THE Atlantic hurricane season got off to an early start last week as the first named storm, Andrea, formed along the south-east coast of the US. Forecasters are betting on a busier than average season, but in honing their predictions they might do well to watch for thunderstorms and lightning over east Africa.
Colin Price of Tel Aviv University in Israel and colleagues used a ground-based network of radio antennas to monitor lightning activity over Africa and measure the size, intensity and location of thunderstorms. In the 2005 and 2006 Atlantic hurricane seasons, they found that over 90 per cent of named storms were preceded about a week earlier by intense thunderstorms over east Africa (Geophysical Research Letters, vol 34, p L09805).
The connection between thunderstorms and hurricanes appears to be disturbances called African easterly waves (AEWs) in the tropical breeze blowing off west Africa into the Atlantic. 鈥淭hunderstorms act like a boulder in a stream, creating an instability in the trade winds passing over Africa,鈥� Price says.
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鈥淭hunderstorms act like a boulder in a stream, creating an instability in the trade winds passing over Africa鈥�
All east African thunderstorms created AEWs, and during the hurricane season about half of these appeared to seed tropical storms. Price notes that high sea-surface temperatures also help to generate tropical storms, but despite this he says that thunderstorm activity over east Africa alone should have hurricane forecasters twitching.
Once a tropical storm has formed, another African weather system seems to influence its intensity. According to Liguang Wu of the University of Maryland, Baltimore, the key is the Saharan air layer (SAL), which blows from the Sahara desert. From an analysis of 58 years鈥� worth of data from satellites and weather stations over north Africa, Wu found that when the SAL was strong and dry, as in the mid-1980s, hurricanes in the Atlantic were less intense (Geophysical Research Letters, vol 34, p L09802).
鈥淒uring active SAL years the dry air can intrude into tropical storm circulation and suppress hurricane convection,鈥� Wu says. In addition, and perhaps counter-intuitively, the stronger SAL winds tend to make it harder for the storm to build.
