Much of the weather-related attention in the United States focuses on the Golden State, where a series of storms this week have dumped between 8 and 10 inches of rain in the mountains around Los Angeles. States of emergency were declared in five counties – three in southern California.
But forecasters note that the effects of El Niño and its La Niña counterpart, which constitute a see-sawing climate phenomenon in the tropical Pacific, touch virtually every continent.
Its influence on global weather patterns on seasonal time scales "far dwarfs everything else we've got," says Mike Halpert, deputy director of the National Oceanic and Atmospheric Administration's Climate Prediction Center in Camp Springs, Md.
This El Niño's ranking: top third for strength
As measured by NOAA criteria, the current El Niño, which arrived last July, has nudged its way into the "strong" category, Mr. Halpert says. Although it is far weaker than similar events in 1982-83 and 1997-98, the current El Niño's intensity places it among the top third of all El Niños since 1950.
El Niño is an unusually deep warm pool of water that builds in the tropical western Pacific, then migrates east toward the west coast of South America. Warm, moist air rising off that pool triggers the formation of thunderstorms, whose strong updrafts represent a crucial part of the atmosphere's heat pump –drawing warm air from the tropics up and toward the poles, where it cools, sinks, and circulates back toward the equator.
The change in location for that vigorous, widely spread region of convection from west to east in turn alters atmospheric circulation patterns. As the circulation patterns shift, so does the distribution of heat and moisture.
These so-called teleconnections are the strongest in the tropics, says Tony Barnston, lead climate forecaster at the International Research Institute for Climate and Society at Columbia University's Earth Institute in Palisades, N.Y. They are somewhat weaker across the US.
Jet stream becomes more southerly
Still, as Southland residents can attest, El Niño's influence can be significant. That influence has less to do with the strength and number of storms, Mr. Barnston says, than it does with where storms make landfall along the Pacific coast.
He explains that a storm-steering river of high-altitude wind called the jet stream crosses North America farther north, on average, in the absence of El Niño. Much of the weather recently striking the Southland would be plowing into the Pacific Northwest instead.
But El Niño's influence pulls the average path of the jet stream to the south, drawing more storms than usual across the southernmost tier of the US. While these regions get a dousing, others – such as the Rocky Mountain states, the upper Great Plains, and much of the Midwest – don't receive as much precipitation as usual. And temperatures flip, with much of the southern tier colder than normal, while states such as Montana, the Dakotas, and Minnesota are warmer than normal (although still frosty).
In the southern hemisphere, where it's now late summer, El Niño brings drier-than-normal conditions to much of southern Africa and northeastern Brazil. One of the hardest-hit regions spans Indonesia and northern and eastern Australia, where precipitation can fall well below normal.
One result: an increase in wildfires in Indonesia, with smoke overspreading much of Southeast Asia. Many of Indonesia's wildfires start after farmers clear forests to grow crops, exposing layers of peat that become tinder-dry. In 2007, scientists working with NASA's Aqua satellite reported finding a strong correlation between spikes in Indonesian wildfires and the peak of El Niño events between 2000 and 2006.
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