El Niño, La Niña and freshwater resources
MICHAEL H. GLANTZ
explains how El Niño
water resource planning
Ever since the beginning of human settlements, people have sought to understand and predict next season's or next year's weather. Will the growing season have good rains for food production? Will there be enough rain to make grassland for pasture or enough river flow for irrigation?
Decision makers in water resources, agriculture, energy and health could all benefit from advance warnings of weather conditions. That is why there is so much interest in El Niño, the name given by the Peruvians to the air-sea interaction in the Pacific Ocean around the equator.
El Niño is usually described as an anomaly, an unusual or abnormal interaction between air and sea in the equatorial Pacific that is not part of
the normal climate system. In fact, El Niño is a normal and expected
part of it. While we can talk of sea surface temperatures in the eastern and central Pacific Ocean departing from a mathematically identified average condition, we should not view an El Niño-related departure from the average as being abnormal and unexpected.
El Niño (a warm event), like its counterpart La Niña (a cold event), is an integral part of the global climate system. It is a recurrent phenomenon with an average return period of four and a half years, but can recur as little as two or as much as ten years apart. Such events have occurred for millennia, and can be expected to continue to occur in the future. Indeed, to go through a decade or two without an El Niño would be truly unusual. Since El Niño does not represent unusual global climate behaviour, we can attempt to prepare for it.
There have been El Niño events throughout the 20th century. However, it was the 1982-1983 event that first sparked serious interest. Researchers and policy makers, in particular in Australia, Brazil, Chile, Ecuador, Peru, Southern Africa and the United States of America, became interested in developing an improved understanding of El Niño and its impact on weather systems.
The onset of a strong El Niño in early 1997 intensified interest, especially in the potential value of planning to mitigate its direct effects. However, we still need a better understanding of the ways El Niño events can form. Nor have we seen all of the ways that they can affect society and ecosystems.
Policy makers, governments and scientists are increasingly focusing on
El Niño as a tool for forecasting climatic conditions months in advance, particularly for identifying the potential for droughts or floods. Droughts can have devastating impacts on communities, affecting food and energy production and, in arid zones, the very survival of humans and animals. Floods can destroy property and kill people. In January 1998, in northern Peru and Kenya, for example, heavy rain had serious consequences.
Timely warning of such extreme events can enable civil defence experts to plan for evacuation of at-risk populations. If reservoir managers are given a reliable forecast of seasonally above average rainfall, they can lower reservoir water levels to make storage room for the expected excess water.
As an event, El Niño grabs media and research attention. But it is only the warm phase of a larger cycle that also includes a cold phase - La Niña. Extreme weather events around the globe have also been associated with La Niña, though many policy makers are unaware of its existence. Some scientists suggest that, in various regions, La Niña-related extreme events are the opposite of those
related to El Niño. For example, in southern Africa drought accompanies El Niño, while rain is associated with La Niña. Researchers are just beginning to realize that they should focus more attention on the cold part of the cycle.
But El Niño is also associated with flooding which can have several adverse effects: it can disrupt farming along river borders; drinking water can become contaminated during excessive rainfall and flooding; and local water delivery systems can be damaged or destroyed, with negative consequences for human health.
Effects on water resources
A number of studies are beginning to correlate El Niño events and changes in water resources. Such studies are currently being undertaken in Australia, Costa Rica, Japan, North America, Panama, Thailand and Viet Nam.
One study, which emerged from a regional training course organized
by the new International Research Institute (for climate prediction) together with Argentina's Facultad de Ingeniería y Ciencias Hídricas of the Universidad del Litoral, has looked at practical methods of seasonal hydrological forecasting and focuses on El Niño's impact
on water resources in Central and South America.
A second study has tried to identify a relationship between El Niño events and river flows in the Iguazu River Basin in Argentina. Such information could be important for the management of the Iguazu River hydroelectric dams.
Another report looks at the influence of El Niño on the variability of river flow in the Brazilian state of Ceará, where rivers are intermittently dry, often for long periods. Knowing that an El Niño is due can enable managers of reservoirs supporting irrigation schemes in the region to hedge their decisions if the El Niño effect is likely, for example, to bring drought. It could perhaps have helped planners in southern Africa during the 1982-1983 El Niño-induced drought, when lack of water led to the closure of factories, with workers laid off for many months.
That same El Niño brought severe flooding to northern Peru and southern Ecuador. There had been no warning. More recently, in November 1997, the northern Peruvian city of Piura, which typically receives about 6 millimetres of rain in November, received almost 600 millimetres of rain. But this time the Government of Peru had been warned of the El Niño and had been able to mobilize engineering and civil defence resources to mitigate the impacts of flooding in the north.
Not every weather anomaly throughout the world that occurs during an El Niño year is caused by the El Niño itself, though in 1997 there was an increasing tendency to blame it for everything. For example, floods in the midwest United States of America during 1993 were blamed on that year's El Niño, even though the evidence for this conclusion was very weak. On the other hand, evidence did support the conclusion that the severe drought in the same region in the summer of 1988 was caused by La Niña.
Extreme record-setting weather events are occurring around the
globe, even in non-El Niño years. Only some parts of the globe are directly influenced and even these areas are not influenced in the same way by each El Niño.
Teleconnections, or long distance effects, between El Niño and regional climate anomalies have been identified both statistically and through observations of direct links between warm surface water in the equatorial Pacific and distant anomalies such as drought in northeast Brazil or Ethiopia.
El Niño can have positive and negative aspects at the same time.
For example, during an El Niño the number of hurricanes and tropical storms along the Atlantic and Gulf coasts and in the Caribbean is noticeably reduced. During the 1997 El Niño, the hurricane season was almost non-existent and no devastating storm occurred. For many this was good news, but not for everyone. The absence of tropical storms also reduces the amount of freshwater that tropical islands receive to carry them through the ensuing year.
The impact of global warming on El Niño is not yet known, though there are hundreds of studies on the possible effects of global warming on the global hydrological cycle and what these effects might mean for regional and local hydrological cycles. Some researchers suggest that dry areas will become drier with global warming and wet areas wetter, others have suggested the opposite.
Knowing when an El Niño will strike, however, is different from forecasting the effect it will have in different areas. Although some kinds of impacts tend to recur in the same places, each El Niño can cause a different set of impacts (droughts, floods, fires). Predicting its arrival is likely to prove relatively easy compared with forecasting what will actually happen to disparate locations around the globe.
But forecasting can provide useful insights. Certain teleconnections do have a tendency to occur when an El Niño has been forecast. In areas where the link seems reliable, authorities can begin to prepare for seasonal changes in water resources.
While research on El Niño has some way to go before scientists can forecast its onset, magnitude and duration with a high degree of reliability, the potential value of information (including but not limited to forecasts) to those involved in water resource management is clear.
Dr. Michael H. Glantz is a Senior Scientist at the National Center for Atmospheric Research, Boulder, Colorado, United States of America.