In most years, the trade winds at the equator blow from east to west, pushing the warm waters of the tropical Pacific Ocean towards the shores of Indonesia and leaving cool water from the ocean depths to well up along the coast of Peru.

But every two to seven years, the trade winds drop or even change direction and push the warm water back toward South America, causing sea surface temperatures in the central and eastern tropical Pacific to rise by several degrees.

That's the phenomenon called El Niño, and its effects on weather patterns are felt tens of thousands of kilometres away from the tropics -- even in the Yukon.

This year's El Niño has already increased sea surface temperatures off the coasts of Ecuador and Peru by about five degrees Celsius, making it the strongest El Niño in the past 50 years.

During an El Niño year, winter temperatures are usually warmer than normal across western Canada and Alaska all the way to the eastern Aleutian Islands, says Eric Taylor of Environment Canada's Aquatic and Atmospheric Science Division in Vancouver. Although the El Niño phenomenon might last up to a year and a half, its effects on northern areas are generally noticeable only in winter.

Environment Canada has compiled data on the six strongest El Niños since 1950. In three of those six years, the average temperature between Nov. 1 and Mar. 31 in the southern Yukon was significantly warmer than usual, says Taylor.

In the warmest El Niño winter in the past 50 years, the average temperature in Whitehorse was -8.5 degrees C., compared with a normal winter average of -13 degrees.

However, Yukoners can't count on a warm winter in an El Niño year, warns Taylor. Two of the six years Environment Canada documented were about average in temperature, and in one the winter was colder than average.

"It's just the chaotic nature of the atmosphere," Taylor says. "El Niño has an effect but it's not the primary effect."

Besides its impact on temperatures, El Niño often affects precipitation in the Yukon. In parts of the Yukon, precipitation records go back to 1900, and they generally show less precipitation in El Niño years, says Taylor.

Of the six strongest El Niño years since 1950, four were quite a bit drier than normal -- anywhere from 18 to 42 millimetres less than the southern Yukon's average precipitation of 83 millimetres between Nov. 1 and Mar. 31.

"That said, precipitation is such a fickle thing," says Taylor. "In one El Niño year, 1990-91, the southern Yukon had 33 millimetres more precipitation than normal."

Predicting the effects of an El Niño is a matter of looking at probabilities, Taylor says. Most of the time, an El Niño will bring the Yukon a warmer and drier winter. But it won't always be warmer and drier.

Even if the winter is warmer on average, that doesn't mean there won't be cold spells, Taylor cautions.

"Usually it means quite a few more days above normal, and that pulls the average up."

The major difficulty in predicting the effects of an El Niño arises from the fact that so many other factors influence general weather patterns. While El Niño is a powerful influence, Taylor says, the other factors combined are even more powerful.

"The atmosphere is one big fluid," he says. "It's like when you drop a glass of water on the floor. It's very difficult to say where every drop of water will go."

For more information about El Niño and climate, contact Environment Canada, Whitehorse, or check out Environment Canada's El Niño web site at http://www.msc-smc.ec.gc.ca/education/elnino/.