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

A. T. Evan et al., Science 324, The role of aerosols in the evolution of tropical North Atlantic Ocean temperature anomalies

Science (8 May 2009), Vol. 324, No. 5928, pp. 778-781; DOI: 10.1126/science.1167404

The role of aerosols in the evolution of tropical North Atlantic Ocean temperature anomalies

Amato T. Evan* (Cooperative Institute for Meteorological Satellite Studies, and Department of Atmospheric and Oceanic Sciences, University of Wisconsin, Madison, WI 53706, U.S.A.), Daniel J. Vimont (Department of Atmospheric and Oceanic Sciences, University of Wisconsin, Madison, WI 53706, U.S.A.), Andrew K. Heidinger (National Oceanic and Atmospheric Administration; National Environmental Satellite, Data, and Information Service; and Center for Satellite Applications and Research, 1225 West Dayton Street, Madison, WI 53706, U.S.A.), James P. Kossin (NOAA/NESDIS/National Climatic Data Center, Madison, WI 53706, U.S.A.) and Ralf Bennartz (Department of Atmospheric and Oceanic Sciences, University of Wisconsin, Madison, WI 53706, U.S.A.)

Abstract

Observations and models show that northern tropical Atlantic surface temperatures are sensitive to regional changes in stratospheric volcanic and tropospheric mineral aerosols. However, it is unknown whether the temporal variability of these aerosols is a key factor in the evolution of ocean temperature anomalies. We used a simple physical model, incorporating 26 years of satellite data, to estimate the temperature response of the ocean mixed layer to changes in aerosol loadings. Our results suggest that the mixed layer’s response to regional variability in aerosols accounts for 69% of the recent upward trend, and 67% of the detrended and 5-year low pass–filtered variance, in northern tropical Atlantic Ocean temperatures.

*e-mail: atevan@wisc.edu

Link to abstract:  http://www.sciencemag.org/cgi/content/abstract/324/5928/778

J-F. Huang, C-D. Zhang & J. M. Prospero, ERL 4 (2009), African aerosol and large-scale precipitation variability over West Africa

Environmental Research Letters, 4  (2009) 015006; doi: 10.1088/1748-9326/4/1/015006

African aerosol and large-scale precipitation variability over West Africa

Jingfeng Huang*, Chidong Zhang and Joseph M Prospero

Rosenstiel School of Marine and Atmospheric Science, University of Miami, 4600 Rickenbacker Causeway, Miami, FL 33149, U.S.A.

Abstract

We investigated the large-scale connection between African aerosol and precipitation in the West African Monsoon (WAM) region using 8-year (2000–2007) monthly and daily Moderate Resolution Imaging Spectroradiometer (MODIS) aerosol products (aerosol optical depth, fine mode fraction) and Tropical Rainfall Measuring Mission (TRMM) precipitation and rain type. These high-quality data further confirmed our previous results that the large-scale link between aerosol and precipitation in this region undergoes distinct seasonal and spatial variability. Previously detected suppression of precipitation during months of high aerosol concentration occurs in both convective and stratiform rain, but not systematically in shallow rain. This suggests the suppression of deep convection due to the aerosol. Based on the seasonal cycle of dust and smoke and their geographical distribution, our data suggest that both dust (coarse mode aerosol) and smoke (fine mode aerosol) contribute to the precipitation suppression. However, the dust effect is evident over the Gulf of Guinea while the smoke effect is evident over both land and ocean. A back trajectory analysis further demonstrates that the precipitation reduction is statistically linked to the upwind aerosol concentration. This study suggests that African aerosol outbreaks in the WAM region can influence precipitation in the local monsoon system which has direct societal impact on the local community. It calls for more systematic investigations to determine the modulating mechanisms using both observational and modeling approaches.

*e-mail: jhuang@rsmas.miami.edu


Link to abstract:  http://www.iop.org/EJ/abstract/1748-9326/4/1/015006