Climatologists offer explanation for widening of Earth's tropical belt

A cool-water anomaly known as La Ni?a occupied the tropical Pacific Ocean throughout 2007 and early 2008. In April 2008, scientists at NASA’s Jet Propulsion Laboratory announced that while the La Ni?a was weakening, the Pacific Decadal Oscillation (PDO) -- a larger-scale, slower-cycling ocean pattern—had shifted to its cool phase. This image shows the sea surface temperature anomaly in the Pacific Ocean from April 14–21, 2008. Places where the Pacific was cooler than normal are blue, places where temperatures were average are white, and places where the ocean was warmer than normal are red. The broad area of cooler-than-average water off the coast of North America from Alaska (top center) to the equator is a classic feature of the cool phase of the PDO. The cool waters wrap in a horseshoe shape around a core of warmer-than-average water. (In the warm phase, the pattern is reversed). Unlike El Ni?o and La Ni?a, which may occur every 3 to 7 years and last from 6 to 18 months, the PDO can remain in the same phase for 20 to 30 years. The shift in the PDO can have significant implications for global climate.Credit: NASA image by Jesse Allen, AMSR-E data processed and provided by Chelle Gentemann and Frank Wentz, Remote Sensing Systems Recent studies have shown that Earth's tropical belt -- demarcated, roughly, by the Tropics of Cancer and Capricorn -- has progressively expanded since at least the late 1970s. Several explanations for this widening have been proposed, such as radiative forcing due to greenhouse gas increase and stratospheric ozone depletion.

Now, a team of climatologists, led by researchers at the University of California, Riverside, posits that the recent widening of the tropical belt is primarily caused by multi-decadal sea surface temperature variability in the Pacific Ocean. This variability includes the Pacific Decadal Oscillation (PDO), a long-lived El Ni?o-like pattern of Pacific climate variability that works like a switch every 30 years or so between two different circulation patterns in the North Pacific Ocean. It also includes, the researchers say, anthropogenic pollutants, which act to modify the PDO.

Study results appear March 16 in Nature Geoscience.

"Prior analyses have found that climate models underestimate the observed rate of tropical widening, leading to questions on possible model deficiencies, possible errors in the observations, and lack of confidence in future projections," said Robert J. Allen, an assistant professor of climatology in UC Riverside's Department of Earth Sciences, who led the study. "Furthermore, there has been no clear explanation for what is driving the widening."

Now Allen's team has found that the recent tropical widening is largely driven by the PDO.

"Although this widening is considered a 'natural' mode of climate variability, implying tropical widening is primarily driven by internal dynamics of the climate system, we also show that anthropogenic pollutants have driven trends in the PDO," Allen said. "Thus, tropical widening is related to both the PDO and anthropogenic pollutants."

Widening concerns

Tropical widening is associated with several significant changes in our climate, including shifts in large-scale atmospheric circulation, like storm tracks, and major climate zones. For example, in Southern California, tropical widening may be associated with less precipitation.

Of particular concern are the semi-arid regions poleward of the subtropical dry belts, including the Mediterranean, the southwestern United States and northern Mexico, southern Australia, southern Africa, and parts of South America. A poleward expansion of the tropics is likely to bring even drier conditions to these heavily populated regions, but may bring increased moisture to other areas.

Widening of the tropics would also probably be associated with poleward movement of major extratropical climate zones due to changes in the position of jet streams, storm tracks, mean position of high and low pressure systems, and associated precipitation regimes. An increase in the width of the tropics could increase the area affected by tropical storms (hurricanes), or could change climatological tropical cyclone development regions and tracks.

Belt contraction

Allen's research team also showed that prior to the recent (since ~1980 onwards) tropical widening, the tropical belt actually contracted for several decades, consistent with the reversal of the PDO during this earlier time period.

"The reversal of the PDO, in turn, may be related to the global increase in anthropogenic pollutant emissions prior to the ~ early 1980s," Allen said.

Analysis

Allen's team analyzed IPCC AR5 (5th Assessment Report) climate models, several observational and reanalysis data sets, and conducted their own climate model experiments to quantify tropical widening, and to isolate the main cause.

"When we analyzed IPCC climate model experiments driven with the time-evolution of observed sea surface temperatures, we found much larger rates of tropical widening, in better agreement to the observed rate--particularly in the Northern Hemisphere," Allen said. "This immediately pointed to the importance of sea surface temperatures, and also suggested that models are capable of reproducing the observed rate of tropical widening, that is, they were not 'deficient' in some way."

Encouraged by their findings, the researchers then asked the question, "What aspect of the SSTs is driving the expansion?" They found the answer in the leading pattern of sea surface temperature variability in the North Pacific: the PDO.

They supported their argument by re-analyzing the models with PDO-variability statistically removed.

"In this case, we found tropical widening -- particularly in the Northern Hemisphere -- is completely eliminated," Allen said. "This is true for both types of models--those driven with observed sea surface temperatures, and the coupled climate models that simulate evolution of both the atmosphere and ocean and are thus not expected to yield the real-world evolution of the PDO.

"If we stratify the rate of tropical widening in the coupled models by their respective PDO evolution," Allen added, "we find a statistically significant relationship: coupled models that simulate a larger PDO trend have larger tropical widening, and vice versa. Thus, even coupled models can simulate the observed rate of tropical widening, but only if they simulate the real-world evolution of the PDO."

Future work

Next, the researchers will be looking at how anthropogenic pollutants, by modifying the PDO and large scale weather systems, have affected precipitation in the Southwest United States, including Southern California.

"Future emissions pathways show decreased pollutant emissions through the 21st century, implying pollutants may continue to drive a positive PDO and tropical widening," Allen said.

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Towns hit by twisters offer rebuilding road map (AP)

By JIM SUHR and JIM SALTER, Associated Press Jim Suhr And Jim Salter, Associated Press – Thu May 26, 10:45 pm ET

JOPLIN, Mo. – Less than a week after one of the nation's deadliest tornadoes wiped a big chunk of Joplin off the map, the city is beginning to shift its focus toward the next challenge: rising from the ruins. And the town that gained fame as a stop along Route 66 can use the road maps drawn by other storm-savaged communities that have endured the same long journey.

Not far from Joplin, tiny Pierce City and Stockton rebuilt piece by piece after tornadoes reduced much of their prized downtowns to rubble, killing four. Greensburg, Kan., did the same, starting over after a 2007 twister leveled the town and killed 11.

The rebirths took years of hard work, partnerships and plain old stubborn faith. Some of the projects are still unfolding. Yet they offer a hopeful testament about beginning anew after a disaster, even if the other towns didn't face the scale of death and decimation rendered by Sunday's tornado that killed at least 126 people and by some estimates injured more than 900.

Joplin's half-mile-wide twister took out the city's main hospital, the high school and possibly thousands of homes. The Walmart was flattened, along with the Home Depot. Hundreds of businesses and industrial buildings were lost. And an untold number of vehicles — from cars to tractor-trailers, even the hospital helicopter — were mangled.

City Manager Mark Rohr said planners are already plotting a comeback, vowing Joplin "will recover stronger than when we began."

Federal aid will help.

President Barack Obama has declared disasters in Jasper and Newton counties, and a key House panel has approved a $1 billion aid package to make sure federal disaster-relief accounts don't run out before the end of the budget year in September.

Rep. Robert Aderholt, an Alabama Republican, said the move would ensure there's enough money for victims of the Joplin tornado, as well as those suffering from flooding along the Mississippi River and last month's twisters that swept across Alabama.

A day after Joplin was crippled, Republican House Majority Leader Eric Cantor told reporters any federal aid to disaster areas may need to be offset by spending cuts. But Sen. Roy Blunt, a Missouri Republican, vowed to make sure Joplin gets all it needs.

Funding questions aside, former Joplin Mayor Ron Richard, now a state senator, was a bit more cautious than Rohr in his vision for the immediate future.

"I wouldn't consider this an economic-development opportunity. This is just survival," Richard said. "We're just going to have to get back to where we were — as close as we can would be the goal. I'm not sure how much time it's going to take."

Julie Johnson felt that way about Pierce City on May 4, 2003, when tornadoes marched into Jasper County on the Missouri-Kansas line and cut a swath east across the state. When the fury had passed, 18 people were dead and some 70,000 structures were damaged or destroyed over 76 counties.

The twister killed a man when the National Guard Armory where many were seeking shelter partly collapsed.

Pierce City, a town of 1,260 people some 35 miles southeast of Joplin, lost more than 80 homes and all but three of its businesses. A nine-block area of its historic downtown was wiped out. The district was once replete with antique shops and eateries housed in buildings that dated to the 1800s.

The city hall, fire station, library and senior center? Gone, along with the grocery store, two convenience shops, the pharmacy and the hardware store.

"We had a lot of historic stuff, two full blocks of historic, beautiful buildings. Only three managed to make it through," said Johnson, the city clerk.

But Pierce City pressed on. A new $4.7 million armory opened on the city's south side, replacing the old building, which now houses a mental-health center. The city got a new firehouse, a City Hall that's a $450,000 replica of an old train depot, and things local leaders never dreamed of having — a Dollar General store, two strip malls and storm shelters at the fire station and at the schools.

"Some things came back bigger and better," said Johnson, who has spent all but four of her 48 years in Pierce City, where the population is down just 128 from a decade ago. "It's never going to be the way it was as far as physical attributes, and we lost the historical charm as far as downtown. But we're back."

So is Stockton, eight years after the lakeside town was brought to its knees by the tornado that killed three people, nearly destroyed the town square and knocked out one-third of its 120 businesses. Hundreds of homes were damaged or destroyed, and the park was obliterated.

Residents wanted to start rebuilding immediately, Mayor Patty Thompson recalls. But the city council barred new construction for a few months, allowing for cool reflection about how the resurrected town should look.

Now, Thompson said, Stockton has a "whole new downtown," its buildings fashioned of dark brick. The community center, about the only thing salvaged from the tornado, has been rehabbed. Trees have been replanted. Residents appear to like it. The population of 1,819 is down just 141 from 2000.

"I'm really proud of how this community pulled together and went forward," said Thompson, 65, a waitress at a Mexican restaurant. "A lot of people could have just moved away and said forget it. But we've just got a beautiful little city."

Scars remain: Some sidewalks buckled by the tornado still need replacing. Replanted trees in the park can only grow so fast, and saplings are being planted in the once-devastated cemetery.

Still, she said, "In eight years, you can say, `Look at how far we've come. We can overcome.'"

Both cities had big-time help. The federal government extended a rare offer to assist with long-term recovery, offering a package that included developing a plan for rebuilding, as well as appointing a project manager to seek out private and government assistance. The Federal Emergency Management Agency eventually doled out millions of dollars in grants to both communities.

In southern Kansas, Greensburg was a town of about 1,500 with a declining population before a tornado roared through in May 2007, destroying 90 percent of the town in winds topping 200 mph.

At the time, Steve Hewitt was a 33-year-old, first-time city manager on the job for only a year. He said the town approached rebuilding as an opportunity and, like Stockton, took some time to think about it.

Greensburg brought in planning consultants from a big city, held community meetings, reworked some zoning ordinances and literally shuffled the town. The school was rebuilt at a different site. City Hall was rebuilt on land that once was a car dealership, which relocated to a higher-traffic spot. And Greensburg rose up as a model green community — more environmentally friendly than it ever had been, including a $50 million school with geothermal systems that rely on the Earth's heat for warmth in the winter and cooling in the summer.

Even so, the recovery is far from total. According to the 2010 census, Greenburg's population of nearly 800 is barely half of what it was a decade ago, as is the number of businesses. Yet to be rebuilt are the town's pharmacy, variety store and movie theater.

"We're seeing a gradual growth, but it comes down to jobs and employment. Economic development is highly important and critical to our situation," said Mayor Bob Dixson, a 57-year-old retired postal worker. "The tax base was gone. The businesses were gone. Schools were gone."

Hewitt, who left Greensburg a few months ago to become the city manager of Clinton, Okla., knows Joplin can follow a similar path to get back on its feet.

"The next weeks and months (in Joplin) will be very dark and grim," Hewitt said. "But in that tragedy, there will be some unique bright spots, because you start to see the future develop. You'll see a sense of community unlike they've probably ever witnessed."

Stockton's Thompson agrees.

"It'd be easy to just give up," she said. "But a lot of people who have lost loved ones still have a lot of loved ones left. The community can pull together and rise from the rubble. They can do it. We did."

___

Suhr reported from St. Louis. Associated Press Writer David Lieb in Jefferson City contributed to this report.

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