia) and Tahiti, These variations are highly coher-and ~:ru: Usually ent with those in sea-surface temperature in the centralEl Nmols fol~ equatorial Pacific. The lower panel shows the departurelowed by its com~ of sea-surface temperature from 22°C.plement, knownas La Nina, a period of low sea~sur~face temperatures and dry c:ondi~tions in the eastern tropical Pacific,and plentiful rainfall in the westerntropical Pacific. Figure 2 (see page13) depicts El Nino and La Ninawhich, together, constitute theSouthern Oscillation.

T owards the end of thenineteenth century, scien-

tists discovered that when atmos-pheric pressure is high over thewestern tropical Pacific, it is lowover the eastern tropical Pacificand vice versa (Walker G.T. and E.W.Bliss. "World Weather Y" MemRoyal Meterol. Soc. 4, 53-84. 1932).This see-saw in pressure, known asthe Southern Oscillation, has a .period of approximately four years,and is associated with fluctuationsin a number of other variables. Forexample, during one phase of theSouthern Oscillation the tradewinds are intense, and rainfall isheavy over the western Pacific butlight over the eastern Pacific.During the complementary phase,the trades are weak, while rainfallis light over the western and heavyover the eastern tropical Pacific.

The reason for this interannualclimate fluctuation became appar-ent in 1957 when, for the first timeduring the occurrence of El Nino,sea-surface temperatures across theentire Pacific were available. Thosemeasurements revealed that thewarming along the shores of Peruassociated with El Nino is not con-fined to a narrow coastal zone butextends thousands of kilometersoffshore and influences an area soenormous that the global atmos-pheric circulation is affected. Thisled Bjerknes (Bjerknes, J.

T he changes in the tempera-ture patterns of the tropical

Pacific Ocean shown in Figure 2induce the Southern Oscillation.Of particular importance arechanges in the regions of maximumsurface temperature. The air oftenrises spontaneously over suchregions, creating tall cumulus tow-ers that provide plentiful rainfalllocally. To sustain the rising

waters of the tropical Pacific(under the region of heavy rainfall)are confined to the west. During EINino, the eastern equatorial Pacificbecomes warm, and the region ofcumulus towers and heavy rainfallshifts eastward. This explanationfor the climate fluctuations associ-ated with the Southern Oscillationleads to another question: Whatcauses the sea-surface temperaturesto change?

Temperature patterns at the sur-face of the ocean reflect subsurfaceoceanic conditions. The ocean iseffectively composed of two layers offluid: a shallow surface layer ofwarm water, some 100 meters deep,above a deep, cold layer thatextends to depths in excess of 4 km.The themiocline, a thin region oflarge vertical temperature gradients,separates the two layers. In theabsence of any winds, the thermo-cline, which is the interface

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between the warm and cold water, ishorizontal. Under such conditions,the warm surface waters are spreaduniformly over the cold layer. Thereis a tendency toward this state dur-ing EI N ifio when the trades areweak. The intensification of thosewinds, during La Nifia, drives thewarm surface waters westward, caus-ing the thermocline to tilt down-

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Oscillation is an inversion, a mirrorimage, of the first part of this tangofor ocean and atmosphere. A slightintensification of the winds drivessome of the warm water westward,thus increasing the temperature dif-ference between the eastern andthe western Pacific. That increasemakes the winds stronger, whichcauses the temperature differenceto become even bigger, and so onuntil La Nina is established.

EI Nino and La Nina are beau-tiful examples of how the wholecan be greater than the sum of theparts. The ocean, by itself, cannotproduce .the Southern Oscillationunless the winds over the oceanfluctuate in a certain manner. Theatmosphere by itself is also inca-pable of producing the SouthernOscillation unless the sea surfacetemperatures vary in a certain man-ner. Together, the ocean and theatmosphere can interact and sponta-neously produce a SouthernOscillation with wind and tempera-ture fluctuations that are perfectlyorchestrated. To ask why EI Nino orLa Nina occurs is equivalent to ask-ing why a bell rings or a taut violinstring vibrates. The Southern Oscil-lation is a natural mode of oscillationof the coupled ocean-atmospheresystem; it is the music of theatmosphere and the hydrosphere.

The atmosphere and the oceanare partners in a dance. But wholeads? Which one initiates the east-ward surge of warm water that endsLa Nina and starts EI Nino?Though intimately coupled, theocean and the atmosphere do notform a perfectly symmetrical pair.Whereas the atmosphere is quickand agile and responds nimbly tohints from the ocean, the ocean isponderous and cumbersome andtakes a long time to adjust to achange in the winds. The atmos-phere responds to altered sea sur-face temperature patterns within amatter of days or weeks; the oceanhas far more inertia and takesmonths to reach a new equilibrium.

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From an atmospheric perspec-tive, changes in sea-surface temper-atures cause the Southern Oscil-lation, including changes in thewinds over the Pacific. From anoceanic perspective, those changesin the winds cause the sea surfacetemperature changes. In responseto this circular argument, it istempting to ask which changedfirst, the winds or the tempera-tures? This is a futile approach,equivalent to asking whether thechicken or the egg came first. It isfar more rewarding to explore thebehavior of the coupled ocean-atmosphere system. Consider a ran-dom disturbance which, at theheight of La N ifia, causes a slightrelaxation of the trades. Thosewinds drive the warm surface waterswestward and cause the region ofhigh temperatures, rising air, andheavy rainfall to be confined to thewestern Pacific. A weakening ofthose winds therefore causes someof the warm water to start flowingback eastward, thus expanding theregion of high surface temperatures.The difference in temperaturebetween the eastern and the west-ern Pacific, which drives the winds,is now smaller, so that the windsbecome weaker. In other words, thechange in ocean temperatures rein-forces the initial weakening of thewind.

Figure 2: A schematic view of LaNina (top) and EI Nino (bottom).During La Nina, intense trade windscause the thermocline to have apronounced slope from east to westso that the equatorial Pacific is coldin the east, warm in the west wheremoist air rises in cumulus towers.The air subsides in the east, aregion with little rainfall except inthe doldrums where the southeastand northeast trades converge.During EI Nino the trades along theequator relax, as does the slope ofthe thermocline when the warmsurface waters flow eastward.

ward to the west so that the coldwater becomes exposed to the sur~face in the east. (See Figure 2.)Hence, from an oceanographic per~spective, the Southern Oscillationcorresponds to a sloshing, back andforth across the Pacific Ocean, ofthe warm surface waters in responseto trade winds that fluctuate fromweak (during El Nino) to strong(during La Nina).

As a result, even more warmwater flows eastward, causing a fur-ther weakening of the winds. Aninitial, cautious retreat by thetrades induces a tentative eastwardstep by the warm surface waters,which hastens the retreat, which inturn emboldens the pursuit. Theinteractions between the ocean andatmosphere amount to an escalat-ing tit-for-tat (a positive feedback)that causes the warm surface watersand humid air to surge across thetropical Pacific. Soon they are hug-ging the shores of Latin America,and £1 Nino has arrived. Once £1Nino is established, the stage is setfor La Nina to make its entrance.This new phase of the Southern

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The state of the ocean atany time is not simply deter,mined by the winds at thattime because the ocean is stilladjusting to and has a memo,ry of earlier winds, a memoryin the form of waves belowthe ocean surface. They prop'agate along the thermocline,the interface that separateswarm surface waters from thecold water at depth, elevatingit in some places, deepeningit in others. These verticaldisplacements of the thermo,cline effect the transitionfrom one phase of theSouthern Oscillation to thenext so that it is of critical -importance to monitor ocean, Figure 3: The array of instruments that monitor oceanic conditions. All theseic conditions in order to measurements are relayed to stations on land, in "real time." The web siteanticipate future develop, (http://www.pmel.noaa.gov/togatao/ftp.html) displays some of the measurements,ments. That is why oceanog, The blue lines show the tracks of commercial ships that deploy instruments whichraphers developed and now ~easure temperature to a depth of a few hundre? meters. The arrows show drift-maintain the measurement Ing buoys which measure temperature and the winds, and whose movementsarray shown in Figure 3. On yield information abou.t surface currents. The yellow dots are tide gauges thatth b . f h . measure sea level which depends on the average temperature of a water column.

e aSiS 0 suc ocearuc mea,.. .. . The red diamonds, representing buoys moored to the ocean floor, show locationssurements, which wer.e avau, where temperature is measured over the upper few hundred meters of the ocean.able on the World Wide Web The red squares indicate where oceanic currents are measured.a few days after the measure,ments had been made, scien,tists were able to anticipate thearrival of the El Nifio of 1997-98months in advance and could alertthe public to the impendingchanges in weather and climate.

therefore concerned with ocean-atmosphere interactions, not onlyin the tropics, but also in higherlatitudes.

that can excite EI Nino very effec-tively, because their surface windshave a spatial structure that coin-cides with those of the SouthernOscillation, are two-week bursts ofwesterly winds that sporadicallyoccur over the far western equatori-al Pacific. Such wind bursts wereinfluential in initiating EI Nino of1997-98. Will the ringing now con-tinue, with another EI Nino mak-ing its appearance in the year 2002?

Evidence that the SouthernOscillation is subject to long-termmodulations, so that it is promi-nent and energetic during somedecades, less so during others, isavailable from coral records thatcover a century or more. One ofthe factors responsible for thismodulation is the time-averageddepth of the equatorial thermo-cline, which depends on exchangesbetween the tropical and the extra-tropical oceans. Current researchon the Southern Oscillation is

mS. George Philander is a profes,sor of geosciences at PrincetonUniversity, where he teachesand does research on the inter,actions between the ocean andthe atmosphere that cause phe,nomena such as El Nino. He isthe author of the recent bookIs the Temperature Rising?The Uncertain Science ofGlobal Warming.

A' bell, to ring, must be

struck, after which itsoscillations last for a while beforedying out. It appears that there areperiods during which the SouthernOscillation "rings" by itself, andperiods when a disturbance (blow)is needed to start it "ringing" again.During the 1980s it seemed to beringing - El Nino of 1982 and of1987 were both followed by LaNina episodes, but El Nino of 1992persisted for several years, as if anew disturbance were needed forthe ocean-atmosphere system tostart ringing again. Disturbances

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