El Niño and La Niña

DEFINITION: El Niño is a quasi-periodic abnormal warming of surface waters in the central and eastern tropical Pacific Ocean; La Niña is a quasi-periodic abnormal cooling of surface waters in the central and eastern tropical Pacific Ocean. Together, they are referred to as the El Niño-Southern Oscillation (ENSO).

The El Niño-Southern Oscillation climate pattern has implications for weather around the world. El Niño and La Niña events affect seasonal temperatures and precipitation patterns in many different regions and are associated with extreme weather such as heavy rainfall, floods, and droughts.

Ecuadoran and Peruvian fishermen gave the name El Niño to a warm, southward-flowing ocean current that would occur off the west coasts of their countries every year around Christmastime (El Niño is Spanish for “the little boy” or, more specifically, “the Christ child”). Originally used to describe a brief, localized, annual phenomenon, the term later became associated with unusually strong ocean warming in this area that occurred every few years, disrupting local fish and bird populations. “Anti-El Niño” ocean cooling events were called either La Niña (the little girl) or El Viejo (the old man).

The term “El Niño” has come to be associated with the large warm-water anomalies covering extensive portions of the tropical Pacific Ocean off the coast of Latin America that persist for many months, and with the related weather effects noted around the globe. It has been known for many years that when warm water appears off the coast of Peru, atmospheric pressure drops over the eastern Pacific and rises over Australia and the Indian Ocean. Because of this relationship, major El Niño events are usually associated with other global weather phenomena, including drought in Africa and Australia and the failure of the Indian monsoon. Scientists know the atmospheric component of this global pattern as the Southern Oscillation. The coupled global oceanic-atmospheric system is called El Niño-Southern Oscillation (ENSO). El Niño is sometimes called an ENSO warm event or the warm phase of ENSO; similarly, La Niña may be referred to as an ENSO cold event or the cold phase of ENSO.

Typically, El Niño and La Niña events occur every three to five years, although the interval may vary from two to seven years. El Niño events tend to last nine to twelve months but have been known to last as long as two, three, or even four years. The typical La Niña event lasts one to three years. Both El Niño and La Niña conditions typically develop during the months of March through June, reach peak intensity during the months of December through April, and diminish during the months of May through July.

Causes and Prediction

When an El Niño condition develops in the eastern Pacific, the sea surface temperature (SST) and rainfall in the eastern tropical Pacific are at their seasonal peaks. Major El Niño occurrences are closely tied to global weather patterns and the circulation of currents in the Pacific Ocean. Variations in Indian monsoon circulation sometimes precede variations in the Southern Oscillation, indicating that there is a possible feedback mechanism linking these phenomena. The period of the Southern Oscillation is irregular, with a return period of about three to four years, so about two El Niños occur per decade. The amplitude of the Southern Oscillation is highly irregular. If some global atmospheric perturbation contributes to the amplitude of the Southern Oscillation while an El Niño is developing, a major El Niño might be expected to occur. However, if a global perturbation subtracts from the amplitude of the Southern Oscillation, the El Niño might be weak.

The point at which scientists decide that a major El Niño condition is occurring has been historically contentious. A network of buoys has been established to augment satellite monitoring of sea surface temperatures in the Pacific Ocean. This network played a key role in the early detection of the 1997–98 El Niño. When sea surface temperatures reach 3 to 5 degrees Celsius (5.4 to 9.0 degrees Fahrenheit) above normal for the season in the eastern equatorial Pacific, scientists can be fairly certain that an El Niño is occurring. The classic El Niño begins off the coast of Peru, slowly propagating westward. Since the 1990s, however, a new type of El Niño has been observed in which the maximum ocean warming occurs instead in the central-equatorial Pacific. Known variously as a central Pacific El Niño, dateline El Niño, warm-pool El Niño, or El Niño Modoki, this type of event occurred in 1991–92, 1994–95, 2002–03, 2004–05, and 2009–10, for example.

During a major El Niño, the normal westerly trade winds subside, and the height of the eastern Pacific sea surface rises. This is coupled with a decline in the height of the western Pacific sea surface, which sometimes causes normally submerged coral reefs in the western Pacific to appear above the ocean surface.

Since historical recording of El Niño events began, long-term variations in their strength have been observed. The 1920s and 1930s experienced only weak El Niño events. In contrast, the El Niños of the 1980s and 1990s were generally moderate to strong. El Niño conditions result from a complex interplay of atmospheric and oceanic forces, and the reasons for the waxing and waning in strength of El Niño events over periods of decades are not understood.

Some scientists have noted that unusual El Niños have followed volcanic eruptions. A major volcanic eruption causes the formation of a stratospheric aerosol layer, which may lead to an increase in solar radiation being reflected back into space. The 1951 El Niño followed the eruption of Mount Lamington in Papua New Guinea; the 1982 El Niño followed the eruptions of El Chichon in Mexico and Galunggung in Indonesia; the 1991 El Niño followed the eruption of Mount Pinatubo in the Philippines. However, while 1951 was a weak El Niño year, both 1982 and 1991 were strong. Although it is tempting to focus on a single parameter such as sea surface temperatures or the period of the Southern Oscillation as a predictor of El Niño, history shows that many factors, both atmospheric and oceanic, contribute to the development of a strong El Niño event.

Environmental Consequences

When unusually warm water appears off the coast of Peru, the local anchovy fishing industry falters. Sport fishing off Baja California, California, and Oregon, in contrast, enjoys a boom, as marlin and other highly prized fish usually found in more tropical southern waters move north. Other marine ecosystems around the globe may also be affected by El Niño conditions. Factors such as increased sea surface temperature, decreased sea level, and salinity changes related to high rainfall affect the algae that protect coral reefs, causing the coral to bleach and die. This, in turn, has negative impacts on fish and plant life within reef ecosystems. The 1997–98 El Niño event was marked by coral reef bleaching in the Indo-Pacific region, the Caribbean, and the Florida Keys. Extensive coral bleaching also occurred around the globe in association with the 2009–10 El Niño.

During El Niño episodes, the intertropical convergence zone, a band of major tropical convection circling the globe, moves southward. This southward shift in precipitation patterns causes torrential rains in some places that are normally dry and dry conditions in places that are usually wet. In the Galápagos Islands, El Niño brings much higher than normal precipitation in March, April, and May. During major El Niños, Peru and Ecuador experience torrential rains and flooding. In Guayaquil, Ecuador, El Niño was blamed for causing more than 3 meters (9.8 feet) of rain between October 1982, and January 1983. During the 1997–98 El Niño, severe flooding occurred in Ecuador along rivers where rain forests had been cleared to establish shrimp farms. By contrast, the moderate El Niño of 2002–03 had little effect on the weather of these two countries.

During many major El Niño events, countries bordering the western Pacific and Indian oceans experience droughts. In El Niño years, India often receives lower-than-normal rainfall, while Sri Lanka’s rainfall tends to be unusually high. During the 1982–83 El Niño, Indonesia and Australia were stricken by drought. Early in the 1990s, southern Africa experienced its worst drought of the twentieth century, probably worsened by the El Niño that began in late 1991. Curiously, the Australian drought associated with the strong 1997–98 El Niño was not as severe as the one that accompanied the moderate 2002–03 El Niño. The 2015–16 El Niño, one of the strongest on record, led to severe droughts in Australia and Indonesia. El Niño years in Japan are associated with mild winters, cool summers, and lengthy rainy seasons.

Many diverse ecological, environmental, and economic events throughout the world are often attributed to El Niño occurrences. Sometimes these events may indeed be related to El Niño, but some occurrences can be attributed to other factors. Sometimes conflicting claims are made about the effects of El Niños. Just as there is no consistent relationship between the failure of the Indian monsoon and El Niño years, other claims about El Niño and weather may hold up during a statistically significant number of years but not in all years.

Although the 1997 El Niño was credited with causing the unusually mild winter of 1997–98 in the eastern United States, the 1976 El Niño was blamed for causing extreme cold in the same region in December 1976 and January 1977. The Sonoran Desert of Arizona and California tends to be wet in El Niño years. The Florida drought of 1998 was attributed to El Niño, although the warm-water anomaly off Peru had virtually disappeared by July, when forest fires were plaguing Florida. Because Texas often experiences more precipitation during the growing season after an El Niño, farmers planted crops requiring more moisture than usual in 1998. The summer of 1998 was unusually dry in Texas and Oklahoma, however, leaving many of the affected farmers to conclude that El Niño-based forecasts might be less than reliable. The failure of the Soviet harvest in 1972 was attributed by some to El Niño. In El Niño years, Moscow frequently experiences very cold winters; December 1997 fit into this pattern.

The years 2014 and 2015 both set new global temperature records, with many scientists suggesting that a record-strength El Niño played a significant role in 2015 and that, together with climate change, ENSO would continue to increase global average temperatures in the near future. However, most scientists also noted that while El Niño certainly contributes to rising temperatures when active, it is mostly a relatively short-term phenomenon, and the larger trend of global warming is chiefly to blame for the overall major temperature surge. Ongoing research explores the interconnection between ENSO and global warming, a topic that often generates controversy. Research has suggested that climate change causes more powerful and volatile ENSO effects and therefore increases the likelihood that the weather patterns will damage ecosystems. El Niño events have also increased in frequency during the early twenty-first century, which has likewise been attributed to climate change.

La Niña

As the warm-water anomalies in the eastern Pacific fade, scientists know that El Niño is ending. When a large body of colder-than-average water establishes itself off the coast of Peru, along with a strong ridge of high pressure, meteorologists announce that a La Niña event is occurring. A La Niña may follow closely on the heels of an El Niño, as was the case with the 2010–11 La Niña, or occur after a year or two of neutral conditions. During a La Niña, the Pacific sea surface height in the eastern Pacific is measurably lower than when an El Niño is occurring. The westerly trade winds are also stronger than normal. During a La Niña, the average temperature of the tropical troposphere may be 1 degree Celsius (1.8 degrees Fahrenheit) lower than during an El Niño. La Niñas tend to be much more variable in strength than El Niños.

During periods when the waters of the eastern Pacific have been observed to be anomalously cold, the Pacific Northwest tends to be wetter and cooler than normal, especially during winter. During a La Niña year, winter temperatures in the southern and southeastern United States are often warmer than normal. In the United States, some link La Niña years to very hot summers; the summer of 1988, which was very hot and dry, is the prototype summer for this weather phenomenon. Tropical cyclones are more common on the northern Australian coast during La Niña events. Widespread flooding in eastern Australia tends to occur early in the calendar year (late summer) during La Niñas. The 2020–21 La Niña, for instance, brought cooler and wetter conditions to Australia, leading to significant flooding. La Niña conditions also tend to be accompanied by more damaging Atlantic hurricanes, while a reduction in hurricane activity is typically associated with El Niño. This was the case during the 2020–21 La Niña, which resulted in one of the most active Atlantic hurricane seasons on record.

Bibliography

Changnon, Stanley Alcide, ed. El Niño, 1997-1998: The Climate Event of the Century. Oxford UP, 2000.

Clarke, Allan J. An Introduction to the Dynamics of El Niño and the Southern Oscillation. Academic Press, 2008.

Cropper, Thomas. "Did El Niño Drive the Record Heat of 2015?" Niskanen Center. Niskanen Center, 25 Jan. 2016. Web. 1 Feb. 2016.

D’Aleo, Joseph S., and Pamela G. Grube. The Oryx Resource Guide to El Niño and La Niña. Oryx Press, 2002.

Glantz, Michael H. Currents of Change: Impacts of El Niño and La Niña on Climate and Society. 2d ed. Cambridge UP, 2001.

Glantz, Michael H. La Niña and Its Impacts: Facts and Speculation. United Nations UP, 2002.

"NASA, NOAA Analyses Reveal Record-Shattering Global Warm Temperatures in 2015." Goddard Institute for Space Studies. NASA, 20 Jan. 2016. Web. 1 Feb. 2016.

Philander, S. George. Our Affair with El Niño: How We Transformed an Enchanting Peruvian Current into a Global Climate Hazard. Princeton UP, 2006.

Rosenzweig, Cynthia, and Daniel Hillel. Climate Variability and the Global Harvest: Impacts of El Niño and Other Oscillations on Agroecosystems. Oxford UP, 2008.

"What Are El Niño and La Niña?" National Ocean Service, National Oceanic and Atmospheric Administration, 16 June 2024, oceanservice.noaa.gov/facts/ninonina.html. Accessed 24 Jan. 2025.