The waters along the coasts of Peru, Ecuador and northern Chile are normally some of the most productive in the world. Nutrient-rich currents rising from the sea-bed support vast populations of microscopic plankton, which in turn provide food for fish and other wildlife. The system is delicately balanced, with dead plankton falling to the sea floor and decaying to provide the nutrients for future upwellings. During an El Niño, plankton populations drop dramatically, disrupting the food chain and leading to starvation among marine life forms. Over the past 40 years, two of the major industries of Chile and Peru, fertilizer and fishing have been directly affected by these changes.
In the first half of this century, fertilizer made from the droppings of guano sea birds living on off shore islands was central to the Peruvian economy. The sea birds fed on anchoveta fish. Guano fertilizer was exported in bulk, and used domestically to improve crop yields. The changes I sea conditions caused by the 1957-58 El Niño led to a massive drop in anchoveta numbers. Millions of guano birds starved to death and their number dropped from 30 million to 16 million. A few years later, Peru began to expand its anchoveta fishing industry. Competition from fishermen for the anchoveta and stresses caused by El Niño caused a further decline in the number of guano birds and ended the dominance of the guano fertilizer industry in Peru.
The fishing industry continued to grow and, by the late 1960s, Peru had become the world's leading fishing nation by weight of catch, with anchoveta making up the bulk. By 1970, 1400 modern boats were bringing in 14 million tonnes of anchoveta a year, one-fifth of the world fish catch. However, the 1972-73 El Niño devastated the Peruvian anchoveta fishing industry and brought the El Niño phenomenon onto the world stage for the first time. During the onset of the 1972-73 El Niño, the warming of the Pacific coastal waters drove large numbers of anchoveta close to the shore
Interest in the impacts of the El Niño phenomenon, as such, goes back at least to the middle decades of the 1800s. At that time, its adverse effects on the guano birds (i.e. seabirds such as cormorants, gannets and pelicans) and on guano production (bird droppings used as a fertilizer for agriculture) had already been observed. Guano deposits that had built up over millennia were "mined" along the Peruvian coast throughout the second half of the nineteenth century, despite the decline in European demand for it. By 1900, Peruvian authorities were alarmed that this valuable export commodity (guano) was being mined at an alarming and unsustainable rate, that it was being mined at a rate faster than it was being produced. As a result, in the first decade of the twentieth century, the Peruvian government established a Guano Administration Company to oversee the protection of the guano bird population and to control the mining of guano (Figure 3.1 and 3.2).
Guano birds along the Peruvian coast live off of the fish populations that dwell near the ocean's surface (these are called pelagic fish), primarily the anchoveta, which is a fish of anchovy family. The cold oceanic conditions in Peru's coastal waters were usually optimal for anchoveta populations but are occasionally perturbed by El Niño. El Niño-related changes in physical, biological and social conditions can be devastating for the anchoveta, and in turn to seabird populations. Some of these processes are described briefly in the following paragraphs.
Physical setting
The rotation of the earth, combined with the winds that tend to blow toward the equator and offshore up the west coast of South America, pushes coastal surface water westward away from the continent and toward the central Pacific. As a result, cold water is drawn up from the ocean's depths to replace the warmer displaced surface water. This process is referred to as coastal upwelling. Coastal upwelling processes create regions in the ocean that are biologically highly productive oases (Figure 3.3).
The upwelling of deep cold ocean water brings a variety of chemicals in to the sumlit later near the ocaen's surface. They are converted through photosynthesis to nutrients for phytoplankton which are at the bottom of the marine food chain. The plants are eaten by zooplankton and fish populations, which in turn are consumed by guano birds.
The coastal upwelling phenomenon usually occurs along the western coasts of continents in both the Northern and Southern Hemispheres. Coastal upwelling regions from around the globe make up about 0.1% or the ocean's surface area but provide around 40% of all the commercial fish captured globally. The cold water that upwells along the coast tends to suppress rain-producing processes in the atmosphere and as a result, upwelling regions are usually found adjacent to coastal deserts. So, although coastal deserts appear to be harsh and barren environments from a societal perspective, the productivity that they did not get on land they found in their adjacent coastal waters. The location of major coastal upwelling regions are shown in Figure 3.4.
Biological setting
Biological productivity in the marine environment can be measured in terms of fixation of carbon by photosynthesis. The productivity of an upwelling ecosystem is measured in part by the amount of nutrients that is brought into the sunlit layer near the surface. According to David Cushing (1982),
"Each upwelling region moves poleward as spring gives way to summer and each is two or three hundred kilometers broad in biological terms, even if the prominent physical processes are confined to a band within about 50 km of the coast." (Cushing, 1982)
Of these highly productive marine ecosystems, Peru's is considered to be one of the best not only in terms of the rate of fixation of carbon but of tonnage of fish caught as well (mostly anchoveta during non El Niño periods). Before 1960, Peru was not noted for its commercial fishing activities, even though its commercial fish catches had doubled each year in the 1950s. However, from the mid-1960s to the first couple of years of the 1970s, it had become the world's number one fishing nation.
When El Niño events occur, coastal upwelling processes in the eastern Pacific are altered to such an extent that behavior within and among species becomes modified in major ways. Anchoveta, for example, disperse and migrate as well as dwell deeper in the ocean. Patterns of reproduction and migration change for the various fish species, with some reproducing less in the temporarily altered marine environment. Warm water species temporarily invade the waters of the western equatorial Pacific. Some fish populations such as sardines fare well in the new but temporary warm surface water environment. More specific to Peruvian interests, the standing stock of anchoveta becomes reduced for a variety of reasons, including higher mortality and lower fertility.
As a result of changes in the behavior of the anchoveta, the fish population becomes much less accessible to the guano birds, which causes the starvation and death of hundreds of thousands to millions of birds, depending on the magnitude and intensity of the particular El Niño episode. Figure 3.6 provides an example of the adverse impacts on Peruvian guano birds of the combination of El Niño events and heavy commercial fishing pressures over the span of a few decades.
Societal setting
The Guano Administration Company, created in 1909 by the Peruvian government to manage the guano resource, and its main supporters among the Peruvian agricultural elite managed for many decades to block the development of large scale commercial anchoveta fishing ventures. Apparently, they were able to argue successfully within the nation's highest political circles that there were not enough fish in Peru's coastal waters to sustain both a viable guano-mining industry and a viable anchoveta fishing sector. Birds and fishermen would be competing for the same resources in order to "survive". It is important to note that the Peruvian anchoveta captured by humans. They were caught to be processed into fishmeal for use as an animal feed supplement for export primarily to the rapidly expanding North American poultry industry. The anchoveta was also a source of fish oil for Peru's domestic market.
In the early 1950s, however, the enterprisers interested in developing a Peruvian commercial fishery finally convinced politicians at the time to allow them to establish a commercial fishing industry, winning out over those who opposed its development. The arguments of Peruvian investors
CONCLUSION
The primary methods used to forecast the impacts of a La Niña event are (A) computer modeling or (B) quantitative or qualitative analyses of past La Niña events and their environmental and societal impacts. These are the approaches of choice by scientists to identify the possible consequences of an ENSO extreme cold or warm event. However, less scientific ways to get the impacts that a La Niña might bring to a given region include projections based on what is likely not to occur under La Niña or normal tropical Pacific sea surface temperature conditions. One could identify El Niño teleconnections considered to be very reliable and then assume that, in the absence of an El Niño, there would be a much lower chance for those El Niño-related anomalies to occur.
For example, the El Niño-associated extreme drought situations in Indonesia, Papua New Guinea and in Australia or the forest fires in Borneo (Indonesia) are much less likely to occur during a La Niña event. So while one might not be able to forecast what will happen during a La Niña event, one can identify what is less likely to happen in some location during La Niña. Here are some examples:
Northern Brazil is less likely to have a drought
Southern Brazil, Uruguay and Argentina are less likely to receive good rains for crop production.
Southern Africa is not likely to have a severe regional drought
East Africa is not likely to have severe flooding
Indonesia, Philippines, Papua New Guinea and Malaysia are likely to have average to above average rainfall
Indonesia is less likely to suffer from uncontrollable tropical fires
Central Chile is not likely suffer flooding
Strong, nutrient-rich coastal upwelling and arid conditions would reappear along the Peruvian coast
The Atlantic hurricane season will become more active
India's monsoon is less likely to fail
Precipitation in the southern part of China is less likely to be excessive
While the mere mention of the possible onset of a La Niña can spark reactions from decision makers in various countries or corporations where those decision makers believe that there is a strong La Niña impact, it is important to keep in mind that: Firstly, a sharp drop in sea surface temperatures in the tropical Pacific as happened in 1998 does not assure that a strong La Niña will follow; Secondly, a La Niña does not always follow an El Niño; Thirdly, La Niña events vary in intensity (weak, moderate, strong and very strong) and that each level of intensity generates its own set of world-wide teleconnections.
El Niño and the Walker Circulation
During an El Niño event, the Walker Circulation becomes modified in a major way. The westward flowing surface winds across the equatorial Pacific basin weaken and in the western part of the basin they reverse and flow eastward. This enables water in the warm pool in the west to spread eastwaed.as the warm water shifts eastward, the sea level in the east increases. With the slowing down of the westward winds, the surface waters of the central and eastern Pacific become warmer. As this occurs, the thermocline also begins to shift, moving upward toward the ocean's surface in the west and deepening in the central and eastern equatorial Pacific. As the thermocline moves downward along the Peruvian coast, upwelling continues but the water brought up to the surface is warmer and less rich in nutrients (Figure 5.4b, p.70).
Meanwhile, the water in the western equatorial Pacific becomes a few degrees cooler, as the surface and subsurface waters in the central and eastern Pacific warm up. Because convective activity (cloud formation) follows the sea's warm surface water, clouds increase in the central and eastern Pacific, while they decline in the west. This displacement in convective activity generates droughts in Australia, Papua New Guinea and Indonesia, typhoons in the central Pacific, and heavy rains along the normally arid coast of northern Peru. These conditions can last from 12 to 18 mouths, until the westward flowing surface winds once again begin to strengthen, causing warm water to flow back toward to the region of the western Pacific warm pool. The sea levels at both ends of the basin begin to change direction now rising in the west and falling in the east, as does the depth of the very important but out of sight thermocline. Strong upwelling returns to the equator and to the eastern Pacific boundary of coastal Peru.
Figure 4.2a,b summarizes the west-to-east interaction between the atmosphere and the ocean in the equatorial Pacific under normal conditions and under El Niño conditions. The precise timing of the beginning of any particular El Niño event may not be well known, although there are several hypotheses about how to detect it. Once started, however, the processes that keep El Niño going, as well as the processes that end it, appear to be better idea some months in advance about its potential impacts on some ecosystems and societies around the globe. This was the case for the 1997-98 event. Even though forecasters missed forecasting its onset, they were relatively more successful in identifying some of its worldwide impacts.
澳洲代写作业 El Nino In The Galapagos Biology Essay
