Temperature anomalies at the ocean surface ( C) during El Nio in 1997

El Nio (literally "flow of the Child Jesus , "because it rises just after Christmas ) means originally a seasonal coastal current off Peru and of Ecuador ^Description

In normal (called anti-Nio or La Nia), the coast of Chile , from Peru and of Ecuador are bathed by the cold Humboldt current , heading north and swept by winds sea, blowing from the south- east to north-west. The latter drove the warm surface waters residual shoreline and cause a vacuum that is filled by an upwelling of deep , is the phenomenon known as upwelling in English. These waters, from a depth of 100-200 m, are rich in carbon dioxide and permit rapid development plankton which attracts fish, birds and, logically, promotes the activity of fishing.

Every year, shortly after Christmas and until April, a small coastal current reverse this movement and flows south. Irregular intervals, this current El Nio is more important and more down south. Cold water is replaced by warmer water and the fish are disappearing from coast ruining the activities of fishermen. In the same period, the coastal areas usually little rain in northern Peru and Ecuador are experiencing heavy rainfall ^{Explanation current}

The explanation is based on the model of the Walker circulation that many scientific missions in recent years have not changed intrinsically. In this perspective, El Nio is caused by a displacement air of the Walker circulation that we get a hard time explaining and returns periodically , modifying the path of an ocean current of comparable size to one and half times that of United States. These changes occur exceptionally some years , on average once or twice a decade , along the coasts of Peru in late winter, around December - January ^{, , , ,} .

The convergence zone of winds, called the intertropical convergence zone (ITCZ), is where is the upward movement in the Walker circulation. Quand les alizs soufflent leur pleine puissance, les remontes d'eau froide des profondeurs (upwelling) le long du Pacifique quatorial refroidissent l' air qui les surplombe. This creates a temperature difference between the eastern Pacific coast and offshore. A breeze regime is thus established between these two areas which creates a subsidence of air along the coast and a descent to sea. Under these conditions, the vapor of water contained in the air near the coast can not condense and form clouds and drops of rain. Thus, the air remains free of clouds during the year "normal" in the eastern Pacific. The rain in the equatorial belt is then largely confined to the extreme west of the basin, near the Indonesia.

But when the trade winds weaken and retreat eastward during the early stages of an El Nio event, the upwelling of deep slows and the ocean warms. This equalizes the temperature between the central and eastern Pacific and cross the Walker circulation to the west. The moist air at the surface of the ocean also warms to the generation of heavy rain when the ITCZ moves eastward. This change in sea surface temperatures is responsible for the eastward shift of the maximum rainfall over the central Pacific. Adjustments weather associated correspond to a decrease of pressure in the central Pacific and East and an increase in pressure in the western Pacific ( Indonesia and Australia ), suitable to a larger withdrawal from the trade winds.

The first sign of onset of El Nio is a considerable strengthening of trade winds from the southeast. They lead to an accumulation of warm waters in the western Pacific, raising the sea level on the Australian coast, and a relative lowering along the South American coast . But when the southerly winds weaken, the water "hot" in the western Pacific invade those of the eastern Pacific. It's the start of El Nio. The latter is connected to a temporary weakening, and very pronounced, the anticyclone present among Pacific ( anticyclone Easter Island ), which reduces the strength of winds from the southeast. Reflux mass of warm water accumulated in the western South Pacific to the east acts as the principle of a seiche effect .

The duration of El Nio is usually about 18 months. After this period, the cold waters spread westward. Then the end of the phenomenon that can be followed by its opposite La Nia. A remarkable correlation between the atmospheric pressures of eastern and western Pacific. When they increase to the west, they decrease to the east, and vice versa. This phenomenon accelerates the surface winds from east to west, Peru until Indonesia or decreases during El Nio .

In 2010, El Nio is still the subject of much research in order to discover the causes of this sailor.

Convective circulation of normal Walker

The decrease in trade winds disrupt the cycle of Walker and let the warm water spreading further east: is El Nio

Strengthening winds stretching the area covered by the Walker circulation and enhanced: La Nia

Study and forecasting of the phenomenon

The beginning of the study of El Nio

Sir Gilbert Walker and the Southern Oscillation

British scientist and head of the Indian Meteorological Service, Sir Gilbert Walker was assigned in 1920 to India to find a way to predict the monsoon Asia. Brilliant scientist, he worked on the task with determination. He got in contact with South American scientists who provided him with the results of their studies on local effects of El Nio. By studying these data climate and weather and that he had at his disposal, he managed to establish in 1923 , a temporal correlation between the returns barometric west and east of the South Pacific. Indeed, he realized that the pressure increased to the west when it decreased to the east (El Nio), and vice versa. Because of this equilibrium and balance, he named the Southern Oscillation phenomenon ( Southern Oscillation in French).

Focusing its research on the Southern Oscillation, Sir Walker is able to determine, again in 1923, an index to which it gave its name. The latter function would be to measure the difference in pressure between the eastern and western Pacific Ocean. When the index, and therefore the gap increased, the pressure was high in the eastern Pacific, and the winds were stronger. When the index was rather low, the winds were less strong, resulting in mild winters in Canada and America West. The whole is accompanied by droughts in Australia , in Indonesia , India and parts Africans.

One of his colleagues attacked this subject in a scientific journal , found "absolutely ridiculous the idea that climatic conditions in parts of the world as distant from each other can be linked together in that way". What Sir Walker replied that there was more precise explanation, but "would likely require structural knowledge of the wind at levels other than the ground." This involved the concepts and methods of observation unknown at the time but the current research methods have actually confirmed the theory of pressure index Walker.

Jacob Bjerknes and ENSO

In the decades that followed, scientists studying climate variations leaned on the enigma of the desert islands of the equatorial central Pacific. These islands, although it received (based on climate statistics US-Canadian) the same amount of rain than their surrounding lush, were hopelessly sterile. In fact, this sterility was due to a change in the pressure index Walker: most of the time, the index of the latter was rather high, resulting in very low or nonexistent annual precipitation. However, during a period that is repeated every two to seven years ago, these islands were under a deluge that lasted several months, from December to mid-June.

The link between this seemingly obvious and strange phenomenon El Nio, however, will be established that during the 1960s , a meteorologist Norway : Professor Jacob Bjerknes. It was first noticed in 1967, the ratio between the observations of Sir Walker and El Nio. The two concurring phenomena at any point, he even had the idea of completing the name of El Nio involving the discovery of the Columbia: the phenomenon is now called ENSO or El Nio Southern Oscillation ( El Nio Southern Oscillation ).

Professor Bjerknes has also established a few years later, the link between changes in temperatures on the surface of the Wed , powerful winds and heavy rainfall that usually accompany the troughs to the east and west Pacific. This corresponds to the phases of an index of low index Walker.

A growing interest in the late ^twentieth century

Since 1982, the date of ENSO have devastated the entire belt of countries in the intertropical belt and even affected the climate of Europe, thousands of scientists and researchers worldwide have tried to understand the phenomenon. During this period, only two programs brought answers to some questions.

TOGA

Launched in 1985, the program of international collaboration Tropical Ocean and Global Atmosphere (TOGA: "Study of the Tropical Ocean and Global Atmosphere Study), led to a better understanding of the coupled ocean-atmosphere. It lasted 11 years and served as the basis for launching its successors. He paid particular attention to changes in coupling caused by El Nio.

WOCE

Program launched five years after the TOGA by 44 countries, including those of the European Union at the time, the World Ocean Circulation Experience (WOCE "Ocean Circulation Experiment Global") was intended to establish a global ocean description. He has established a climate model that can predict more or less years that hit the ENSO phenomenon.

CLIVAR and GODAE

The result of these programs was made by the Climate Variability and Predictability Program (CLIVAR "Syllabus forecasting and climate change"), who studied the climate and ocean- ice -atmosphere global , and the Global Ocean Data Assimilation Experiment (GODAE), which in 2003-2005, preparing the establishment of a global climate monitoring and prediction.

The 2000s

TOPEX / Poseidon

After early infancy, the study of El Nio experienced a real boom in the XXI ^century. New technologies and new resources made available to researchers allowed to make significant progress in the analysis of the phenomenon.

Institute of Development Research

In 2000 , the Institute of Development Research (IRD) has launched the ECOP ( Study Climate tropical Pacific Ocean ) to study climate variations due to ENSO and its opposite, La Nia. The same year, the IRD launched as well, with a budget of 132 000 , the program PALEOCEAN which, in turn, studied corals. The technical core of coral , recently developed, allowed him to use corals as paleothermometers. These contain the uranium and strontium , which amount present varies with the surface temperature of the sea, and is measured by spectrometry. These elements dating corals and witnesses attest to the fluctuation of sea level over the years. They reveal the presence of cardiomyopathy , living organisms whose state reflects the impact of ENSO on the environment.

A very special satellite

In 1992 , the NASA and the Centre National d'Etudes Spatiales (CNES) joined forces to launch the satellite Topex / Poseidon with the rocket Ariane 4. The vehicle of 2.4 tonnes was sent to an altitude of 1336 km , making a tour of the Earth every 112 minutes , and can observe up to 90% of the oceans. CNES and NASA began the 50 000 daily measurements of Topex / Poseidon available to the scientific community in July 1993. More than 600 scientists from 54 countries exploited these measures, distributed through two databases: one located in the United States , the other, the center AVISO , located in Toulouse. This center produces a monthly CD containing all the data collected by satellite, or nearly 2 million monthly measurements.

In October 2005 , a technical incident has cost the satellite's ability to maneuver in orbit, putting themselves unable to acquire new scientific data. The satellite has completed its mission on 5 January 2006 , after 13 years in space and over 60,000 revolutions around the Earth.

Weather

Observations of TOPEX / Poseidon inserted themselves in several major international scientific programs, including WOCE, TOGA, CLIVAR, and GODAE (with MERSEA its component European ). Meteorological agencies, too, drew from satellite data. Thus, these measures soon proved indispensable, and it became evident that a new program should take over from TOPEX / Poseidon.

The Jason Program

Since it was launched by Delta II on 7 December 2001 , the satellite Jason-1 , the successor to Topex / Poseidon, delivers usable data in real time (approximately 3 hours after receipt of data). The Jason program was designed as a series of satellites. Thus, the Jason-2 satellite, whose launch is planned for 2008 , began its development phase in 2004. The Jason-1 is 5 times lighter than Topex / Poseidon (only 500 pounds for 3 meters wide) and about 2 times less expensive. It provides accuracy at least equal if not greater, than its predecessor, because of the collaboration between his actions and those taken directly to the ocean surface of the Earth, by ships or specialized tags weather.

Data altimetry also provide near real-time ocean observations to enable the development of weather forecasting. Thanks to measures of Jason-1, Meteo-France and provides regular bulletins on the state of the ocean but also alerts in case of deteriorating weather. Jason-1 fits into the project of Oceanography Operational Mercator, launched in 1997 and became a public interest group in 2002 (partnership between CNES, CNRS / INSU , the IFREMER , IRD, Mto-France and SHOM ). Mercator can perform real-time monitoring of the oceans (making weekly newsletters of the sea state ), but also long-term predictions about phenomena bioclimatic such as El Nio.

Comments

Road washed away in Peru during heavy rains during the El Nio winter 1997-98

El Nio causes many changes climate. The oceans and atmosphere are in constant interaction. The changes induced on the surface temperature of Wed will affect the winds.

Thus, the El Nio of 1982-1983 produced dramatic effects on the continents. In Ecuador and northern Peru approximately 250 cm rain fell during 6 months. Farther west, the anomalies of wind had baffled typhoons their normal routes, to Hawaii or Tahiti unprepared for such weather.

Effect of El Nio and La Nia in the North Pacific and North America

The phenomenon can affect shock wave conditions climate in the most remote areas of the globe. This message is conveyed by global movements of regions of tropical rains, which then affect the wind structures on the planet. The clouds bring rain tropical twist air above them (8 to 16 km above sea level). The winds that are formed in the air above the clouds will determine the positions of the monsoons and roads cyclones and belts of strong winds between the hot and cold regions on the surface of the Earth. During El Nio years, when the rain area usually centered over Indonesia moves eastward toward the central Pacific, the waves present in the upper layers of the atmosphere are affected, causing climate anomalies in many regions.

The impacts of El Nio on the climate in temperate latitudes are most obvious during the winter. For example, most El Nio winters are mild on the Canada West and parts of north-western United States , and wet over the southern United States (from Texas to Florida ). El Nio also affects temperate climates in other seasons. But even during the winter , El Nio is one of many factors that influence the climate in temperate regions.

Thus, the 1997 version of El Nio caused the drought and forest fires in Indonesia , heavy rains in California and floods in the south-eastern United States. The estimated average temperature of the earth surface, for land and sea areas has also increased. End of December 1997 , a storm dumped record-breaking 25 cm of snow in the southeast United States. Waves up to 4 meters high swept south of San Francisco. Severe storms have caused epidemics in Florida with tornadoes reaching 400 km / h. According to the UN report, El Nio in 1997-98 made many thousands of dead and wounded, and caused damage estimated between 32 and 96 billion .

In June 2002, some effect of El Nio was felt in the tropics of South America. Violent storms , the worst of the past eight decades , have softened the Chile. Towards the end of December , the Australia suffered the worst drought in a century called the "super-dry". Deadly storms have also unleashed on the west coast of the United States. Five full days of heavy rains and high winds.

A phenomenon considered to be global

Global effect of El Nio.

In the 1990s a correlation between hot weather and global climate change in the short term has been highlighted. One result is the discovery of the extension of El Nio in the tropical regions of the Indian Ocean and the Atlantic Ocean. It was made possible through an analysis of the surface of the oceans with more than 650 000 measurements made by boat. The amount of data used covers a period of approximately fifteen years. It was thus observed, 12 to 18 months after the end of El Nio in the Pacific , a cyclical warming of the surface of the equatorial Atlantic Ocean. This appears to be a passive response to changes in atmospheric pressure and winds (caused by El Nio) in the region. This reaction of the Atlantic Ocean is not really explained so far, but suggests the spread of the global consequences of El Nio.

The fact that El Nio is now considered a global phenomenon, affecting the three major tropical oceans, should help explain the disruptions of climate on the entire planet. The changes in temperature can therefore ocean, at the local level, changing the absolute humidity of the atmospheric circulation, causing increased rainfall in the surrounding areas. This gives an intuitive idea of the mechanisms that cause the effects observed mostly in the Pacific region, but also to a lesser extent in the rest of the world. This change is especially large, frequent and sustained that the energy of the atmosphere increases with temperature, for greenhouse.

El Nio contributes to these abnormalities or changes in the relative humidity in a way that is not explained well, but we're pretty sure he is responsible and in this sense that the moisture can be considered one of the engines of the atmosphere. The applications of these researches will then better prevent the consequences now undeniable of El Nio on the global climate system.

References

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