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 hurricane Ivan have given invaluable information that could help predict the surges.
To understand how winds transfer their energy to the water to cause the surges, oceanographers usually try to measure wind speeds above the ocean surface, but large waves make this difficult. 鈥淲e鈥檇 really prefer to measure ocean current speeds directly under hurricanes,鈥� says William Teague of NASA鈥檚 Stennis Space Center in Hancock county, Mississippi. 鈥淏ut you can鈥檛 tell where in the sea to lay your instruments before the storm strikes.鈥�
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. The readings revealed a surprise. As the wind speeds increased to about 32 metres per second, the ocean currents beneath them also sped up. But at even higher wind speeds, the currents died down (Science, vol 315, p 1707).
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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. The findings should help predict the size of storm surges more accurately from measurements of wind speeds.
