La Nina System on Tap?

Water temperatures and sea surface levels indicate that a visit from La Niña, El Niño's sister, may be on the way.

As water warms, it expands. When it cools, it contracts. These actions cause sea surface levels to rise and fall, and the changes tell scientists about the ocean's temperature.

La Niña is characterized by a cooling of equatorial waters in the Pacific, which affects weather over most of the ocean. As 2005 came to an end, sea-surface height changes in the tropical Pacific exhibited characteristics of a developing La Niña.

This image, produced from measurements taken by the U.S.-French Jason satellite, shows sea surface height relative to normal ocean conditions on Dec. 31, 2005, and reveals a continuation of the gradual cooling of the central equatorial Pacific in the last few months of 2005.

The white areas indicate sea surface height between 6 to 13 inches (14 and 32 centimeters) above normal. The red areas indicate a height about 4 inches (10 centimeters) above normal. The green areas indicate normal conditions. The purple areas are 6 to 7 inches (14 to 18 centimeters) below normal. The blue areas are 2 to 5 inches (5 to 13 centimeters) below normal.

La Niña is essentially the opposite of El Niño. During La Niña, the trade winds--easterly winds that blow through the tropics and subtropics--are stronger than normal. Likewise, the cold water that normally exists along the coast of South America carries into the central equatorial Pacific. La Niña reappears every three to five years. At the beginning of 2006, scientists were still unsure whether this cold pool would evolve into a long-lasting, strong La Niña condition.

--Bjorn Carey

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Credit: NASA/JPL Ocean Surface Topography Team