

Ask Tom Why

October 15, 2006|By Tom Skilling

Dear Tom,

What's the difference between an El Nino and a La Nina?

Gregg McGinn

Dear Gregg,

El Nino is the name given to the warming of the equatorial waters of the central and eastern Pacific west of South America. It begins when the Pacific trade winds weaken, allowing the warm surface water in the western Pacific to spread eastward. The result is an altering of atmospheric circulation patterns that cause weather extremes in much of the world. Moderate to strong El Ninos tend to bring cool falls and mild winters to Chicago, so if the developing El Nino strengthens, it may telegraph the nature of the remainder of fall and upcoming winter here. La Nina, the opposite of El Nino, is caused by a strengthening of the easterly Pacific trade winds. This results in lower than normal equatorial sea surface temperatures west of South America due to upwelling of cold water by the increased off-shore flow.



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