El Nino-Southern Oscillation Essay ⋆ Environment Essay Examples ⋆ EssayEmpire

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The El Nino-Southern Oscillation (ENSO) is a phenomenon that occurs in the tropical Pacific Ocean approximately every two to five years and typically lasts nine to 12 months. ENSOs, and the opposite condition, called La Nina, represent severe disruptions of the normal weather patterns over the Pacific and have significant impacts on weather around the world. These events typically start around August, reach their peak intensity in December through April, and dissipate in the spring and early summer. However, particularly strong events can persist for up to four years.

Anatomy of the Phenomenon

Under normal conditions, the northeast trade winds push surface waters westward across the tropical Pacific ocean. These waters warm as they absorb solar energy and pile up in the western Pacific near Australia and Indonesia. Here, the warm water contributes to low air pressure, and the resulting convection, along with evaporation from the warm ocean, creates plenty of rainfall. At the same time, the eastern Pacific (off the coast of South America) normally has cool surface temperatures, due to cold surface currents flowing toward the equator from higher latitudes. In addition, cold water is brought up to the surface to replace the water that has moved westward, in a process known as upwelling. Because the ocean surface tends to be cold in the eastern Pacific, the air pressure tends to be high and there is little rainfall. However, the upwelling brings nutrients to the upper layer of the ocean, so even though the land is dry the ocean is extremely productive and fisheries thrive.

When an ENSO occurs, this normal condition is altered. In the ocean, warmer-than-normal surface waters move eastward along the equator, and sea surface temperatures become unusually warm in the eastern tropical Pacific Ocean (along the equator between the International Date Line and South America). Off the coast of Peru, fisherman historically observed unusual warm currents around Christmastime and referred to them as El Ninos, referring to the Christ Child. The name was later extended to refer to the entire warming event.

The warm waters in the eastern Pacific cause lower than normal air pressure in that region, while the unusually cool surface waters in the western Pacific create high pressure. This is the Southern Oscillation part of the phenomenon, and can be thought of as a seesaw-like shift in the air pressure pattern across the tropical Pacific. Because the pressure pattern is reversed across the tropics during the ENSO event, the trade winds slow down or even reverse. The oceanic and atmospheric parts of the phenomenon reinforce each other: weaker trade winds allow more warm water to accumulate in the eastern ocean, while warm water in the east contributes to a weakening of the trades.

The reverse of the El Nino pattern is referred to as a La Nina and is characterized by unusually cold sea surface temperatures along the equator, lowerthan-normal pressure in the western Pacific (near Indonesia and Australia), higher pressure in the central and eastern Pacific (near South America), and stronger-than-normal trade winds. La Ninas frequently occur immediately after El Nino events. Although the names El Nino and La Nina are common, it is increasingly preferred among atmospheric scientists to refer to these events as the “warm phase ENSO” and “cold phase ENSO,” respectively.

Human Impact

ENSO events have been occurring for at least 5,000 years based on paleoclimatic and archaeological evidence and have had significant impacts on human societies around the world. The most obvious impacts are seen around the Pacific Basin, where ENSO events bring heavy rainfall, flooding, and mudslides to the normally dry west coast of South America. During the 1982-83 event, one of the largest El Ninos on record, approximately 600 people were killed in Ecuador in Peru. At the same time, the cold upwelling off the coast of South America was cut off, and fisheries declined. The economically important Peruvian anchovy industry was decimated during the 1982-83 event and has yet to fully recover. In the western Pacific, El Nino brings lower-than-normal rainfall and drought to Indonesia and northern Australia. During the 1997-98 event, another extremely strong El Nino, forest fires in drought-stricken Indonesia resulted in billions of dollars in damage and serious air pollution that was responsible for at least one deadly airline crash.

ENSO events have significant weather impacts outside of the Pacific Basin as well. When the warm ocean water shifts eastward, the main area of low surface pressure and convective storminess shifts eastward as well. This shift in pressure patterns results in altered patterns in the upper-level winds, which flow west-to-east at the top of the troposphere. Because the upper-level winds play a major role in determining where storms will form and move, they link the tropical Pacific to the rest of the world.

In the United States, ENSO events are associated with unusually wet and mild spring conditions across the southern half of the country, along with drier and warmer conditions across the northern half. ENSO events frequently produce strongerthan-normal upper-level winds across the southern United States and out into the tropical Atlantic Ocean. These upper-level winds disrupt the formation of hurricanes in the Atlantic Basin, and so El Nino years tend to have a reduced chance of hurricanes in the Gulf of Mexico and the Atlantic Coast. However, the strong jet stream over the southern United States gives this region a greater likelihood of severe weather, including tornadoes. Also, El Nino contributes to increased hurricane frequency in the eastern Pacific.

La Nina events tend to have the opposite effect on the United States. During a La Nina, the jet stream is shifted northward. As a result, the northwest coast and the Midwest often experience wetter-than-normal winter and spring weather, while the southern half of the country is warmer and drier than usual. Severe weather and tornado outbreaks are less likely than usual in the southern states. However, La Ninas produce upper-level patterns that are favorable for hurricane formation in the Atlantic and these storms become more likely to make landfall on the Gulf and Atlantic coasts.

Globally, El Nino events have been linked to increased precipitation in eastern equatorial Africa and parts of the Indian Ocean, while Brazil, India, southeastern Africa, and Madagascar generally experience drought. The opposite patterns tend to occur during La Nina events.

Ongoing Study of ENSO

Although ENSO events have been occurring for thousands of years, a full understanding of the phenomenon had to wait until the latter part of the 20th century, when a sufficient amount of observational data in the tropical Pacific became available. The earliest piece of the puzzle was provided by Gilbert Walker, who first identified the Southern Oscillation while seeking an explanation for the occasional failure of the monsoon in India (which resulted in devastating famine when the rains did not arrive). In the 1950s, Jacob Bjerknes made the connection between the Southern Oscillation and the sea surface warming in El Nino episodes. Bjerknes hypothesized the complex ocean-atmosphere linkages known as ENSO in 1969. His hypothesis was able to be tested during the strong ENSO of 1977-78, and again during the event of 1982-83. Subsequent events have provided opportunities to refine our understanding of the process and impacts of this major source of variability in the global climate. One of the major questions still remaining is what impact global warming will have on the intensity and frequency of ENSO events. Thus far, there is insufficient evidence to determine whether or not any relationship exists.

Bibliography:

Edward Aguado and James Burt, Understanding Weather and Climate, 3rd ed. (Prentice-Hall, 2004);
Brian Fagan, Floods, Famines, and Emperors: El Nino and the Fate of Civilizations (Basic Books, 1999);
Glenn McGregor and Simon Nieuwolt, Tropical Climatology, 2nd ed. (Wiley, 1998);
Greg O’Hare, John Sweeney, and Rob Wilby, Weather, Climate, and Climate Change: Human Perspectives (Prentice-Hall, 2005);
William Stevens, The Change in the Weather: People, Weather, and the Science of Climate (Delta, 1999).