What is La Niña?

From The National Center for Atmospheric Research and The Tampa Tribune.

A La Niña effect may be defined as a drop in average sea-surface temperatures to more than 0.4 degrees C (0.7 degrees F) below normal, lasting at least six months, across a specified part of the eastern tropical Pacific (5 N- 5 S latitude, 120-170 W longitude).

When La Niña forms, the hurricane season is affected as the cooling water creates dramatic changes in the upper-level air currents that play a major role in storm development.

Though El Niño caused worldwide weather problems, it also snuffed out hurricanes. Winds 35,000 to 45,000 feet in the atmosphere shifted to come from the west, basically shearing the tops from developing storms.

During La Niña, high-level westerly winds either weaken or shift to come from the east, allowing more storms to develop, said Jerry Jarrel, director of the National Hurricane Center in Miami.

The 1995 and 1996 hurricane seasons, sandwiching the last La Niña, were the most active back-to-back seasons on record, combining for 20 hurricanes, said Christopher Landsea with the Hurricane Research Division of the National Oceanographic and Atmospheric Administration.

In 1957, 1965 and 1991, El Niños disappeared rapidly without La Niñas forming. However, in 1969, 1972 and 1987, La Niñas were in place by July.

Even if it is too late to affect a hurricane season, La Niña can have a profound impact on fall and winter weather in Florida and the United States. During one, Florida can expect winter - already a dry season - to be warmer and drier.

``It will be drier with fewer cold fronts and more sunshine,'' Landsea said. ``Four or five months with no rain could have a dramatic effect on the potential for fires.''

Some cold fronts still could wander into the state even if temperatures are warmer on average, as expected, Kousky said. ``There will be a lot more variability this winter. Chances are you will experience some colder periods than you had last year.''

The northern part of the country can expect a colder winter, he added, while the Midwest can expect less rain or snow. Droughts struck that region in 1988-89 and 1995-96 following La Niñas.

``It accentuates the normal pattern,'' Kousky said. ``Areas that are cold will be colder. Areas that are warm will be warmer.''

La Niña conditions may persist for as long as two years.

Frequently asked questions

What's the difference between La Niña and El Niño?
Both terms refer to large-scale changes in sea-surface temperature across the eastern tropical Pacific. Usually, sea-surface readings off South America's west coast range from the 60s to 70s F, while they exceed 80 degrees F in the "warm pool" located in the central and western Pacific.

This warm pool expands to cover the tropics during El Niño, but during La Niña, the easterly trade winds strengthen, cold upwelling off Peru and Ecuador intensifies, and sea-surface temperatures there fall as much as 7 degrees F below normal. Like its counterpart, La Niña tends to peak during the Northern Hemisphere winter. El Niños were present 31% of the time and La Niñas 23% of the time from 1950 to 1997.

What are La Niña's main weather and climate impacts worldwide and across the United States?
In many locations, especially in the tropics, La Niña produces the opposite kinds of climate variations from El Niño.

For instance, parts of Australia and Indonesia are prone to drought during El Niño but are typically wetter than normal during La Niña. However, there are also more complex U.S. effects due to the influence of the North Atlantic oscillation and other climate variables.

Hurricanes:
Hurricanes are more likely to form across the Atlantic Ocean and Gulf of Mexico during La Niña than El Niño. In fact, the Atlantic's two busiest back-to-back seasons on record -- 1995 and 1996 -- occurred on either side of the last La Niña.

Tornadoes:
Despite the intense, frequent tornadoes during this past El Niño spring across the South and East, some research has shown that outbreaks of violent tornadoes east of the Mississippi River are actually more likely during springs that follow La Niña. Examples include the Jumbo Outbreak of April 3-4, 1974, which produced a record 148 tornadoes; the Palm Sunday tornadoes of 1965; and the 76 killer tornadoes (a record) that occurred in 1909.

Precipitation:
The southern and central United States tend to be drier than normal during a La Niña winter. Major droughts accompanied the last two La Niñas in the Midwest (1988-89) and southern plains (1995-96). The Pacific Northwest tends to be wetter during La Niña than El Niño.

Temperature:
On average, north-south temperature contrasts are increased during La Niña winters. Cooler than normal conditions become more likely across the Northwest and warmer than normal conditions across the South and East.

Why haven't we heard much about La Niña before now?
Partly because La Niña's effects on the immediate coast of South America are benign rather than destructive, the phenomenon wasn't recognized until much later than El Niño.

Research on La Niña (Spanish for "the girl") began only after the phenomenon was recognized and named in the 1980s. It is also called El Viejo ("the old man") or the anti-El Niño.

When have La Niñas occurred?
The answer varies depending on the definition used.

According to the National Centers for Environmental Prediction, this century's previous La Niñas began in 1904, 1908, 1910, 1916, 1924, 1928, 1938, 1950, 1955, 1964, 1970, 1973, 1975, 1988, and 1995. These events typically continued into the following spring. Since 1975, La Niñas have been only half as frequent as El Niños.

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