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tropics is the Madden–Julian Oscillation(MJO), a wave-like disturbance in theatmosphere with a period of 30–60 daysthat originates over the Indian Ocean11. Itcould have been that the ocean got a healthykick from the MJO at just the right time tosend it on a course towards record hightemperatures7. The tropical Pacific was pre-conditioned to the onset of an El Niño bythe build-up of excess heat in the westernequatorial Pacific due to stronger thannormal trade winds in 1995–96. However,beginning in late 1996, the MJO was par-ticularly energetic, and several cycles ofthe wave amplified through nonlinearocean–atmosphere interactions as theypassed over the western Pacific. This set inmotion a series of positive feedbacksbetween the ocean and the atmospherewhich reinforced initial MJO-inducedwarming.

Another possibility is that the ENSOcycle may be interacting with the PacificDecadal Oscillation (PDO) — which, as thename implies, is a naturally occurring oscil-lation of the coupled ocean–atmospheresystem in the Pacific basin with a period ofseveral decades12. In association with thePDO, sea-surface temperatures have gener-ally been higher in the tropical Pacific fromthe mid-1970s. Since then, there have beenmore El Niños than La Niñas, the early1990s was a period of extended warmth inthe tropical Pacific, and two super El Niñoshave occurred. The PDO may be one of the

reasons for the observed decadal modula-tion of the ENSO cycle, because it affectsthe background conditions on which ENSOevents develop. From that perspective, thestrength of the 1997–98 El Niño may bebut one manifestation of a linkage betweeninterannual and decadal climate variationsin the Pacific.

Global warming trends are yet anotherpossible influence on the ENSO cycle. Thewarmest years on record were, in order,1998 and 1997. The 1997–98 El Niñocontributed in part to these record highs,because global mean temperatures generallyrise a few tenths of a degree Celsius fol-lowing the peak El Niño warming as thetropical Pacific loses heat to the overlyingatmosphere13,14. Underlying these extremetemperatures, however, is a century-longwarming trend that may well be due toanthropogenic greenhouse-gas warming15.

Some computer models suggest thatglobal warming may be slowly heating upthe eastern equatorial Pacific Ocean, asobserved over the past 25 years16. Otherspropose that ENSO events may be strongeror more frequent in a warmer climate17.The superposition of ENSO variations onincreased warming due to CO2 and a warmphase of the PDO could produce tempera-ture fluctuations like those seen in the equa-torial Pacific since the mid-1970s, includingthe extreme temperatures associated withthe 1997–98 El Nino18. But computer mod-els used in global change studies are limited

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Figure 1 Taking the pulse of El Niño. The 1997–98 event was followed through the El Niño/SouthernOscillation (ENSO) Observing System, which was set up to monitor and predict ENSO variations.The Earth-based components, shown here, largely relay data in real time via satellites. The maincomponents are a volunteer ship programme (blue tracks); an island and coastal tide-gauge network(cream); and systems of drifting (orange arrows) and moored buoys (red). Complementing thisnetwork are satellites that provide data from space with near-global coverage. They include theUS/French TOPEX/Poseidon mission; the European Space Agency Earth Remote Sensing satellites;US Department of Defense satellites; and NOAA’s polar-orbiting weather satellites. Taken together,this ensemble of instrumentation delivers data on surface and subsurface temperature, wind speedand direction, sea level, and current velocity. The ENSO Observing System was completed in 1994 atthe end of the ten-year international Tropical Ocean Global Atmosphere programme. It is now beingcontinued in support of operational climate forecasting as well as research on ENSO dynamics.

100 YEARS AGOWhat constitutes the natural prey of thelion in his wild state is, I believe, adisputed point. The majority of people,probably, are of opinion that he isextremely fastidious in his tastes; others,again, assert that he will eat almostanything. Certainly, it is only reasonableto suppose that a lion sufficiently underthe impulse of hunger will eat “almostanything”! Years ago I was present onmore than one occasion when animateddiscussions on this point took placebetween two notable Africanecclesiastics — both since dead — BishopSmythies and Archdeacon Maples (hewas then), both of whom had travelled agood deal in Africa — Maples moreespecially — and had seen something ofthe habits of lions. Bishop Smythiesdefended the former theory; ArchdeaconMaples — a most talented andentertaining man — the latter, saying hehad known instances of lions killingporcupines, and adding that he believedthe porcupine to be specially endowedwith the power to propel his quills intohis assailant when so attacked. At thisjuncture, Bishop Smythies generally lostpatience and declined to continue theargument. Had Bishop Smythies lived, itwould have interested him … to knowthat in March last, at the Salt Stream,two days’ march N. W. of Kibwezi, I shota fine old lion in whose left fore-pawwere deeply buried the tips of threeporcupine quills.From Nature 13 April 1899.

50 YEARS AGOIt is announced that Prof. J. W. McBain,who has recently retired from the chair ofphysical chemistry at Leland StanfordUniversity, California, has been appointedthe first director of the National Chemicallaboratory of India. It is difficult to thinkof a better choice. While his manyEnglish friends have not seen so much ofhim since he left the University of Bristol,yet they have been able to follow hissteady development of the concept of themicellar structure of colloidalelectrolytes, a chapter in physicalchemistry which is particularly his own.… We must recollect that we are alsoindebted to him for coining the word‘sorption’, thus directing attention to thecomplexity of the interactions between agas and a solid.From Nature 16 April 1949.