NewsMay 27, 2011

Looking to the Tropics for Drought Relief

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As La Niña subsides and hurricane season begins, will drought-stricken states finally get relief?

By Tom Yulsman

One usually doesn’t cast a hopeful eye to the tropics during hurricane season. Yet that’s precisely what National Oceanic and Atmospheric Administration (NOAA) scientists did earlier this week during a briefing on the deadly and expensive drought afflicting much of the southern United States.

Texas and New Mexico have been hit particularly hard, with upwards of 90 percent of all counties in severe to exceptional drought (the latter being the most dire category). From January through April, New Mexico averaged a scant 0.79 inches of precipitation, making this the third-driest such period on record. And over the last month, the state has received less than 5 percent of its normal precipitation.

In the Lone Star State, 34 fires scorched nearly 62,000 acres in just the seven-day period ending May 27 — a follow on from a series of blazes in April that destroyed more than 400 homes. Austin has seen more than its share of woes. The elevation of Lake Travis, from which the city draws its drinking water, is 20 feet below average for this time of year, says Kris Wilson, a former television meteorologist and current senior lecturer in journalism at the University of Texas. In April, he came perilously close to losing his home in a wildfire. 

Flames burn out of control at Possum Kingdom Lake on April 15, 2011. Credit: SSG Malcolm McClendon, Texas National Guard/flickr.

“I’ve never seen it quite so dry as it has been,” he says. “And it’s not done, I don’t think.”

During their briefing, NOAA scientists agreed. While there have been some pockets of relief, “drought has intensified since late April in Texas, New Mexico and Oklahoma,” said Victor Murphy, climate program manager for NOAA’s National Weather Service Southern Region. And dry conditions are expected to continue in Texas for at least one more month, and possibly more. 

Murphy says drought conditions could begin to ease in New Mexico by mid-June, when the annual summer monsoon typically begins. And Texas could eventually get some relief during this summer’s hurricane season, which starts June 1.

To be sure, he’s not hoping for devastating hurricanes — “just areas of disturbed weather, or weak tropical storms, that drop a lot of rain and don’t do a lot of damage.”

But when scientists are pegging their hopes for drought-relief on tropical storms, it’s a clue that something exceptional is taking place.

So just what is going on?

Murphy and other experts point towards strong La Niña conditions as the most significant factor driving both the feast of precipitation in the Midwest, and the famine of moisture in the South. “Basically the La Niña would be far and away the most prominent actor, or bad player if you will,” he says.

La Niña is one half of what scientists call the El Niño-Southern Oscillation, or ENSO, a seesawing climate pattern involving an interplay of sea surface temperatures, atmospheric pressures, winds, and rainfall in the tropical Pacific Ocean. While ENSO is a natural source of climate variability, studies hint that manmade climate change may alter its behavior.

An image from the Landsat-5 satellite, showing the area burned by the Wildcat Fire in Texas. By May 5, the fire had consumed about 160,000 acres. The burned land shows up as dark red in this image. Credit: NASA Earth Observatory.

When a La Niña develops, a natural disparity in atmospheric pressures across the Pacific intensifies, and trade winds blow even more strongly than normal from east to west. This pushes warm surface waters into the far western Pacific, corralling them there and enhancing tropical storminess in the region, bringing a heightened risk of floods to Indonesia and northeastern Australia. Meanwhile, surface waters in the central and eastern Pacific become cooler, and the region drier.

Like the standing waves and whitewater that form around a boulder in a rushing stream, atmospheric airflow around a large area of storminess is disturbed. So when La Niña pushes storminess to the far western Pacific, weather patterns elsewhere also shift.

That’s exactly what happened throughout the world during the recent La Niña, which began in mid-June (after an unusually rapid transition from it’s opposite, El Niño). The La Niña quickly became quite intense, contributing to a stronger than normal summer monsoon in Asia that was implicated in Pakistan’s severe summer flooding, as well as the record-breaking rain and floods in Queensland, Australia late in the year.

Meanwhile, the La Niña was affecting the jet stream over the Pacific. “That affected downstream weather patterns over the United States,” says Gerry Bell, a meteorologist with NOAA’s Climate Prediction Center.

During the winter and spring, the jet stream has been very strong and persistent across the middle of the country, a major factor in the Midwestern floods and record tornado outbreaks. But the main storm track bypassed places like Texas and New Mexico, leading to exceptionally dry conditions there.

Researchers have found some evidence for changes to the El Niño-Southern Oscillation, including a possible increase in its variability and a shift to a new kind of El Niño. This raises an obvious question: Are human activities contributing to any shifts in the pattern, frequency, or intensity of La Niñas and El Niños?

“The effects of climate change on ENSO itself are not yet clear, and our models are not yet good enough to say,” says Kevin Trenberth, a climate scientist at the National Center for Atmospheric Research. “But ENSO involves moving heat around, and so it is likely that there is an effect of warming,” he argues.

Trenberth says ENSO events are having more of an impact on the world’s weather than they used to: “The effects of ENSO in terms of floods and droughts around the world are undoubtedly getting worse,” Trenberth says. And he believes that has been a factor in the continuing intense drought in the southern United States.

This image, from NASA’s Terra satellite, compares plant health between April 7 and April 14 to average conditions seen between 2000 and 2010. Brown colors depict places where plants were less leafy or more sparse than normal, while better-than-normal conditions are green. In mid-April, plants throughout the state showed clear signals of drought stress. Credit: NASA Earth Observatory.

In Texas, those bone-dry conditions have combined  destructively with soaring temperatures, setting the stage for a lethal series of devastating fires. According to the Texas Forest Service, since mid-November at least 10,555 fires have charred 2.7 million acres (and there have been news reports of two firefighters being killed). That’s an area equivalent to three quarters of Connecticut.

In April, Kris Wilson was worried that his home would be one of them. “The greatest urban fire risk in the U.S. is where I live — in the woods west of Austin,” he says. On Friday, April 15, that risk was driven home when ash began falling on him as he was gardening. He looked up to see flames and plumes of smoke. “I loaded up the car with photos, documents, and kitty cats,” he says.

The fire came within a mile of his house but was contained before he had to evacuate. Two days later, a second, much larger fire ignited. But luckily for Wilson, this one was farther away.

And fortunately for all residents of the southern United States, La Niña conditions in the Pacific are now essentially over. But relief probably will not come quickly. Even when the temperature of surface waters in the Pacific returns to average, it takes some time for tropical rainfall patterns to respond. “Until they do, there will be a continuing impact on jet stream winds,” Bell says.

While the summer monsoon could help New Mexico before the end of August, it’s not a sure bet. And for Texans, there’s the looming hurricane season to look to.

According to NOAA's recently released 2011 Atlantic Hurricane Season Outlook (which includes the Gulf of Mexico), there is a 70 percent probability of 12 to 18 named storms, compared to the seasonal average of 11.

Bell, NOAA’s lead seasonal hurricane forecaster, points out that active hurricane seasons do tend to bring more land-falling storms. (Not always, as was the case last season, but this is a matter of averages.) So his advice for residents along the Gulf Coast, including those in drought-stricken areas of Texas, is to prepare for hurricanes. “You should have your plan in place right now,” he says.

Flooding is the leading cause of death in these as well as smaller tropical storms. Let’s just hope that Texas doesn’t get that kind of “relief.”