MICHAEL H. GLANTZ
examines the El Niño phenomenon and discusses
suggestions that recent events may be related
to global warming
El Niño events disrupt human activities around the globe more than any other climate-related process apart from the natural flow of the
seasons. Every four and a half years, on average, anomalously warm water appears along the equator in the eastern and central Pacific Ocean and tends
to cause unexpected - and undesirable - shifts in weather systems both in tropical regions and, to a lesser extent, in North America and elsewhere in
The name El Niño (EN) was originally used to describe a seasonal warming of sea surface waters off the western coast of South America, centred
on Peru. But meanwhile the Southern Oscillation (SO) takes place in the atmosphere on the other side of the Pacific. This atmospheric phenomenon is a
seesaw-like change in sea-level pressure across the Pacific basin: when the pressure is low in the region round Darwin, Australia, it is usually high
in the region of Tahiti. These two processes (EN+SO) together produce ENSO, a Pacific basinwide phenomenon that disrupts weather around the globe.
Although El Niño referred originally to a local phenomenon and ENSO to the basinwide event, many people, including scientists, now use these
These events can be weak, moderate, strong or - rarely - extraordinary. A weak event covers the eastern portion of the equatorial Pacific and has sea
surface temperatures 1 or 2oC above average. A strong one covers a large part of the equatorial Pacific and sea surface temperatures increase by 3 or
4oC or more.
Normally, warm ocean water builds up in the western part of the equatorial Pacific Ocean near Australia, the Philippines and Indonesia. This build-up
results from strong westward flowing winds blowing across the Ocean from the Peruvian coast. Every so often these winds weaken - and sometimes reverse
and blow towards the east - which allows a warm pool of water to form in the central and eastern part of the Ocean.
As sea surface temperatures rise, evaporation from the warm water increases, leading to cloud formation and, ultimately, rainfall. The rain follows
the warm water. So areas that are normally wet - such as Indonesia, the Philippines and northeast Australia - become plagued with drought, while areas
that are normally dry - such as the west coasts of Peru and Chile and the Galapagos Islands - become excessively wet. The resulting shifts in high and
low pressure systems around the globe can frequently bring drought to southern Africa, Ethiopia, India, northeast Brazil, southern Peru and Bolivia,
and Central America.
During an El Niño, there are usually warm dry winters in the Pacific northwest of North America, from Oregon to British Columbia, and in the
Canadian Prairie provinces. Meanwhile the northeast of the continent has mild wet winters, and the states that border the Gulf of Mexico from Texas to
Florida usually get wet and cool ones. There is also an apparent sharp reduction in the number of hurricanes that form in the tropical Atlantic.
The last extraordinary event took place in 1982-1983, covering much of the tropical Pacific with relatively high sea surface temperatures. Its impacts
around the globe were so devastating that popular magazines like Reader's Digest and National Geographic felt compelled to publish
stories about it for the first time.
This, the biggest El Niño in a century, took the entire research community by surprise, at a time when it felt it had come to understand the
process. It was bigger than expected, its impacts were broader than expected and it started at an unexpected time of the year. Its impacts on climate
around the globe drew the attention of researchers and policy makers to the fact that forecasting El Niño events several months in advance
could provide government planners and individuals with advanced warning of possible climate-related anomalies.
Research on El Niño was elevated to higher levels: governments provided more funding and more researchers took an interest. The controversial
El Niño event(s) of the early 1990s captured the passive interest of potential users of the forecasts. The most recent event (starting in 1997)
has been compared to the devastating one of 1982-1983, and so has prompted decision makers in many sectors of society to try to identify ways to use
such climate-related information in their management activities. In many parts of the world, it seems, El Niño information has come of age.
Originally El Niño was mainly of interest to local farmers and fishermen in Peru. During events the numbers of anchovies fell heavily. Millions
of fish-eating birds used to die, their carcasses washing up along the shore, and the amount of the birds' guano, an excellent fertilizer, would be
sharply reduced. Interest broadened in the early 1970s, after the Peruvian anchovy fishing industry, then the world's biggest fishery in terms of the
weight of landings, collapsed, partly through El Niño and partly through overfishing, with disruptive impacts on international trade.
Today, El Niño is under great scientific scrutiny. It is monitored daily by satellites, as well as by moored buoys and 'ships of opportunity'
that happen to be crossing the Pacific. Weather satellites keep constant vigil, monitoring sea-level changes, ocean surface temperatures, cloud
formation, etc. The Internet has scores of web sites devoted to the phenomenon.
Considerable effort has been devoted during the past 15 years to improving understanding of the science of El Niño so as to be able to forecast
its onset several months to a year in advance. An international global research programme from 1985 to 1994 - Tropical Ocean and Global Atmosphere -
greatly improved monitoring capabilities. There have even been a few forecasting successes - for example, of the 1986-1987 and 1991-1992 events.
Unfortunately, there have also been recent forecasting failures, such as the unexpected return of El Niño in 1993.
A considerable body of usable information about El Niño already exists for several parts of the globe and it could enable decision makers in
agriculture, fisheries, energy production and public safety to make somewhat better informed decisions. Unfortunately many are not yet aware of the
potential value of using historical El Niño information in their decision processes.
Not every region of the globe can identify a direct influence on its local and regional weather and climate, but that should not deter the search for
an improved understanding of El Niño. The influence of El Niño in East Africa may remain unclear, but Kenya's coffee trade is affected
by what happens to the crops of competitors like Colombia, Brazil and Indonesia, which are directly impacted by the phenomenon. The same logic applies
to other commodities traded in the world marketplace, such as cocoa, sugar, palm-oil and fishmeal. Information about El Niño and its impacts on
distant regions should be viewed as a valuable commodity. Forewarned is forearmed.
Expecting the unexpected
Scientists and the public alike have been surprised by just about every El Niño event. Researchers have been studying El Niño as a
basinwide phenomenon only since the mid-1970s and so we have not as yet witnessed all the ways it can form and manifest itself. Have we yet seen the
biggest event, or witnessed all possible combinations of impacts that might occur during an El Niño? Just when scientists feel they understand
it well enough to produce reliable forecasts many months in advance of its onset, it takes a different form. So we must expect El Niño-related
surprises to continue for a long time yet.
Most recently, two researchers at the National Center for Atmospheric Research in Boulder, Colorado - Kevin Trenberth and Timothy Hoar - have
suggested that El Niño's 'strange' recent behaviour between 1991 and 1995 results from the influence of global warming on the Pacific. They
concluded: 'Both the recent trend for more ENSO events since 1976 and the prolonged 1990-1995 ENSO event are unexpected given the previous record,
with a probability of occurrence about once in 2,000 years. This opens up the possibility that the ENSO changes may be partly caused by the observed
increase in greenhouse gases.' However, other researchers have challenged their conclusions and the issue remains fertile ground for scientific debate
In November 1991, in an attempt to separate speculation from science, UNEP's World Climate Impact Assessment and Response Strategies Programme
sponsored what was perhaps the first workshop to focus on the El Niño/climate change issue.
Participants representing various perspectives noted that El Niño would probably continue to occur with global warming and that computer models
suggest that events could become more intense. They also said that present knowledge suggests that the frequency of El Niño events is less
likely to change: there are likely to be long and short El Niño events in the future, as in the past.
Australia's Bureau of Meteorology Research Centre held a similar workshop in 1993 addressing similar issues and participants came up with similar
responses. But it is important to note that these answers are speculative at present. More research is needed not just on El Niño and its
impacts but on global warming's potential influence on it.
Dr. Michael H. Glantz is a Senior Scientist in the Environmental and Societal Impact Group, National Center for Atmospheric Research, Colorado. He
is the author of Currents of Change: El Niño's Impact on Climate and Society, published by Cambridge University Press in 1996.