El Nino And The Walker Circulation Biology Essay

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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.

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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.

Since 1960, the total catch by weight of Peruvian anchovy has exceeded that of any other single fish species in the world, but the annual catch has varied considerably (Figure 5). In the decade between 1962 and 1971, annual yields averaged 9.7 million tonnes, but the catch dropped dramatically in 1972 and averaged only 1.3 million tonnes in the decade between 1976 and 1985. Such large variability in fishery yields makes economic planning very difficult in a country that relies heavily on the foreign exchange from this one source.

One cause of the large fluctuations in anchovy catch is a phenomenon called El Niño, an intrusion of warm, nutrient-poor water from the vicinity of the equator southward along the coast of Peru. The water is transported by the Peru Countercurrent, and in extreme cases may extend as far as 12oS latitude. The intruding water is less dense than the Peru Coastal Current, both because it is warmer and because it is less saline (contains a lower concentration of dissolved salts). This lighter water overrides the Coastal Current water in a nutrient-poor layer up to 30 meters deep. During El Niño, the coastal wind system sometimes slackens, and upwelling may cease. Alternatively, the coastal wind system and upwelling may continue as usual, but because the nutrient-poor upper layer is so much deeper than usual, the upwelled water comes from above the nutricline and so is low in nutrients. From a biological standpoint, upwelling has become ineffective. El Niño conditions have been known to persist for as long as 12 to 18 months.

Biological effects

During El Niño there is a dramatic drop in photosynthetic rates in the coastal current system, in extreme cases by more than an order of magnitude. This decline in photosynthesis impacts the food supply of virtually all organisms in the coastal current ecosystem.

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Fish-eating birds are significant and highly visible predators of the anchovy. These birds, regionally known as guano birds, sometimes number in the millions and live on islands off the coast of Peru. Their droppings, or guano, are used as fertilizer. Guano birds include cormorants, gannets or boobies and pelicans. Anchovies account for 95% of the cormorants' diet and 80% of the diets of gannets and pelicans. Guano birds have a limited ability to fish at depth, so when El Niño occurs, the guano bird population is seriously impacted. The weaker swimmers and divers are unable to find food even if the schools of fish do nothing more than disperse into deeper water. Census data confirm that weak, immature guano birds are decimated by El Niño. In mild or moderate El Niño events, the adult guano birds are still able to find enough food for themselves but not enough to sustain their young as well. They abandon their nests, and their nestlings starve. This may be appropriate evolutionary behavior for relatively long-lived birds, because it allows the adults to use available food for survival rather than reproduction. However, in severe El Niño events, significant numbers of adult guano birds die as well. After the severe El Niño of 1957 the guano bird population, then estimated to be about 27 million, dropped to 6 million. Millions of dead birds washed up on the Peruvian coast. The population gradually recovered in subsequent years and had reached 17 million just prior to the El Niño of 1965 (Figure 6). The population dropped to fewer than 5 million in 1966 and to a few hundred thousand after the 1972-73 El Niño.

During a strong El Niño year large numbers of marine organisms also die off the coast of Peru, including fishes, squids, and even turtles and small sea mammals. In some years the decomposition of all these dead bodies may consume the oxygen in the winter, and as a result toxic and foul-smelling hydrogen sulfide gas bubbles up through the water and blackens the paint on boats. Although some geographical redistribution of the anchoveta population may occur during El Niño years, the catch of anchovies by Peru's southern neighbor Chile does not increase during El Niño events. In year when Peru's anchoveta catch has dropped below 2.0 million tonnes, the catch of anchoveta by Chile has never exceeded 0.32 million tonnes. A strong El Niño drastically reduces or eliminates the fertilization of the ocean off Peru by upwelling. The result is a radical drop in the supply of food for all animals in the ecosystem.

Case study - Peru

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

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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

Niño Regions

Scientists have identified five regions in the equatorial Pacific that they consider to be worthy of special attention with regard to the observations and monitoring of El Niño processes. Their locations are shown in Figure AAA. Researchers are increasingly focusing on environmental changes in the Niño3.4 region in order to identify the onset of an El Niño event. Each region provides different kinds of information about either El Niño or the Southern Oscillation.

Figure AAA. Map depicting four regions in the equatorial Pacific Ocean identified as important locations for monitoring the wind and sea surface temperature changes associated with the El Niño process. Source: Australian Government Bureau of Meteorology (2011).

Niño1 is the region of coastal upwelling off the coasts of Peru and Ecuador. It is sensitive to changes in the ocean and the atmosphere, both seasonally and especially during El Niño episodes. Coastal upwelling processes in Niño1 are particularly sensitive to changes in air-sea interaction in the central and eastern equatorial Pacific.

Niño2 represents the Galapagos Islands region of the equatorial Pacific. Equatorial upwelling processes in this area are also sensitive to seasonal, as well as El Niño-induced, changes in the marine environment. Niño2 is a transition zone between the central and eastern equatorial Pacific, sensitive to changes in either Niño1 and Niño3, or both.

Niño3 is in the central equatorial Pacific, where there is a large El Niño signal but not much sensitivity to seasonal changes in air-sea interaction. It is in this region where information on changes in surface wind has been used by Mark Cane and Stephen Zebiak to project the likely onset to El Niño events. According to Cane (1991), "a warming in this region is thought to influence the global atmosphere strongly. It is probably the best single indicator of an ENSO episode likely to affect global climate".

Niño3.4 is relatively new region in the tropical Pacific increasingly used by more researchers to correlate changes in sea surface temperatures and surface winds there to climatic anomalies around the globe. Many researchers now use changes in Niño3.4 instead of Niño3 in their El Niño forecast modeling activities. It overlaps the Niño3 and Niño4 regions, as shown in Figure AAA.

Niño4 encompasses part of the western equatorial Pacific known as the warm pool. Here, sea surface temperatures are the highest in the Pacific. During an El Niño event, there is a relatively small change in sea surface temperatures (cooling). However, that small change is important, because the warmest water at the ocean's surface and the cloud producing processes that tend to follow it move away from the western Pacific toward the central and eastern Pacific. Hence during El Niño there are dry conditions in several countries in the western Pacific and very wet conditions in northern Peru and southern Ecuador.

Australia has suffered severe droughts in its northern and eastern regions during many El Niño years. These droughts have reduced crop yields, killed livestock, eroded soils and encouraged destructive bush fires.

1982-83

El Niño event of 1982-83 coincided with a drought that may have been the worst for 100 years. The 1982 winter rains (June - September) failed over the south-eastern grain and pasture areas of Australia. In northern New South Wales and southern Queensland, the summer rains (December 1982 - February 1983) also failed.

The American Heritage dictionary defines an anomaly as a deviation from the normal order and as something unusual or irregular. El Niño is an anomaly in terms of changes in sea surface temperatures and in sea level pressure. The 1982-83 El Niño can, therefore, be described as an anomalous anomaly. In addition to being unusual, it is considered by the scientific community to be the most extreme this century.

The sea surface temperatures associated with this El Niño were way above normal in the central and eastern equatorial Pacific (reaching 4 degree Celsius or more above normal in some areas). Those concerned about natural disasters and their socioeconomic impacts also considered it to have been a very extreme event, because of numerous destructive climate anomalies that occurred around the world at the time. In the wake of this particular El Niño event, many world leaders, the public, and the media, for the first time, were forced to pay more consistent attention to this phenomenon.

In 1982, several months prior to the onset of the 1982-83 El Niño, some researchers published an article on what they considered the "typical" El Niño. The canonical El Nino, as they called it, was in fact a composite of the features of several El Niño events that took place from the early 1950s to the late 1970s. Until then, it appeared that El Niño events had generally followed a similar pattern of growth and development from onset to decay. In less than a year, however, that notion was challenged with the occurrence of an unexpected out of phase onset of the 1982-83 event. According to Harrison and Cane,

"The warm event in the Pacific in 1982-83 was unusual in many respects. Rather than exhibiting surface warming first along the northeast coast of South America in the spring, sea surface temperatures first significantly exceeded climatological values along the equator in the eastern central Pacific during late summer."

(Harrison and Cane, 1984, p.21)

In addition, Rasmusson and Arkin (1985), in reviewing what had happened in 1982-83, wrote that the timing of the warming of water in the central Pacific was typical and that the warming along the Peruvian coast followed instead of preceded that warming. In any event, the coastal warming occurred at the normal time of the year.

The 1982-83 El Niño differed in both timing and location from the set of post-World War II events used to compile the characteristics of a "typical" event. For example, anomalously warm sea surface temperatures appeared first in the central Pacific instead of off the coast of Peru. The warm sea surface temperatures moved eastward toward the South American coast, instead of first appearing along the coast and then moving in a westward direction away from it. It emerged later in the year (between June and August) than the expected typical El Niño. The winds along the Peruvian coast did not weaken as expected when the El Niño began, even though a weakening of the westward flowing winds was considered to be a necessary, although not sufficient, condition to spark El Niño's

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.