Aided by satellite monitoring, scientists at the University of Wisconsin’s Cooperative Institute for Meteorological Satellite Studies have updated their study of cyclone formation in the eastern Atlantic Ocean off of Africa’s west coast. Their findings, released last month, appeared about the same time as tropical Cyclone Ivan struck Madagascar in the Indian Ocean east of the African continent. The International Red Cross and the UN humanitarian agency OCHA have stepped up emergency appeals for millions of dollars to help Madagascar recover from the tropical storm, which caused severe flooding and left at least 84 people dead and several hundred thousand homeless.
While ingredients and conditions for storms forming over the Indian Ocean differ from the catalysts that can precipitate storms over the Atlantic, climate specialists are hoping to enhance the predictability and understanding of tropical storm formation in general. Researcher Evan Amato in Madison, Wisconsin explains that the latest studies over West Africa and the eastern Atlantic show a unique climatological link between African dust outbreaks, water temperatures, and the frequency and intensity of tropical cyclones.
“When you have a lot of dust particles suspended in the atmosphere over the Atlantic Ocean, essentially they just reflect sunlight back out to space. They’re like millions of tiny little mirrors, and so that’s energy that could have been used to warm up the ocean that’s now being reflected back out to space. And so, what happens is that in years when there’s an anomalously high amount of dust storm activity, you get immeasurable cooling of the ocean temperatures,” he notes.
During dry years when airborne dust particles drift southward to cover the West Africa coastal zone where humid ocean air can breed tropical storms, cooling atmospheric conditions can lower ocean temperatures and reduce the chances of storms developing. But in years when heavier precipitation covers a greener land mass, reduced quantities of aerosol dust allow more sun to radiate and heat up ocean temperatures. Amato says such seasonal indicators can help researchers understand several months in advance whether or not ocean temperatures, coupled with wind speeds, moisture, and dust content will be ripe for storm formation.
“Probably one of the biggest impacts of this recent study is that dust storm activity, even before hurricane season starts, say in May and June, is really giving us an indication of the environment. If we get a lot of dust storm activity and a lot of dust suspended out over the Atlantic Ocean, it’s going to cool the ocean temperatures and that’s going to have an impact on hurricane activity, later in the season, even into September, directly through the ocean temperatures,” says Amato.
Scientists recognize the unique conditions over West Africa that permit them to study this phenomenon from space. Wisconsin’s Amato says the combination of a vast dust-generating Sahel desert region, bordered by a tropical, moist area to the south which fuels hurricane formation, produces many different kinds of atmospheric phenomena that, coupled with warm ocean temperatures, can generate conditions that may or may not be ripe to cultivate a tropical storm.
“All of the wind patterns, all of the storm patterns that we see across the tropical north Atlantic are all being generated over Africa, over the Ethiopian highlands. They’re being modified as they travel across Chad, across Mauritania. And really, West Africa is just such a fascinating and unique place in the world. There’s this super, dry desert, that’s just massive, and then to the south of that is this very tropical, moist area. All kinds of interesting atmospheric phenomena are set up when you have very dry, warm air to the north of these very tropical air masses, when you have these strong gradients of temperature and moisture. That just tends to fuel these storms that are being initiated over the Ethiopian highlands, and so by the time they get out over the Atlantic, they’re these very strong storms that are very likely to be able to form into hurricanes,” he notes.
Amato says there can be fluctuations from season to season, but his 25-year findings show promising signs for identifying cyclical patterns in hurricane formation. He notes an overall trend that generally between 1982 and 1994, higher dust concentrations coincided with reduced cyclone activity, while the remaining recent years of the study, extending up until 2006, exhibited lower dust levels and greater cyclone activity, particularly in 1996, 2004, and 2006. But the Wisconsin researcher cautions that measuring year to year changes is not a straight forward process. He says that other factors like human industrial development, environmental factors like volcanic eruptions that can generate huge amounts of aerosols, changes in Sahelian vegetation levels and a rise in carbon dioxide levels over the next few decades may alter the entire scheme of findings from the past 25 years and leave room for much future investigation.
