La Niña

La Niña refers to anomalously cool central to eastern equatorial Pacific surface waters. These climate events occur every 2 to 7 yrs. (years) and last anywhere from about half a year to 3 yrs., with peak strength in the boreal winter season. La Niña is also often synonymous with the “high” phase of the Southern Oscillation, with the El Niño comprising the “low” phase. During the high phase, sea-level pressure is higher than normal over the Eastern Pacific (e.g., Tahiti) and lower than normal over the Maritime Continent (e.g., Darwin, Australia). La Niñas have a large impact on certain regional climates but have not been extensively studied in geography or the atmospheric sciences. While El Niño (“little boy”) has been part of the Peruvian vernacular for centuries, La Niña (“little girl”) was coined by George Philander in the mid 1980s, implying its opposite nature. This definition coincided with a La Niña event from [Page 1755]October 1984 to September 1985, which followed on the heels of the very strong 1982–1983 El Niño and marked the first cold period since the mid 1970s. Recent La Niña events, as defined by the National Oceanic and Atmospheric Administration, have occurred in 1988–1989, 1995–1996, 1998–2001, and 2007–2008.

La Niñas often follow strong El Niños. According to the delayed-oscillator theory, “warm” eastward-traveling Kelvin ocean waves and “cold” westward-traveling Rossby ocean waves are generated by westerly wind stress from the El Niño, and when these waves reflect at the boundaries of the Pacific and recross the ocean, their damped signal forms the subsequent La Niña. In general, La Niñas tend to be weaker than their climatological brothers, with the largest temperature anomalies reaching about −2.0 °C (Oceanic Niño Index). The basic climate system of the equatorial Pacific is warm, deep waters in the west and cool, shallow waters in the east with easterly surface winds. Since La Niña simply amplifies this base state, it is much more difficult to get strong anomalies than when there is a reversal of the system during an El Niño. La Niñas also affect the biogeography of the Pacific. The shoaling of the Eastern Pacific Ocean mixed layer is accompanied by vigorous upwelling. More nutrients are brought to the ocean surface during La Niña, causing widespread primary productivity blooms, which support a diverse marine ecosystem.

La Niñas change the global atmospheric circulation. While El Niños theromodynamically increase the instability of the tropical atmosphere, La Niñas concentrate convection over the Maritime Continent, which receives copious rainfall. Thus, the zonal atmospheric circulations, or Walker Circulation, are dynamically altered, with much of the Pacific dominated by subsidence and dry conditions. These changes in the Pacific are communicated to the rest of the globe through “teleconnections”—a combination of variations in the meridional Hadley Circulation, “atmospheric bridge” mechanisms, and atmospheric Rossby wave trains (Figure 1).

In boreal winter over the United States, the southern states are anomalously dry and warm, the northeast is wet and cool, a band from the Pacific coast of Alaska to the Northern Great Plains is cool, and the Ohio Valley is wet. Also in this season, northeast Brazil tends to experience wetter (cooler) conditions than normal. Japan and West Africa are anomalously cool, and southeast Africa is wet and cool. Conversely, parts of equatorial Africa are drier than normal. In boreal summers, there is no La Niña signal in the extratropical Northern Hemisphere, but West Africa remains cool, the Caribbean becomes cool and wet, and most of South Asia is cooler than normal, with India also becoming wetter than normal. Also in this season, the South Pacific island nations experience warm conditions, the west coast of South America is cool, and Argentina, Uruguay, and Southern Brazil become dry. Just as El Niño and La Niña are near mirror images in the Pacific Ocean, many of the teleconnections are seen as opposites as well. However, some scientists dispute this linearity assumption. Finally, generalities about the impacts of La Niña are problematic because every event is different. Figure 2 shows the evolution of global precipitation and temperature anomalies during the strongest La Niña in the satellite era.

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