What is El

Every few years the temperature of the surface waters in the eastern tropical Pacific rises quite significantly. The strongest warming takes place between about 5° S and 5° N, and from the west coast of Peru (a longitude of about 80° W) almost to the dateline, at 180° W, as illustrated in figure 6.3. The warming is significant, with a difference in temperature up to 6°C from an El Niño year to a non-El Niño year. The warmings occur rather irregularly, but typically the interval between warmings ranges from three to seven years, as illustrated in figure 6.4.

The warmings have become known as El Niño events, or even (with a little violence to the Spanish language) El Niños. The name derives from the fact that the warm waters off Peru appear at about christmastime, and the name el Niño is Spanish for the Christ child.3 The warmings typically last for up to a year, sometimes two, and appear as an enhancement to the seasonal cycle, with high temperatures appearing at a time when the waters are already warming. Although there is no universally

(a) Normal (December 1996)

(a) Normal (December 1996)

(b) El Nino (December 1997)
(b)-(a)

Figure 6.3. The sea-surface temperature in December of a normal (i.e., non-El Niño) year (December 1996, top panel); in a strong El Niño year (December 1997, middle panel); and their difference (bottom panel). A normal year is characterized by a cold tongue of water in the eastern tropical Pacific, which disappears in El Niño years.5

Figure 6.3. The sea-surface temperature in December of a normal (i.e., non-El Niño) year (December 1996, top panel); in a strong El Niño year (December 1997, middle panel); and their difference (bottom panel). A normal year is characterized by a cold tongue of water in the eastern tropical Pacific, which disappears in El Niño years.5

Figure 6.4 Top: A time series of the sea-surface temperature (SST) in the eastern equatorial Pacific region (specifically, in the so-called Niño 3 region). The spiky curve shows the annual means, and the dots represent Decembers. The smoother curve shows the SST after the application of a 20-year low-pass filter, and the top of the gray bar is the 1876-1975 mean. Bottom: A similar plot for the negative of the Southern Oscillation Index (SOI), the anomalous pressure difference between Tahiti and Darwin. Particularly large El Niño events can be seen in 1877-78, 1982-83, and 1997-98.6

Figure 6.4 Top: A time series of the sea-surface temperature (SST) in the eastern equatorial Pacific region (specifically, in the so-called Niño 3 region). The spiky curve shows the annual means, and the dots represent Decembers. The smoother curve shows the SST after the application of a 20-year low-pass filter, and the top of the gray bar is the 1876-1975 mean. Bottom: A similar plot for the negative of the Southern Oscillation Index (SOI), the anomalous pressure difference between Tahiti and Darwin. Particularly large El Niño events can be seen in 1877-78, 1982-83, and 1997-98.6

agreed-upon definition of an El Niño, an event is often regarded as having occurred when there is a warming of at least 0.5°C averaged over the eastern tropical Pacific lasting for six months or more.4 Rarely can a year be described as truly normal; rather, the ocean temperatures tend to fluctuate between warm El Niño years and years in which the equatorial ocean temperatures are colder in the east and warmer in the west, with those years that are particularly anomalous this way having become known as La Niña events (la niña, without capitalization, is Spanish for young girl; there is no female equivalent of the Christ child, el Niño in Spanish). We have direct observational evidence—that is, compilations of measurements of the sea-surface temperature from ships and buoys—of El Niño events for more than a century, but the events have almost certainly gone on for a much longer time, perhaps millennia or longer. We know this through a variety of proxy records; tree rings provide some of the most detailed information. As we will discuss later on, El Niño events bring anomalous rainfall and temperature throughout the equatorial Pacific and western North America, affecting the tree-ring characteristics and providing convincing evidence of the occurrence of El Niño events for the past several hundred years.

Corals also provide a good record of El Niño events because they have skeletal growth bands that, rather like tree rings, provide an accurate annual chronology. The isotopic composition of oxygen contained in the skeletons responds to both SST and rainfall, thus providing a record of El Niño that goes far into the past. Indeed, one recent analysis of corals from Papua New Guinea suggests that El Niño events have occurred for the past 130,000 years—that is, even over the last ice age!7

Our knowledge of El Niño seems to have begun with observations of the SST off the coast of Peru, but it is now understood that the phenomenon itself is Pacific-wide and also involves the atmosphere. Observations of the winds and the surface pressure in the equatorial Pacific show that these tend to covary with the SST. In particular, during El Niño events the equatorial Pacific trade winds, which normally blow toward the west, become much weaker and may even reverse. One useful measure is the pressure difference between Darwin, at the far north of Australia (12° S, 130° E), and Tahiti (17° S, 150° W), an island in the Pacific; the record of this difference is known as the Southern Oscillation. (There is nothing truly special about these locations vis-à-vis El Niño, but pressure measurements have been made there for a long time.) We see in figure 6.4 that the Southern oscillation and the SST record of El Niño are highly correlated.

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