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“At least 85% of intense hurricanes and two-thirds of all Atlantic hurricanes in the study developed from AEWs that formed after thunderstorms in eastern Africa, the researchers found.”
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NATURE
11 May 2007;

Lightning spurs hurricanes
Link shows storms in Africa can cause havoc in the United States.

Harvey Leifert

Heavy lightning in eastern Africa apparently perturbs the westward trade winds across the African continent.

Getty

What creates an Atlantic hurricane? The most devastating ones are spurred by intense thunderstorms in the Ethiopian highlands, according to new research.

The link between lightning strikes and hurricane formation should give researchers a heads-up about when a nasty hurricane might form, weeks before it could make landfall in the United States, says Colin Price of Tel Aviv University in Israel. Today, scientists apply various models to predict storm tracks and strength, but only once they form over the Atlantic Ocean. “This is what is unique about our work,” Price says. “We look at the initial stages of these devastating storms before they have become hurricanes.”

Price and his colleagues at Israel’s Open University studied the 2005 and 2006 hurricane seasons, which were markedly different from each other. In 2005 there were a record 28 named storms, including the catastrophic Hurricane Katrina, while 2006 brought only 10 named storms – a 64% reduction. Summertime lightning activity in eastern Africa, mainly in the Ethiopian highlands, was also quite different in each of the years, the researchers found, with 23% less activity in 2006 over 2005.

The two phenomena are linked, says Price, an atmospheric scientist who has long studied lightning.

Wind interrupted

Heavy lightning in eastern Africa apparently perturbs the westward trade winds across the African continent, Price writes in Geophysical Research Letters (1). He likens the process to the effect of boulders in a stream: “The boulder produces undulations and turbulence downstream, and the bigger the boulder, the larger the turbulence. Over Africa, thunderstorms act as our boulders.”

So the larger the thunderstorm, the greater the atmospheric turbulence, says Price. This turbulence, in turn, creates low-pressure areas known as African easterly waves (AEW). About half of these systems are known to generate tropical storms as they head westwards over the Atlantic. Various factors, including sea surface temperature, dust and wind shear above the Atlantic, then determine whether those storms strengthen into hurricanes.

The team mined data about lightning strikes from the World Wide Lightning Location Network, whose ground stations monitor the very low-frequency electromagnetic signals that lightning emits. They found that all periods of intense lightning in eastern Africa monitored in both 2005 and 2006 were followed by an AEW low-pressure area.

Striking impact

Only a fraction of these AEWs go on to make hurricanes or cause damage in the United States. But of the big hurricanes that do form, the vast majority seem to have been born of lightning.

At least 85% of intense hurricanes and two-thirds of all Atlantic hurricanes in the study developed from AEWs that formed after thunderstorms in eastern Africa, the researchers found.

Price suggests that forecasters and emergency response personnel keep an eye on major thunderstorms in eastern Africa during the upcoming hurricane season, in order to gauge its likely severity.

Earle Williams, a lightning expert at the Massachusetts Institute of Technology in Cambridge, who was not involved in the study, says the study highlights the benefits of having global lightning data available on a continuous basis.

References
1. Price C., Yair Y. & Asfur M. Geophys. Res.
Lett. 34 , L09805 doi:10.1029/2006GL028884 (2007).

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