Literature Review

 

 

The El Nino phenomenon, commonly referred to as El Nino-Southern Oscillation or ENSO, is a massive disruption in the ocean-atmosphere system of the eastern Pacific Ocean that has significant and far-reaching consequences on the environmental status of almost the entire world. El Nino results from a series of environmental shifts that combine to create changes in both the ocean itself and more importantly in the atmosphere. Up until 1997, information regarding this environmental wonder was scarce, however due to the intensely developing El Nino of present day there is the opportunity to truly understand almost every key aspect of this incredible phenomenon.

In order to truly understand the causes of El Nino it is necessary to understand the normal status of the area affected by ENSO. The trade-wind system of the Pacific Ocean, which creates what are referred to as prevailing winds, is a vital part of ENSO. Near to the equator stands a system of low pressure known as the doldrums. Nearby, some 30 degrees north and south of the equator are the primary high pressure systems known as the horse latitudes. Typically, due to unequal heating, the sun heats the tropical regions of the Western Pacific resulting in a rising of the air in the atmosphere of these regions. This risen air moves from the horse latitudes toward the doldrums whereupon reaching the equator it is deflected westward as a result of the coriolis effect. The coriolis force is the "additional force or acceleration acting on the motion of bodies in a rotating system of reference." (Encarta). When applied to the earth, this system plays a plays a pivotal role in regard to the direction in which the trade winds travel. As a result of the coriolis effect, the winds are deflected along the earths equator in the direction of the earths rotation, which is clockwise in the northern hemisphere, and counter clockwise in the southern hemisphere. Hence, the northeast and southeast trade winds (named for their point of origin) are created and the normal trade wind system, which blows winds westward along the equator of the Pacific is in effect. As a result of these westward winds, there is a shift in the water depth as surface water is carried west due to currents that are kept in motion by these prevailing winds. These large ocean spanning currents are known as gyres. As the surface water moves west, it is replaced by much cooler subsurface water in a process referred to as upwelling. As a result the sea level in the eastern Pacific along the equator, is typically a .5 meter lower than that of the western pacific, where the warm surface water has been gathered into what is called the warm pool. Water temperature is also affected by these trade winds. Typically the water temperatures for the western part of the equator are as much as eight degrees Celsius higher than in the east around South America due to the upwelling of these cooler subsurface waters. The warm pool is fueled with more warm water and the cycle continues. Another typical effect of the trade winds is a high level of rainfall over Indonesia and Southeast Asia caused by the humidity of the winds as they make their long journey westward across the Pacific. During El Nino however, this entire system will undergo a transformation ultimately affecting the enviroment worldwide.

 

This image displays the normal movement of water in the pacific under normal conditions.

Photo Courtesy of UMASS

 

 

El Nino " contributes to significant weather changes around the world" (Environmental News Network). The initial cause of El Nino is the diminishing and even reversal of the trade winds, an event that normally occurs in the middle of Spring. This reversal results in immediate atmospheric alterations. The large amounts of surface water that had previously been in the western Pacific as a result of the westward flowing trade winds return to the east. This tropically heated surface water becomes concentrated near the equator of the eastern Pacific where it then spreads up and down the coasts of North and South America. The so-called warm pool is essentially relocated. The rising air that results from the warm pool also moves east and the heated air begins to be pumped into the upper atmosphere over the eastern pacific instead of the west. The high and low pressure systems are relocated as well and the new eastward flowing trade winds are fueled defying the coriolis effect. This relocation of the warm pool and the high/low pressure systems also fuels the immense tropical thunderstorms associated with El Nino that are usually restricted to the western pacific. Another way of viewing this process is on the basis of planetary scale tidal waves called Kelvin waves. Ocean surface water is constantly moving as a result of the trade wind system and slowly over periods of time the water builds up in the western pacific. However, this process cannot continue forever, as there is a limit to the mass of water in the western pacific. When waves hit a wall they change direction and this is no different on a planetary scale with kelvin waves. At a given point all of the water that has been moving westward will reach a limit and change direction and a eastward moving kelvin wave will result. These tidal waves are of such great mass and move so slowly over such a great span that they are invisible to the human eye. The shift of warm surface water that occurs as a result of El Nino is in essence a Kelvin wave that is moving back and forth from the western to the eastern Pacific. It is the speed of this Kelvin wave and its intensity that respectively generate the time span in between El Nino's and prove how long they last. This perspective also helps to explain the absence of El Nino from the Atlantic and Indian Oceans. The narrower width of these oceans creates a shorter distance for Kelvin waves to travel. As a result, these oceans are able to adapt more rapidly to any large scale variation of surface water and atmospheric temperature. These Kelvin waves also help to explain the lesser known phenomenon known as La Nina. As El Nino ends the process will reverse itself and as the Kelvin wave reverses, it leaves in its path the much cooler subsurface water. As this removal of water takes place in a much shorter period of time than under normal conditions the result can be seen by extremely cold water temperatures along the eastern Pacific. La Nina is essentially the return to normal pre El Nino conditions but in a very short time period. In order to further have a complex understanding of the El Nino phenomenon it is important to understand the history of ENSO.

 

Under El Nino conditions the path of the water is

reversed in a kelvin wave bringing the warm water east.

Photo courtesy of UMASS

 

 

 

 

 

The first historical record of El Nino was in 1567. It was later named by Peruvian fisherman "El Nino" after the Christ Child, because it most frequently occurred around Christmas. Up until the early 1920's, El Nino was though to be a primarily isolated event, however, it was at this time discovered by Sir Gilbert Walker that this assumption was false. Walker, a British scientist in charge of the Indian Meteorological Service, was studying the frequency of monsoons in India when he made the connection between patterns of rainfall in South America and ocean temperature. He made the initial discovery that "when pressure is high in the Pacific Ocean it tends to be low in the Indian Ocean..." (Kessler). He also discovered the shift in atmospheric pressure between Tahiti and Australia leading him to name this aspect of the atmospheric phenomenon the Southern Oscillation. Although Walker had made a huge discovery, he was scoffed for what seemed to other scientists an impossible connection over such widely separated regions. It would not be until over 50 years later that his theories could be proved with the incorporation of trade winds into Walker's initial equation. This revelation was achieved by Jacob Bjerknes, a Norwegian meteorologist, who was a professor at the University of California. It was he that discovered physical evidence that Walker's theory of Southern Oscillation was in fact, not only connected to El Nino, but a division of the same phenomenon. Hence the name that is now prevalent among the scientific community, El Nino-Southern Oscillation or ENSO was conceived. This new understanding of El Nino only created more questions, for although the phenomenon itself was beginning to be understood, there was still an absolute lack of knowledge regarding predicting its frequency and intensity. Even today, these aspects of ENSO are not fully understood.

El Nino results in a drastic change of the environmental patterns of the world and as a result there are severe consequences. The cold uprising of air in non-ENSO conditions stays too cool and dense to form rain clouds. The dry conditions that typify high pressure systems prevail in the eastern Pacific during regular non-ENSO years. Monsoonal rains are restricted to the western Pacific where a low pressure system is preserved due to the high water surface and air temperature. When the warm water migrates to the east the air warms and disrupts the equilibrium in air temperatures. The warmer air rises affecting the atmosphere and essentially creating a low pressure system. With this change in atmospheric pressure comes the monsoonal rains that are typical of low pressure systems. The same atmospheric changes take place in the western Pacific but in reverse as the removal of the warm pool affects the upper atmosphere creating a high pressure system in which rain clouds cannot form. Rainfall is significantly effected on both sides of the Pacific. Areas in the western Pacific such as Southeast Asia depend on these monsoonal rains for their survival. As a result of ENSO, they face serious drought and as their crops fail, starvation. These areas are also not prepared for the dryness that emanates from ENSO and as a result there are frequently severely destructive brush fires. The other side of the spectrum is affected in an equally negative way. South American fisherman depend on the cold water of the eastern Pacific which is nutrient-rich and supports high levels of marine life and ecosystems. In a sense their crops fail too as a result of El Nino for they depend on being able to fish in order to survive as much as the Southeast Asians depend on being able to farm. Many eastern Pacific countries are also not prepared for the monsoonal rain that arrives with El Nino. This lack of preparedness leads to severe flooding and ultimately to more death. This warm water also proves to be ideal grounds for the formation of hurricanes. Hurricanes tend to thrive on warm water and diminish in cold. As a result of El Nino, the season for hurricanes is greatly extended and small tropical storms are given the potential to form into massive hurricanes. A common misconception is that the environmental effects of El Nino are limited to the Pacific. However, this is false as the effects of El Nino are far reaching and worldwide. The warm air that is pressed into the atmosphere does not limit itself to the specific area of the warm pool. At some stages this warm air is pushed as high as 50,000 feet into the atmosphere which causes it to affect not only the trade winds but also the high altitude jet streams. These high altitude jet streams span the entire globe and frequently cause erratic weather changes. However due to the complexity of these high altitude jet streams it is not clear whether this erratic behavior is a direct result of El Nino. One such example is the lack of hurricanes in the western Atlantic in what is normally a strong hurricane season. This typically occurs during the ENSO years however due to the fact that weather conditions have only been recorded for about ten ENSOs this phenomenon cannot be absolutely proved as being a direct result of an ENSO. Essentially, "[w]hile some parts of the world prepare for heavy rains and floods, others face an impending drought, poor crop yields and starvation" (Roach). Due to a lack of knowledge, one of the most efficient methods of discovering the effects of El Nino in the Pacific is referring to past El Ninos.

The El Nino of 1776-77 resulted in some of the coldest temperatures in recorded history. The eastern United States faced one of its most severe winters in history. In fact, "[p]olar regions were so cold, [even] the bears did not hibernate" (Mayell). Snow was recorded as far south as Miami and in Buffalo snow drifts were reported as being two stories high. The El Nino of 76-77 gave meteorologists the information the felt was needed for ENSO prediction, but unfortunately for the world, they were significantly wrong displaying the ultimate unpredictability of ENSO. The El Nino of 1982-1982 was the most severe in history up to that point and it caught the experts entirely by surprise. The normally clear sign of easterly trade winds did not occur and evidence of a possible El Nino was non-existent until late November, more than two months later than usual. The consequences of this failure to prepare were severe. Australia faced one of its most devastating droughts in history and brush fires ravaged the countryside. Severe floods punished most of South America and when all was said a done, between 8 and 13 billion dollars worth of damage had been done and over 2,000 had lost their lives. The El Nino's that followed specifically 1986-1987 and the longest in history 1990 to 1995 were unusually weak and questions regarding what caused the duration of and in between these phenomenon and what made one stronger than the next continued to be asked. True scientific understanding was far from being achieved as many prominent theories were disproved one after the other. El Nino involves a complex combination of oceanic and atmospheric events and as a result its origins are unknown. Information regarding El Nino itself is readily available yet many questions remain unanswered. That is why in order to further understand El Nino it is important to understand the difference between theories and facts.

This animation compares the increase in water temperatures

of three past El Ninos and

El Nino 97-98. Clearly, El Nino 97-98 rivals in

intensity to the great 83-83 El Nino.

animation courtesy of NOAA

Throughout the years in which El Nino has been studied many theories have been formulated regarding its origin. We now know many of these theories to be false. The volcanic eruption of Mount Chichon in Mexico in February of 1982 prior to the 1983-1983 El Nino created what became one of the most common disillusions regarding the origin of El Nino. The eruption of Mount Pintubo in the Phillipines in 1991 fueled this theory however it was not long before technology and computer models were able to disprove this speculation. Another common belief during the late 80s early 90s was that deep ocean vents and chasms which do in fact affect water temperature were the origin of ENSO. Again, the progression of technology displayed that these vents resulted in water temperature change over centuries, way too long of a time period to have any direct affect on El Nino. A more recent speculation involves the process of Global Warming. However, due to the fact that Global Warming itself is unproven, in no way can it be linked to El Nino. However, if in fact the affects of Global Warming are as far reaching as some scientists believe, its future effects on ENSO could be astounding. For the time being though El Nino must be accepted for what it is, which a far from understood incredibly complex phenomenon. There is probably no one origin for this event and perhaps one does not exist. It is highly possible that El Nino results from nothing other than ocean itself. Perhaps in the next several months as what is considered one of the most powerful ENSOs ever occurs, many of these questions will be answered.

El Nino 1997 is the most scientifically important ENSO ever, as due to increased technology, every aspect of it can be studied. Also, since "the sea surface temperatures will remain warm enough to impact global weather patterns into the Spring," (NOAA Climate Forecast) the time period for which it can be studied is immense. What was initially the Tropical Ocean Global Atmosphere program has been developed into a fully operational El Nino observing system. Designed for studying primarily environmental science and the affects of pollution, TOGA became the perfect source for studying ENSO with its complex system of satellites, moored buoys, drifting buoys and other ocean analysis. Many other such programs are being developed and are taking full advantage of the numerous satellites, and informational buoys in place. The US/French TOPEX/Poseidon Satellite was launched in 1992 to monitor relationships between atmospheric pressure and water level and temperature. This closely monitored information is now paying off as the first true factual data regarding El Nino is being recorded. The Tropical Atmospheric Ocean Array is yet another example of how technology is leading to detection and understanding of El Nino. TAO consists of 70 buoys across the Pacific Ocean retrieving real time data on the ocean and the atmosphere. It is supported by an international array of scientists and as all resources are combined, hopefully the most clear picture of El Nino will emerge yet. Agencies all over the world are dedicating full time status to observing and understanding El Nino and maybe in the years to come billions of dollars and thousands of lives will be saved as a result. The effects of the 1997 ENSO are already clearly being seen. The warm tropical ocean waters first appeared in Spring of 1997 and quickly strengthened. Satellite and buoy observation have recorded an average water temperature of 82 degrees Fahrenheit along the eastern Pacific as much as 8 degrees higher than normal. This abnormally warm spans an area as large as 9.5 million square miles and it continues to grow. California, Texas and Florida are expected to see as much as 200 percent their normal rainfall. As early as summer the effects of ENSO were beginning to appear as there was significantly less rainfall in Indonesia and a dramatic decrease in hurricane activity in the Atlantic and Gulf of Mexico. Some areas of Chile received their annual rainfall in a single day months before El Nino 1997 has even reached its peak. Fish migration has been extremely disrupted as species must push northward in the eastern Pacific in search for food. Tuna have been spotted off California and Salmon have been seen as far north as Canada. Many seals and sea lions are expected to be beached for lack of food and whole corral reef ecosystems will be destroyed. The effects of El Nino will most likely show up in the global economy as well, as crop failure and the collapse of the South American fishing industry will consequently lead to raised prices worldwide. El Nino is essentially a force to be reckoned with which is why a full understanding of this phenomenon is so important.

ENSO's complexity stems primarily from the fact that it is one of many environmental phenomenons that affects are atmosphere most of which are not fully understood. Each El Nino that occurs is different from the last because i occurs in combination with many other environmental factors. El Nino can never be fully understood until the earth and its environment are fully understood. Each ENSO is unique and unless we can understand the factors that cause this uniqueness we will never understand the ENSO itself. The value in studying El Nino lies not only in understanding El Nino but also in hopefully coming to truly understand the environment itself. El Nino is just a small piece in a very large puzzle but perhaps it is the missing link that could allow for whole puzzle to be put together and prove to be the ultimate connection regarding environmental science and life on earth.