| What is an El Niņo?
In the strictest sense, an El Niño is the appearance of unusually
warm waters (named for the Christ-Child) in the eastern Pacific around Christmas-time.
In a broader sense, however, an El Niño is the radical alteration
of the entire Pacific climate
system. Climatologists speak of El Niño as having two phases: a cool
(or normal) phase and a warm phase (what a South American fisherman would
consider an El Niño event). In a cool phase, strong southeasterly
trade winds push eastern Pacific surface waters westward, allowing cool
nutrient-rich bottom waters to upwell or come to the surface. These waters
are some of the most productive in the world, supporting enormous plankton
and fish populations. The central Pacific is extremely dry during cool phases;
Kiritimati (Christmas) Island and its neighbors receive less than 20 cm
(8 inches) of rain most years and are truly desert islands. The western
Pacific during cool phases is typified by two features: a pool of extremely
warm water stretching eastward to about 170 degrees W, and an accompanying
belt of low pressure and high precipitation known as the Indonesian
Low that covers portions of Asia, Oceania and Australia. Another belt
of high precipitation known as the Intertropical
Convergence Zone or ITCZ lies several degrees north of the Equator and
east of the International Date Line.
In the warm phase, the trade winds weaken or even reverse, and less eastern
Pacific surface water is pushed westward. Nutrient pumping in the eastern
Pacific is curtailed as less nutrient-rich bottom water reaches the surface,
causing fish populations to decline precipitously. Warm waters spread
across the Pacific, pushing sea
surface temperatures (SST's) up by 3-5 degrees C in the Galapagos
Islands. The Intertropical Convergence Zone moves south and west, while
the Indonesian Low follows the warmer waters east. Kiritimati Island,
once dry as a bone, is deluged with 50-125 cm (20-50 inches) of rain a
year during warm events. Barometric pressure in Darwin, Australia rises
as higher pressure replaces the Indonesian Low. During particularly severe
warm events, winds in the western Pacific actually reverse from their
usual easterly direction to become mild westerlies. In short, the differences
between warm and cool phases of ENSO are often as clear as night and day.
Photo Credits:
Thomas.G. Andrews
NOAA Paleoclimatology Program
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