Dust storms formed in Sahara linked to hurricane activity in Atlantic

Dust storms formed in Sahara linked to hurricane activity in Atlantic

Scientists are trying to understand what's causing the increasing number of more powerful hurricanes from the Atlantic Ocean in the last years. Some see it as part of a natural cycle in which hurricanes get worse for a decade or two before dwindling again.

Recent studies have pointed a potential relationship between warming sea surface temperatures caused by global warming and increases in the strength and number of hurricanes. Recently, a surprising link between a reduced hurricane activity in the Atlantic and thick clouds of dust that periodically rise from the Sahara Desert and blow off Africa's western coast has been noticed. Hurricanes are triggered by heat and moisture, and it seems that dry dust storms repress them before they fully develop.

Three hypothesis were formulated about how sand storms could affect hurricanes: dry air of sand storms could block the rising streams of air needed to increase the hurricane's power; midlevel winds accompanying the Saharan air cause wind cut, preventing rising currents from growing into storms or warmth absorbed by the dust in the air can stabilize the air currents' conditions, blocking rising air.

However, the dust storms could change a hurricane's direction further to the west increasing the possibility that it would reach the United States and Caribbean Islands. "This correlation between dust storms and hurricane activity are especially strong for the past few years," said study leader Amato Evan of the University of Wisconsin-Madison.

"In 2004, we saw an increase in dust activity and a decrease in hurricanes. In 2005, it was the exact opposite," Evan said.

"2005 was the busiest hurricane season on record. It included 26 named storms and 13 hurricanes, including Katrina, hailed as the most destructive U.S. storm ever. Preliminary analyses of this year's dust activity also support the theory," he continued.

The 2006 hurricane season has been weaker than forecasted, and this correlated with a high dust storm earlier in the year. "At the beginning of the year, we saw a lot of dust storms-just really continuous. Then, probably a few weeks ago, when we started seeing some of those hurricanes that were forming up in the Atlantic, we saw a real lack of dust activity," he told.

The dust storms form at the contact between hot, dryer desert air from the Sahara with cooler air from the Sahel (the region at the South of Sahara) to form winds that lift sand and dust into the atmosphere, where they are caught by strong trade winds and blown westward, across the Atlantic Ocean.

Some years, millions of tons of dust clouds that can cross the oceans in as little as five days, being detected in Caribbean and Florida, while in others there are almost no dust storms.

The researchers used satellite images to study the amount of African dust blown out over the Atlantic for the years 1982-2005 and compared that with tropical storm activity. They found an inverse relationship between tropical storms and dust. "While we cannot conclusively demonstrate a direct causal relationship, there appears to be a robust link between tropical cyclone activity and dust transport," they concluded.

"During periods of intense hurricane activity dust was relatively scarce in the atmosphere. In years when stronger dust storms rose up, on the other hand, fewer hurricanes swept through the Atlantic."

"While a great deal of work has focused on the links between [hurricanes] and warming ocean temperatures, this research adds another piece to the puzzle."

"People didn't understand the potential impact of dust until satellites allowed us to see how incredibly expansive these dust storms can be," Evan said in a statement.

"Sometimes during the summer, sunsets in Puerto Rico are beautiful, because of all the dust in the sky - well that dust comes all the way from Africa."

Co-author Jonathan Foley, also of the University of Wisconsin, added: "These findings are important because they show that long-term changes in hurricanes may be related to many different factors."

"What we don't know is whether the dust affects the hurricanes directly, or whether both [dust and hurricanes] are responding to the same large scale atmospheric changes around the tropical Atlantic," says Foley.

"That's what future research needs to find out."

Link: http://news.softpedia.com/news/Dust-Storms-Formed-in-Sahara-Linked-to-Hurricane-Activity-in-Atlantic-37736.shtml

R. Bennartz: Airborne dust reduction plays larger than expected role in determining Atlantic temperature

Airborne dust reduction plays larger than expected role in determining Atlantic temperature

A dust storm off the coast of Morocco was imaged by NASA's MODIS Aqua meteorological satellite on March 12, 2009. A new study by UW-Madison researcher Amato Evan shows that variability of African dust storms and tropical volcanic eruptions can account for 70 percent of the warming North Atlantic Ocean temperatures observed during the past three decades. Since warmer water is a key ingredient in hurricane formation and intensity, dust and other airborne particles will play a critical role in developing a better understanding of these storms in a changing climate. (Credit: Photo: courtesy Amato Evan)


ScienceDaily, March 28, 2009 — The recent warming trend in the Atlantic Ocean is largely due to reductions in airborne dust and volcanic emissions during the past 30 years, according to a new study.

Since 1980, the tropical North Atlantic has been warming by an average of a quarter-degree Celsius (a half-degree Fahrenheit) per decade. Though this number sounds small, it can translate to big impacts on hurricanes, which thrive on warmer water, says Amato Evan, a researcher with the University of Wisconsin-Madison's Cooperative Institute for Meteorological Satellite Studies and lead author of the new study. For example, the ocean temperature difference between 1994, a quiet hurricane year, and 2005's record-breaking year of storms, was just one degree Fahrenheit.

More than two-thirds of this upward trend in recent decades can be attributed to changes in African dust storm and tropical volcano activity during that time, report Evan and his colleagues at UW-Madison and the National Oceanic and Atmospheric Administration in a new paper. Their findings will appear in an upcoming issue of the journal Science and publish online March 26.

Evan and his colleagues have previously shown that African dust and other airborne particles can suppress hurricane activity by reducing how much sunlight reaches the ocean and keeping the sea surface cool. Dusty years predict mild hurricane seasons, while years with low dust activity — including 2004 and 2005 — have been linked to stronger and more frequent storms.

In the new study, they combined satellite data of dust and other particles with existing climate models to evaluate the effect on ocean temperature. They calculated how much of the Atlantic warming observed during the last 26 years can be accounted for by concurrent changes in African dust storms and tropical volcanic activity, primarily the eruptions of El Chichón in Mexico in 1982 and Mount Pinatubo in the Philippines in 1991.

In fact, it is a surprisingly large amount, Evan says. "A lot of this upward trend in the long-term pattern can be explained just by dust storms and volcanoes," he says. "About 70 percent of it is just being forced by the combination of dust and volcanoes, and about a quarter of it is just from the dust storms themselves."

The result suggests that only about 30 percent of the observed Atlantic temperature increases are due to other factors, such as a warming climate. While not discounting the importance of global warming, Evan says this adjustment brings the estimate of global warming impact on Atlantic more into line with the smaller degree of ocean warming seen elsewhere, such as the Pacific.

"This makes sense, because we don't really expect global warming to make the ocean [temperature] increase that fast," he says.
Volcanoes are naturally unpredictable and thus difficult to include in climate models, Evan says, but newer climate models will need to include dust storms as a factor to accurately predict how ocean temperatures will change.

"We don't really understand how dust is going to change in these climate projections, and changes in dust could have a really good effect or a really bad effect," he says.

Satellite research of dust-storm activity is relatively young, and no one yet understands what drives dust variability from year to year. However, the fundamental role of the temperature of the tropical North Atlantic in hurricane formation and intensity means that this element will be critical to developing a better understanding of how the climate and storm patterns may change.

"Volcanoes and dust storms are really important if you want to understand changes over long periods of time," Evan says. "If they have a huge effect on ocean temperature, they're likely going to have a huge effect on hurricane variability as well."

The new paper is coauthored by Ralf Bennartz and Daniel Vimont of UW-Madison and Andrew Heidinger and James Kossin of the National Oceanic and Atmospheric Administration and UW-Madison.

Link:  http://www.sciencedaily.com/releases/2009/03/090326141553.htm

Less dust from Africa warms the tropical North Atlantic Ocean, creating conditions for more hurricanes

Less dusty air warms Atlantic, may spur hurricanes

by Alister Doyle, Environment Correspondent, Reuters, March 26, 2009

OSLO (Reuters) -- A decline in sun-dimming airborne dust has caused a fast warming of the tropical North Atlantic in recent decades, according to a study that might help predict hurricanes on the other side of the ocean.

About 70% of the warming of the Atlantic since the early 1980s was caused by less dust, blown from Saharan sandstorms or caused by volcanic eruptions, U.S.-based scientists wrote in the journal Science.
Clouds of dust can be blown thousands of kilometers (miles) and reflect some of the sun's rays back into space.

"Since 1980 tropical North Atlantic Ocean temperatures have been rising at a rate of nearly 0.25 °C (0.45 °F) per decade," they wrote on Thursday.

In the past, the rapid temperature rise had been blamed on factors such as global warming or shifts in ocean currents. Warmer temperatures may spur more hurricanes, which need sea surface temperatures of about 28 °C (82.4 °F) to form.

A sea temperature difference of just one degree Fahrenheit separated 1994, a quiet hurricane year, from a record 2005 when storms included Hurricane Katrina, which devastated New Orleans, according to a University of Wisconsin-Madison statement.

"We were surprised" by the big role of dust on Atlantic temperatures, said Ralf Bennartz, a professor at the university and a co-author of the study written with experts at the National Oceanic and Atmospheric Administration.

COOLER
In past decades "there was much more dust blowing from (Africa) onto the Atlantic and cooling the sea and ... potentially suppressing hurricane intensity," he told Reuters. No other ocean receives so much dust.

More droughts in Africa in the 1980s, for instance, meant more dust in the air, he said of the study of satellite data and climate models. Annual emissions of dust from North Africa have been estimated at between 240 million and 1.6 billion tonnes.

Bennartz said the scientists were trying to work out, for instance, if wet weather in North Africa could mean less dust and in turn point to fewer hurricanes battering the United States or Caribbean islands.

Big volcanic eruptions were El Chichon in Mexico in 1982 and Mount Pinatubo in the Philippines in 1991. Both dimmed the sun.

The study suggests that only 30% of the warming of the Atlantic can be explained by factors other than dust, for instance global warming blamed by the U.N. Climate Panel on emissions of greenhouse gases from burning fossil fuels.

"This makes sense, because we don't really expect global warming to make the ocean (temperatures) increase that fast," said Amato Evan, a researcher at the University of Wisconsin-Madison who was lead author.
Bennartz said it was unclear how climate change might affect overall dust amounts blown from Africa this century.

(Editing by Janet Lawrence)

Link to article:  http://www.reuters.com/article/environmentNews/idUSTRE52P5T520090326