warns that La Niña may be
succeeding El Niño, and calls for research to understand and anticipate both phenomena
'If Winter comes, can Spring be far behind?'
So what are these capricious siblings? El Niño is an anomalous warming of the central and eastern equatorial Pacific Ocean. A natural pool of warm surface water covering thousands of square kilometres of the western Pacific is normally held in place by seasonal easterly trade winds and a favourable horizontal air-pressure structure. But occasionally (approximately every three to four years) it moves eastwards as the wind and pressure patterns are reversed, and accumulates near the coasts of Central and South America. But it is more than an ocean temperature anomaly. It manifests major interactions between the oceans and the atmosphere which extend beyond the equatorial Pacific. For example, the west Indian Ocean was warmer in 1997 than at any time in the last 100 years, contributing to the torrential rainfall that brought flood, death and economic disaster to eastern Africa.
The phenomenon was first recognized off the coast of Peru when warm water replaced the normally nutrient-rich colder upwellings, and so provoked the departure of the economically important anchovy and sardine - and of the fish-eating seabirds whose abundant droppings fed the guano fertilizer industry. This happened at Christmas time and so the phenomenon was called El Niño, the Spanish for 'Christ child'.
La Niña, 'the little girl', is the other side of the coin. The re-establishment of the Pacific trade winds cools the warm water to seasonal norms or, sometimes, makes it abnormally cold. The ocean thermocline moves up, raising cold water to the surface layers. Surface heat is lost to the atmosphere, possibly contributing, in a small way, to global warming.
1997 witnessed unprecedented warming in the equatorial Pacific. Then in May 1998, sea surface temperatures fell 8oC in 30 days, a rate beyond all past records. (In 1988, during the most significant La Niña to date, it took two months for the ocean to cool 4oC). By July, the easterly trade winds had become well established and a tongue of cold water, 2oC below normal, was developing in the central Pacific. This is indicative of an imminent La Niña, although forecasters could not confirm with absolute certainty that the rapidly cooling water would continue to get cold enough for a very strong La Niña event. (Forecasting the magnitude of an event is far more difficult than forecasting that it will occur.) Ocean/atmosphere modellers have limited experience with La Niña; there have only been two cold events since 1970 - in 1973-1975 and 1988-1989 - whereas the better-researched El Niño has occurred seven times in the same period.
El Niño and La Niña events appeared roughly equally until the 1970s and it is suggested that anthropogenic global warming may be bringing more frequent occurrences of El Niño. Examination of coral formation and similar sources confirms that both phenomena have been regular components of the historical climate record. (Some scientists consider only two climatic states; El Niño and non-El Niño, but the majority of researchers accept La Niña as a significant departure from normal, although its associated weather anomalies have a greater similarity to normal climatology than El Niño's.) Although both originate in the Pacific, neither they nor their impact remain entirely local. Their effect on the global atmosphere is so strong that high altitude jet streams are shifted from familiar patterns, and seasonably familiar pressure patterns and associated wind circulations migrate to different locations, changing regional weather. These mechanisms are described as teleconnections. The results, though often differing from event to event in some regions, frequently have enough common features to allow long-term seasonal forecasting of the anomalous weather, permitting strategies to be developed and applied to mitigate them.
The recent El Niño predictably brought extensive drought to Southeast Asia and provided the conditions for the uncontrollable forest fires in Indonesia. It reduced the propensity for hurricanes to menace the Caribbean and the southeastern United States. Conversely, it encouraged winter storms along the California coast and an unusually warm winter in south-central Canada. It is also blamed for unprecedented rainfall in eastern Africa, although it was record high sea-surface temperatures in the western Indian Ocean that provided the direct influence rather than the state of the Pacific Ocean temperature profile.
The effects of La Niña are mostly opposite to those of its sibling and are generally most dramatic during the northern hemisphere winter. Previous occurrences of La Niña have brought torrential rain to Southeast Asia - welcome news for those combating the widespread forest fires of 1998. La Niña has been associated with an active Indian monsoon and it is usually wet in southeastern Africa during the southern hemisphere winter. This year, the monsoon has certainly lived up to expectations, but no unusual rainfall has occurred in southeastern Africa and it is likely that the still abnormally warm Indian Ocean is exhibiting a greater influence. Indeed, one of the more intriguing features of the developing La Niña is how the still lingering remnants of El Niño (as well as other dynamical influences, including those in extra-tropical regions) modify or even mask its likely teleconnections.
Drought in Australia, induced by El Niño, should give way to above-average rainfall, related to La Niña, aiding grain harvests. In Canada, by contrast, La Niña's expected dry summers and bitterly cold winters (cutting temperatures on the prairies to between -30 and -40oC) should respectively reduce grain yields and increase power consumption for heating. While 1997-1998 saw few Atlantic hurricanes, La Niña is likely to spawn a significant number, some of which will make expensive landfalls on the southeast United States and some Caribbean islands. In Brazil, farmers will brace themselves for late frosts, hitting coffee and citrus production. Past experience with La Niña events makes these weather anomalies probable, but not certain. Climate is not stationary, and varies so much between years and decades that occurrences of La Niña and El Niño do not precisely replicate previous ones. There is also the uncertain influence of climate change.
Global warming has probably contributed to reducing the numbers of occurrences of La Niña compared with those of El Niño. Statistical analysis also indicates that the bigger El Niño events of the past 20 years have coincided with recorded global warming, but how much of the El Niño can be attributed to it cannot be determined at present. Nor is it clear whether the tendency for fewer occurrences of La Niña will continue as climate changes, or whether large El Niño events will become more frequent.
What is certain is that there will be future anomalous ocean temperature events in the Pacific and other ocean basins and that the world's weather will respond in unusual, if hopefully predictable, ways. But there are, however, considerable uncertainties about the cause, periodicity and magnitude of El Niño and, particularly, La Niña events and of the seasonal weather anomalies that can be attributed to them.
The weather events of 1997-1998 - and what we have yet to suffer in 1999 as La Niña bites - demonstrate the urgency of devoting time and energy to better understanding these phenomena and to using the information to make social and economic decisions that anticipate future events. For when the issues are understood and the effects can be anticipated with certainty, we may be able to view occurrences of El Niño and La Niña as opportunities rather than - as now - with dread.
Peter Usher has recently retired as Chief of UNEP's Atmosphere Unit