Energy transferred from storm winds to coastal ocean water decreases after the winds reach hurricane speeds, a finding that could help storm trackers better predict the effects of storms like Katrina.

Wind stress on the ocean surface peaks at about 32 metres per second, or 72 mph, then decreases, researchers from the Naval Research Laboratory at Stennis Space Center in Mississippi reported in the current issue of the journal Science.

"This means that the effect of hurricane winds on the ocean surface is weaker than previously thought," William Teague, a Navy oceanographer and study author, said yesterday in an interview.

Navy researchers used storm data gathered from instruments measuring waves, tides and currents on the outer continental shelf in the northeastern Gulf of Mexico.

The instruments survived Hurricane Ivan, a Category Four storm that hit the Gulf Coast on September 15, 2004, and recorded almost complete profiles of the ocean current speeds.

Measurements of the ocean currents from those instruments led to the finding of the 72-mph peak, according to the study.

"It is one piece of the puzzle that may help us better track and predict hurricanes," Teague said.

Measurements from the atmosphere aren't reliable for predictions in major storms because sea spray and breaking waves can skew wind instrument readings, the researchers wrote. — Bloomberg News