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Kevin E. Trenberth, Head, Climate Analysis Section, US National Center for Atmospheric Research
What is El Niño? Why is it often blamed for everything from hurricanes to droughts, forest fires, floods and even the Ice Storm of 1998? How could human-forced climate change make it worse, possibly to the point of upsetting global climate patterns?
How to Make an El Niño
In essence, an El Niño event happens when an enormous puddle of warm water, which normally sits near Indonesia, sloshes across the Pacific to take up residence around Peru. For centuries, Peruvians have known El Niño to recur every three to seven years, rarely lasting longer than one year each. Usually they were followed by a strong cooling of the equatorial Pacific, a complementary phase termed "La Niña."
These three images illustrate how seawater temperatures in a slice through the equatorial Pacific changed during the large 1997-1998 El Niño and La Niña. Warm water is represented by red-orange, while cooler water is blue-green.
A World of Grief
El Niño impacts are far-reaching, affecting temperature and precipitation as well as generating extreme weather events worldwide. The 1997-98 El Niño has been implicated in many disasters (see box), while the 1998 La Niña followed up with her own, including floods in Bangladesh and China which displaced 230 million people.
El Niño in North America
While El Niño is sitting off the west coast of the Americas, it shifts the flow of air over North America, altering weather patterns across the continent. Notice the small jetstream which forms over the southern USA.
In Canada this produces weather significantly drier and warmer than normal, with more frequent droughts in the summer. The southern jetstream produced by the 97/98 El Niño set the stage for the ice storm that devastated eastern Canada. During a La Niña the air flow pattern relaxes, and abnormally cool, wet conditions prevail from BC to southern Quebec.
The Changing Face of El Niño
Although it is impossible to predict the effects of global warming on the frequency of El Niños, all indications seem to be that they are becoming stronger, more common, and are no longer disappearing completely.
In other words, the Pacific doesn't seem to be reverting to "normal" any more.
Detailed statistical analyses show that the increase in frequency of El Niños observed since the 1970s is very unlikely to be part of a natural cycle (e.g. Trenberth and Hoar, Geophysical Research Letters, Dec 1, 1997). It has been estimated that the observed pattern is 99.9% likely to be due to human induced climate change.
The emerging pattern is one dominated by El Niño conditions, with each event lasting for a few years, interspersed with intense La Niñas. In Canada this would mean - in general - prevalent warm, dry, drought-prone conditions punctuated with wet, flood-producing years.
However, it is also possible that climate change will entail a shift to an entirely different circulation pattern in the Pacific, away from the El Niño/La Niña cycle as we've known it historically. The consequences of such a profound and sudden change are currently unpredictable, but would certainly wreak havoc with the world's weather patterns.
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