INTRODUCTION
Weather and climate variability
Every year around the world, there are extreme climate-related issues with droughts occurring in some places and floods in others. The summer of 1988 witnessed a severe drought on the agricultural heartland of North America, meanwhile an extremely low stream flow in the basin of the Mississippi River. A few years later, in the summer of 1993, a period of rainstorm led to major flooding along the upper Mississippi and lower Missouri River and many of their tributaries in the United States (Figure 1.1).
Figure 1.1 Great flood along Mississippi River, USA, 1993. Source: NASA (2005).
In early 1990s, newspaper headlines announced that drought-related food shortages in southern Africa put about 80 million Africans at risk of famine. In August 1992, Hurricane Andrew destroyed southern Florida leaving an estimated US$30 billion in destruction. In early 1995, extreme flooding occurred in western Europe, that shaking the confidence of countries such as the Netherlands in their ability to prevent natural catastrophes, and challenging their false belief that scientific and technological developments had buffered their societies from the consequences of such periods of extremely heavy rainfall (Annamalai, 2007).
Despite the fact that climate fluctuates on seasonal, annual, decadal, century and even longer time scales, in some years there are many extreme meteorological events and resulting in societal problems, such as droughts, floods, fires, frosts, ice storms or blizzards. One such period was 1972-73, when severe droughts occurred in widely dispersed locations such as Australia and Indonesia, Brazil and Central America, India and in parts of sub-Saharan Africa, at the same time, heavy flooding occurred in Kenya, southern Brazil, and parts of Ecuador and Peru (Roberts, 2009). At the time it was suggested that some of these widely dispersed climatic extremes might have a common geographic origin in changes in sea surface temperatures in the tropical Pacific Ocean (El Niño or EN), and changes in atmospheric pressure at sea level across the Pacific basin (Southern Oscillation or SO). These combined changes have come to be commonly referred to as El Niño events in the popular media and as ENSO (El Niño Southern Oscillation) events in much of the scientific literature.
Very briefly, an El Niño event can be described as the appearance from time to time of warm sea surface water in the central and eastern Pacific Ocean near the equator. Folklore suggests that the term "El Niño" was used by Peruvian sailors and fishermen as a label for the annual appearance of warm water along the western coast of their country by December of each year. In some years, the warming along the coast did not dissipate within few months but lingered for more than a year (Kumar, 1997). This was also called "El Niño". In recent decades, the term "El Niño" has been broadened to include all kinds of anomalous sea surface warming in the equatorial Pacific. Scientists now believe that El Niño events are associated with many anomalous weather extremes around the globe.
ENSO and worldwide climate
The associations or linkages between El Niño events and unusual changes (called anomalies) in normal climate patterns around the globe have been referred to as "Teleconnections" (Bell, 2009). These include known, as well as perceived, connections between ElNiño events and changes in distant weather or climate-related processes. For example, there appears to be an association between El Niño events and droughts in various parts of the globe: north Australia, southeastern Africa, northeast Brazil, parts of India, central America and so forth. There also appear to be linkages between El Niño events and a reduced number of tropical hurricanes occurring in a given tear along the east coast of the USA as well as in the locations of tropical cyclones off the east coast of Australia, where they tend to shift equatorward by several hundred kilometers.
THE SCIENTIFIC BACKGROUND OF ENSO
The importance of upwellings
Upwelling has a profound effect on ocean temperatures and life along coastlines. As the ocean temperatures decrease with depth, therefore, upwelling brings cold water to the surface, and this water is rich in nutrient. These nutrients are an important food source for marine organisms living near the surface. These organisms, in turn, area a source of food for fish and birds, and millions of people across the world depend upon the abundance of life in and near regions of upwelling ceases because of a change in wind patterns, the result can be catastrophic for coastal regions. One example of this, which turns out to have implications for weather and climate worldwide, is El Niño.
AIMS & OBJECTIVES
Aims: To find out the potential environmental and economic impacts of El Niño Southern Oscillation
Objectives: 1. Assessment of historical impact of ENSO
2. Assessment of current situation
3. Potential impacts of future ENSO events
Impacts associated with the 1982-83 El Niño
Most of the major weather anomalies occurring in 1982 and 1983 around the world, especially droughts and floods in the tropics, were linked by one observer or another to the occurrence of an El Niño. Several articles, maps, and charts relating to El Niño appeared in the popular press, suggesting the extent of the worldwide, continent-wide, national, and local impacts of this El Niño. Caution must be used, however, in attributing any particular anomaly or impact to a specific El Niño. Furthermore, the severity of societal impacts will vary according to the level of societal vulnerability to such extremes. Climate-related anomalies can also result from a variety of local and regional conditions, even in the absence of El Niño events. The following examples of the alleged societal impacts of the 1982-83 El Niño are taken from newspaper reports:
Indonesia was plagued with severe drought, resulting in reduced agricultural output (especially rice), famine, malnutrition, disease, and hundreds of deaths. This drought came at a bad time, in the sense that this country had been making great strides toward self-sufficiency in food production. In the few years immediately preceding the 1982-83 El Niño, Indonesia had begun to emerge as a rice exporter. This drought, however, coupled with worldwide recession, huge foreign debts, and declining oil revenues, set back Indonesia's economic development goals for the near term.
In 1982-83, Australia was in the midst of its worst drought this century up to that time. Agricultural and livestock losses, along with widespread bush fires mainly in the southeastern part of the country, resulted in billions of dollars of lost revenue. The El Niño exacerbated this situation. An Australian journalist wrote that the drought was not just a rural catastrophes, it was a national disaster.
The eastern part of the USA was favorably affected by its warmest winter in 25 years and the fewest hurricanes of the century up to this date. According to an estimate by the National Oceanic and Atmospheric Administration, energy savings were on the order of US $500 million. (The opposite was the case, however, during the cold winter that accompanied the 1976-77 El Niño.) Also in 1982-83, the USA was adversely affected by devastating coastal storms and mudslides along the southern California coast, flooding in the states, reducing corn and soybean production. Salmon harvests along the United States Pacific Northwest coast were also down sharply due to reduced coastal upwelling and a general warming of the ocean's water, which pushed salmon populations further north into Canadian waters and into the hands of Canadian fishermen.
South America experienced many and varied impacts. In addition to the highly publicized damage to infrastructure such as roads, railroads, and bridges, and agricultural production in Peru and Ecuador as a result of heavy flooding during the 1982-83 El Niño, there were severe droughts in southern Peru and Bolivia. A major drought continued in Northeast Brazil, adversely affecting food production, human health, and the environment. The drought prompted migration out of the region into the Amazon and into the already crowded cities along the coast and to the south. There were also destructive floods in southern Brazil, northern Argentina, and Paraguay.
Large expanses of Africa were affected by drought. For example, the West African Sahel was, once again, plagued by a major drought. Although the human and livestock deaths resulting from this drought appeared to be lower than those that occurred during the 1972-73 El Niño, the situation with food production was considered extremely poor. The view that the Sahel was in the midst of a long term trend of below average rainfall that began in 1968 gained some credibility.
Southern Africa has witnessed some of its worst droughts, including that of 1982-83, during this century. For example, in 1983 the Republic of South Africa, a major grain producer in the region, was forced to import about 1.5 million tonnes of corn from the USA to replace what was lost in their drought. Zimbabwe, a regional supplier of food, was also devastated by drought and was forced to appeal for food assistance from the international community. Likewise Botswana, Mozambique, Angola, Lesotho, and Zambia, and the so-called Black National Homelands in the Republic of South Africa had their economies devastated by the drought of 1982-83.
In addition to these impacts, the El Niño of 1982-83 was blamed for droughts in Sri Lanka, the Philippines, southern India, Mexico, and even Hawaii, along with severe, unseasonal typhoons in French Polynesia and Hawaii. It was also credited with having a role in suppressing hurricane activity along the Atlantic seaboard. In 1983, many of these events were record-setting extremes: the worst typhoon, the most intense rainfall, the warmest winter, the longest drought, and the fewest hurricanes making landfall on the eastern USA, all occurred in this year.
El Niño has also been associated with indirect societal and environmental effects. However, indirect effects are even more difficult to attribute to an El Niño, as they could be the result of other causes. In 1982-83, these effects tool the form of dust storms and bush fires in Australia, the Côte d'lvoire, and Ghana. In the USA, the 1982-83 event was blamed for such health effects as encephalitis outbreaks in the East (the result of a warm, wet spring providing the proper environment for mosquitos), and increase in rattle-snake bites in Montana (hot, dry conditions at higher elevations caused mice to search for food and water at more densely populated lower elevations; the rattlesnakes followed the mice), a record increase in the number of bubonic plague cases in New Mexico (as a result of a cool, wet spring that created favorable conditions for flea-bearing rodents), an increase in shark attacks off the coast of Oregon (because they followed the unseasonably warm sea temperatures). Even an increase in the incidence of spinal injuries along California's coast was blamed on El Niño (as a result of swimmers and surfers being unaware that the floor of the ocean along the coast gad bee changed as a result of the violent wave action that accompanied coastal storms).
Positive impacts
There has been an overwhelming tendency to focus on the adverse impacts of El Niño on human activities. However, with regional shifts in temperature and precipitation, one can expect that some regions as well as some human activities will benefit from those shifts.
The impacts of La Niña
Researchers have created a rule is that the impact of La Niña are generally the opposite to those of El Niño. For example, droughts tend to accompany El Niño events in Australia, Indonesia and the Philippines, whereas heavy rains and flooding tend to accompany La Niña in these locations. Southern Africa tends to be drought-plagued during El Niño, but very wet during La Niña episodes. Figure 5 shows a correspondence between peaks in Multivariate ENSO Index (MEI) and annual mean rainfall at Australia.
Figure 5. Multivariate ENSO Index (MEI) and annual mean rainfall at Australia. Positive MEI value (>0.5) indicates an El Niño event, negative value (<-0.5) indicates a La Niña event. Source: Australian Government Bureau of Meteorology (2010).
The actual worldwide impacts of cold events depend on the intensity of particular La Niña. Some researchers have suggested that the 1988-89 La Niña was a strong one (Moon et al, 2003; Quan et al, 2004; Wheeler, 2008), while others have suggested that the 1988-89 La Niña was only moderate (Wang et al, 2007). Much more research needs to be done on how best to classify the intensity of La Niña events (or El Niña events). The composite maps shown in Figure 5.7a and b are produced by National Oceanic and Atmospheric Administration (NOAA), provide a statistically based generalization of the potential impacts of ENSO cold extremes. By noting the months in which impacts are likely occur.
Which extremes can be blamed on El Niño or La Niña?
Societal contributions to atmospheric greenhouse gases through the burning of fossil fuels (coal, oil and natural gas, etc.), tropical deforestation and the use of fertilizer (NOx) and refrigerants (chlorofluorocarbons or CFCs), have been linked to a global warming of the atmosphere. In the summer of 1988, a major drought took place in the Midwest United States, which has since been referred to as the most expensive natural disaster in US history. Some researchers quickly blamed the severity of that drought on human-induced global warming, for example, James Hansen (1988) suggested that the effects of global warming on regional and local climates world become more frequent as well as more visible in the near future. The 1988 drought, he argued, was consistent with what one might expect from gobal warming. At that time, however, a hypothesis was proposed by atmospheric scientist Kevin Trenberth: the Midwest drought was a result of La Niña conditions in the equatorial Pacific (Linden, 1988).
Was the 1988 Midwest drought really produced by prevailing La Niña conditions thousands of kilometers away? If that were the case, then there would have been a good change that a major Midwest drought would accompany the 1998-2000 La Niña. However, can such a conclusion be made with confidence? While there is some evidence that a La Niña summer in North America is likely to be hotter and drier than normal, there is not enough hard evidence to make that fairly specific geographic teleconnection with certainty.
Care must be used in identifying previous La Niña (or El Niño) events that are to be used as analog years. Because there have been relatively few La Niña events in the past 60 years, we do not know the full range of ways that La Niña might affect regional climates indifferent parts of the world. Identifying a specific La Niña year from the historical record that might be considered to be similar to an impending La Niña year raises expectations about the increased likelihood of a repeat of the societal impacts that occurred during those previous years. If the selection of an analog year is wrong, however, then those expectations about potential damages world have been false expectations, because these damages are not likely to occur.
Identifying El Niño/La Niña years is very important for those who look for El Niño/La Niña analogs to forecast with some degree of reliability the impacts that might occur and to develop strategies to cope with the societal impacts of the ENSO cycle. There are long time series of sea surface temperature, sea level pressure, thermocline depth and outgoing long-wave radiation to identify the ENSO warm or cold events.
