No matter where you live, this has been a time of weather extremes. Fires in the western and southern U.S., drought in the southwest, rain in the northeast, and heat waves in various parts of the country have marked our warmest year on record so far.

The main culprit is the now familiar phenomenon known as La Niña. But while forecasters say her days are numbered and the pattern of record setting weather is drawing to a close, there are other forces at work heating things up.

Dog days of summer

The cool and rainy conditions this summer in the northeast may seem to be at odds with the fiery conditions in the west, but they’re all part of the same cycle. "Typically, whenever you get one extreme on one coast you get another extreme on the other coast, and this general band of heat in mid-latitudes is very closely linked with this La Niña," says Ants Leetma, director of the National Weather Services Climate Prediction Center.

The "string of pearls" image: NASA

This band of heat is one of several that stretch around the world, according to Leetma. "The bands aren’t necessarily continuous," he explains. "They’re almost like pearls on a necklace. Where you get a pearl you end up getting heat. In the in-between places is where you get cold."

Although La Niña normally lasts six months to a year, this particular one started in 1998. Now it’s finally showing signs of diminishing. "We’re looking at La Niña near neutral," says Leetma. "If you look at the United States as a whole, and you look at our forecast, I think the rainfalls are going to be more normal than the last three years."

Part of a larger trend

But even though La Niña seems to be diminishing, the other reason for the heat in many parts of the country won’t be going away any time soon. "Of course there is the overall background warming trend," says Leetma. "We can’t deny the fact that the planet is warming up slowly."

Global temperature change since 1880. image: NASA

Temperatures in the equatorial Pacific, which normally oscillate every 40 to 70 years, are warmer than normal, Leetma says. "It’s a little bit like a La Niña that doesn’t go away," he says.

Federal weather statistics show temperatures worldwide are rising more than one degree every one hundred years. That may be small by everyday standards, but it’s quite significant when viewed over the long term.

While the gradual process of global warming is ongoing, the disappearance of La Niña means that the weather this coming winter shouldn’t hold too many surprises. "All the indicators and the models are saying that this coming winter looks more normal," says Leetma.

Sibling Rivalry

La Niña and her counterpart El Niño are two extremes of a climate cycle known as El Niño/Southern Oscillation, or ENSO. While El Niño is characterized by a warming of the tropical Pacific, La Niña happens when cooler subsurface water is brought to the top.

The effects of both phenomena can be felt around the world, but they generate opposite weather patterns. So places that would be flooded during El Niño are plagued by drought during La Niña.

While the havoc wrought by El Niño is by now well-known—flooding in California and tornadoes in Florida are just two examples of the last one in 1997/98—scientists have only been studying La Niña since the 1980s.

Elsewhere on the web:

Climate Data Online

Answers to La Niña FAQ

Information on global warming from NOAA

Climate of 2000 from NOAA

Drought Monitoring from the Climate Prediction Center

Heat wave information from the National Weather Service

Global Climate Change

Map of U.S. drought severity

National Fire News

National Weather Service

Temperature extremes by state