is La Niña?
La Niña is characterized by unusually cold ocean temperatures in the Equatorial Pacific, compared to El Niño, which is characterized by unusually warm ocean temperatures in the Equatorial Pacific.
Shown below is the Reynolds sea surface temperature in the equatorial Pacific from Indonesia on the left to central America on the right (20N - 20S, 100E - 60W).
Normal Equatorial Pacific Ocean surface temperatures (December 1993) are shown in the middle panel, including cool water, called the 'cold tongue', in the Eastern Pacific (in blue, on the right of the plot) and warm water in the Western Pacific (in red, on the left). Strong La Niña conditions during December 1998 are shown in the top panel. The Eastern Pacific is cooler than usual, and the cool water extends farther westward than is usual (see the blue color extending further to the left). Strong El Niño conditions, in December 1997, are shown on the bottom panel, with warm water (red) extending all along the equator. El Niño and La Niña are opposite phases of the El Niño-Southern Oscillation (ENSO) cycle, with La Niña sometimes referred to as the cold phase of ENSO and El Niño as the warm phase of ENSO.
In the left hand panel, you see the sea surface temperature at the Equator in the Pacific Ocean (Indonesia is towards the left, South America is towards the right). Time is increasing downwards from 1986 at the top of the plot, to the present, at the bottom of the plot. The first thing to note is the blue "scallops" on the right of the plot, in the eastern Pacific. These indicate the cool water typically observed in the Eastern Pacific (called the "cold tongue"). Cold tongue temperatures vary seasonally, being warmest in the northern hemisphere springtime and coolest in the northern hemisphere fall. The red color on the left is the warm pool of water typically observed in the western Pacific Ocean. El Niño is an exaggeration of the usual seasonal cycle.
During the El Niño
in 1986-1987, you can see the warm water (red) penetrating eastward in
the Spring of 1987. There is another El Niño in 1991-1992, and
you can see the warm water penetrating towards the east in the northern
hemisphere spring of 1992. The 1997-1998 El Niño (at the bottom)
is unusually strong.
Notice the very cool water (blue), in the Eastern Pacific, in 1988-1989, and the somewhat less cool water in 1995. These are La Niña events, which occur after some (but not all) El Niños. Typically, El Niño occurs more frequently than La Niña. A list of El Niño and La Niña years is provided by the National Center for Environmental Prediction (NCEP).
El Niño and La Niña events vary in strength. For example, the La Niña in 1988 was stronger than the La Niña in 1995, and the 1997-1998 El Niño is unusually strong.
At higher latitudes, El Niño and La Niña are among a number of factors that influence climate. However, the impacts of El Niño and La Niña at these latitudes are most clearly seen in wintertime. In the continental US, during El Niño years, temperatures in the winter are warmer than normal in the North Central States, and cooler than normal in the Southeast and the Southwest. During a La Niña year, winter temperatures are warmer than normal in the Southeast and cooler than normal in the Northwest. See U.S. La Niña impacts from the National Weather Service. Also see this graphically in plots of temperature and rainfall anomalies in El Niño and La Niña years from Florida State University. An anomaly is the value observed during El Niño or La Niña subtracted from the value in a normal year.
If you have an MPEG animation viewer, and sufficient memory, you can view an animation which shows the changes in monthly sea surface temperature in the tropical Pacific Ocean. A Java animation is also available. Notice the weak La Niña peaking in December 1995, and the strong El Niño building in 1997. The animation is about 1 Megabyte in size. As you view this animation, you will see a weak La Niña peaking in December 1995. The bottom panel in the animation, labeled anomalies, shows how much the sea surface temperature for each month is different from the long term average for that month. The green color in the anomalies plot indicates that the temperature of the water is slightly cooler than is normal for that month. A strong El Niño is shown by the warm water spreading from the western Pacific to the eastern Pacific during 1997. The red color in the anomalies plot indicates that the temperature of the water is much warmer than is normal for that month.
El Niño was originally recognized by fisherman off the coast of South America as the appearance of unusually warm water in the Pacific ocean, occurring near the beginning of the year. El Niño means The Little Boy or Christ child in Spanish. This name was used for the tendency of the phenomenon to arrive around Christmas. La Niña means The Little Girl. La Niña is sometimes called El Viejo, anti-El Niño, or simply "a cold event" or "a cold episode". There has been a confusing range of uses for the terms El Niño, La Niña and ENSO by both the scientific community and the general public, which is clarified in this web page on definitions of the terms ENSO, Southern Oscillation Index, El Niño and La Niña. Also interesting is the Web page Where did the name El Niño come from?