— Regional map and instrument locations. Bathymetry (in metres) is shown for the north-eastern Gulf of Mexico. Acoustic Doppler current profiler (ADCP) moorings are denoted in black dots (M1 to M6). The yellow triangle is Buoy 42040, National Data Buoy Center. Hurricane Ivan's path is indicated by the dashed red line. Two inserts in the expanded map are a colorized infrared image of Hurricane Ivan taken by the National Oceanic and Atmospheric Administration's GOES-12 satellite (lower right) and a Barny mooring containing the ADCP current meter (upper right) (Image: Science)
This high resolution satellite image of the eye of Hurricane Ivan is from NASA's Moderate Resolution Imaging Spectroradiometer (MODIS), taken at 1850 GMT on 15 September 2004 (Image: William Teague, Naval Research Laboratory, Stennis Space Center)
Hurricanes can wreak havoc by creating a storm surge a huge wall of water that slams ashore. Now, instruments fortuitously placed in the path of 2004's 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 NASAs Stennis Space Center in Hancock County, Mississippi, US.
Unless you get lucky. In September 2004, Hurricane Ivan passed over instruments that Teagues team had deployed to measure the movement of water in the Gulf of Mexico. We thought the instruments would be destroyed, but all 14 survived intact, 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.
Violent 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, 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)