Hurricanes can wreak havoc by creating a storm surge 鈥� a huge wall of water that slams ashore. Now, instruments fortuitously placed in the path of 2004鈥檚 Hurricane Ivan have provided invaluable information that could help predict these surges.
To better understand how winds transfer their energy to the water to cause the surges, oceanographers would ideally like to measure ocean current speeds directly under a hurricane. But it is hard to predict exactly where to place instruments before a storm strikes, says William Teague of NASA鈥檚 Stennis Space Center in Hancock County, Mississippi, US.
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Unless you get lucky. In September 2004, Hurricane Ivan passed over instruments that Teague鈥檚 team had deployed to measure the movement of water in the Gulf of Mexico. 鈥淲e thought the instruments would be destroyed, but all 14 survived intact,鈥� says Teague.
As the wind speeds increased to about 32 metres per second, the ocean currents beneath them also sped up, the readings showed. But, surprisingly, as wind speeds grew even higher, the currents died down.
鈥淰iolent winds generate breaking waves, sea spray and foam, making it harder for the hurricane to get a foothold on the water surface and drag it along,鈥� says Teague. So, slower wind speeds over the ocean are actually more likely to generate storm surges than ones faster than 32 metres per second.
The findings should help predict the size of storm surges more accurately from measurements of wind speeds, he says.
Journal reference: Science (vol 315, p 1707)
