Background ScenarioFlooding rains and warm weather in Peru wipe out the anchovy harvest. Torrential downpours and mud slides besiege southern California while the Northeast United States has fewer hurricanes and a mild winter. Droughts strike Indonesia, Africa, and Australia--all within the period of the same few months. Could all these events possibly be related?
The entire phenomenon of El Niño (warm) and La Niña (cold) episodes are extremes of what is often referred to as the ENSO (El Niño Southern Oscillation) cycle. ENSO is a natural part of our climate system arising from interactions between the ocean and the atmosphere. The cycle has an average period of about four years, although the period has varied between two and seven years in the historical record. This cycle encompasses changes in ocean surface and subsurface temperatures, tropical rainfall, atmospheric winds and air pressure. During El Niño episodes the equatorial sea surface temperatures (SSTs) are abnormally warm from the date line eastward to the South American coast. However, there is a strong annual cycle in the actual SSTs across the eastern equatorial Pacific, such that SSTs are sufficiently warm (approximately 28 degrees C or 82 degrees F) to support persistent tropical rainfall and convective activity in this region for only part of the year.
What is the difference between El Niño and La Niña?
El Niño is characterized by unusually warm ocean temperatures in the Equatorial Pacific compared to La Niña, which is characterized by unusually cold ocean temperatures in the Equatorial Pacific.The term El Niño was originally used by fishermen along the coasts of Ecuador and Peru to refer to a warm ocean current that typically appears around Christmastime and lasts for several months. Fish are less abundant during these warm intervals, so fishermen often take a break to repair their equipment and spend time with their families. In some years, however, the water is especially warm and the break in the fishing season persists into May or even June. Over the years, El Niño has come to be reserved for these exceptionally strong warm intervals that not only disrupt the normal lives of the fishermen, but also bring heavy rains.
As the warm surface water is pushed away from the coasts of Peru, colder, nutrient-rich water rushes up to take its place. The result is some of the coolest water found in the lower latitudes (sometimes dipping to 68 degrees F), and a plentiful plankton-filled feeding ground for the anchovy population on which Peruvians count on for much of their economic survival. In the west, the warm water that's pushed along raises sea levels. By the time winds reach Micronesia, the sea level has risen about three feet and the water has warmed about seven degrees F.
During the past 40 years, nine El Niño’s have affected the South American coast. Most of them raised water temperatures not only along the coast, but also at the Galapagos islands and in a belt stretching 5000 miles across the equatorial Pacific. The weaker events raised sea temperatures only one to two degrees Fahrenheit and had only minor impacts on South American fisheries. But the strong ones, like the El Niño of 1982-83, left an imprint, not only upon the local weather and marine life, but also on climatic conditions around the globe. Absolutely. In fact, it can happen about once every four to seven years - with varying intensity. And it all can be attributed to the same event: the El Niño/Southern Oscillation (ENSO).
El Niño is characterized by a dwindling, or sometimes even a reversal of trade winds. Normally the winds blow east to west across the southern Pacific. These winds travel along the surface of the sea and bring warm surface water along with them to the western coasts.
But during an El Niño, these trade winds relax, or even reverse, as was the case during the devastating 1982-1983 El Niño. Warm water sloshes back east in a vast, slow wave. Along the Peruvian coast, warm water builds up, driving the thermocline (the buffer zone between the upper layer of water and the frigid ocean below) down. The cooler, rich waters drop along with the thermocline, driving the anchovy population down with it, or killing off a large portion. (During the 1972 El Niño, the anchovy population dropped from 20 million to 2 million). This in turn reduces the number of marine birds who feed on the anchovy. The birds' excrement (guano) produces deposits on the islands off the Peru coast, which in the form of fertilizers is another important economic asset of Peru.
Among these consequences are increased rainfall across the southern tier of the US and in Peru, which has caused destructive flooding, and drought in the West Pacific, sometimes associated with devastating brush fires in Australia. Observations of conditions in the tropical Pacific are considered essential for the prediction of short term (a few months to 1 year) climate variations. To provide necessary data, NOAA operates a network of buoys which measure temperature, currents and winds in the equatorial band. These buoys daily transmit data which are available to researchers and forecasters around the world in real time.
In normal, non-El Niño conditions, the trade winds blow towards the west across the tropical Pacific. These winds pile up warm surface water in the west Pacific, so that the sea surface is about 1/2 meter higher at Indonesia than at Ecuador. The sea surface temperature is about 8 degrees C higher in the west, with cool temperatures off South America, due to an upwelling of cold water from deeper levels. This cold water is nutrient-rich, supporting high levels of primary productivity, diverse marine ecosystems, and major fisheries. Rainfall is found in rising air over the warmest water, and the east Pacific is relatively dry. The observations at 110 W (left diagram of 110 W conditions) show that the cool water (below about 17 degrees C, the black band in these plots) is within 50m of the surface.
This annual cycle in SSTs strongly affects the timing and eastward extent of tropical rainfall during the El Niño. As a typical El Niño develops, above-normal rainfall tends to extend eastward to just east of date line during September-November. The lack of rainfall farther east coincides with the minimum in SSTs across the eastern equatorial Pacific at this time of the year. As the El Niño strengthens during December-January tropical rainfall extends well east of the date line, and by March-April rainfall typically covers the entire eastern Pacific east of the date line. This period coincides with the annual maximum in SSTs across the eastern equatorial Pacific. This shift in the pattern of tropical precipitation weakens the large-scale monsoon circulation systems of Australia/ Southeast Asia, South America/Central America and Africa.
During El Niño (bottom panel of the schematic diagram), the trade winds relax in the central and western Pacific leading to a depression of the thermocline in the eastern Pacific, and an elevation of the thermocline in the west. The observations at 110W show, for example, that during 1982-1983, the 17-degree isotherm dropped to about 150m depth. This reduced the efficiency of upwelling to cool the surface and cut off the supply of nutrient rich thermocline water to the euphotic zone. The result was a rise in sea surface temperature and a drastic decline in primary productivity, the latter of which adversely affected higher trophic levels of the food chain, including commercial fisheries in this region. The weakening of easterly tradewinds during El Niño is evident in this figure as well. Rainfall follows the warm water eastward, with associated flooding in Peru and drought in Indonesia and Australia. The eastward displacement of the atmospheric heat source overlaying the warmest water results in large changes in the global atmospheric circulation, which in turn force changes in weather in regions far removed from the tropical Pacific.
La Niña, the "sister" of El Niño, also refers to a set of anomalous climate conditions in the tropical Pacific, but with anomalously cool SSTs, strong east-to-west trade winds, exceptionally heavy rainfall in usually rainy areas near the western Pacific, and very dry weather in usually dry areas near the eastern Pacific. In many ways, the climate anomalies associated with La Niña are opposite those that characterize El Niño. During La Niña episodes the equatorial sea surface temperatures (SSTs) are abnormally cold from the date line eastward to the west coast of South America, and tropical rainfall and convection tends to be focused over the western equatorial Pacific and Indonesia. Little rainfall is typically evident over the eastern equatorial Pacific, as SSTs remain well below 28oC in this region throughout the episode. This pattern represents an amplification of climatological mean conditions, which feature heavy rainfall across Indonesia and little-to-no rainfall over the eastern equatorial Pacific. This persistent pattern of tropical rainfall contributes to stronger than average monsoon systems over Australia/ Southeast Asia, South America/Central America and Africa.
The last strong La Niña event occurred in 1988-89. Historically, El Niño and La Niña events have usually alternated with a period of about 2-7 years. However, since the late 1970's El Niño years have outnumbered La Niña years by a factor of about 2 to 1. Below are two tables that show the documented years when El Niño and La Niña events occurred.
El Niño Years
1902-1903 1905-1906 1911-1912 1914-1915
1918-1919 1923-1924 1925-1926 1930-1931
1932-1933 1939-1940 1941-1942 1951-1952
1953-1954 1957-1958 1965-1966 1969-1970
1972-1973 1976-1977 1982-1983 1986-1987
1991-1992 1994-1995 1997-1998La Niña Years
1904-1905 1909-1910 1910-1911 1915-1916
1917-1918 1924-1925 1928-1929 1938-1939
1950-1951 1955-1956 1956-1957 1964-1965
1970-1971 1971-1972 1973-1974 1975-1976
1988-1989 1995-1996
