Paleo Slide Set: Coral Paleoclimatology
Pacific climate system during ENSO's two modes, cold or "normal," and warm.

What is an El Nino? In the strictest sense, an El Nino 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 Nino is the radical alteration of the entire Pacific climate system. Climatologists 
speak of El Nino as having two phases: a cool (or normal) phase and a warm phase (what a South 
American fisherman would consider an El Nino 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