Storm-force winds exert less pull on ocean than expected

This 15 September 2004 NASA satellite image shows Hurricane Ivan. A snapshot of ocean conditions taken during Hurricane Ivan in 2004 has yielded new clues about the dynamics of storm surges that could help meteorologists make more accurate predictions, a study released Thursday.

Weather forecasters typically rely on data about surface winds and turbulence to try and figure out just how much a storm has churned up the ocean and what the resulting storm surge will look like -- the kind of surge that walloped New Orleans after Hurricane Katrina in 2005. But when US Navy researchers studied data gleaned from deep-sea sensors in the Gulf of Mexico as the eye of Hurricane Ivan passed overhead on September 15, 2004, the wave patterns they observed were quite different from what they anticipated.They expected the waves to build in speed and intensity in line with increasing wind speeds, based on the formulations used in current ocean circulation and storm surge computer models.The first-of-a-kind readings showed that the winds did indeed exert a drag effect on the waves at speeds below 72 miles per hour (32 meters per second), but above that point the wave speeds steadily declined.The investigators suspect that once the winds hit this point, which is considered the threshold for hurricane-force winds, the spray, foam and bubbles from breaking waves reduce the "drag" effect allowing the hurricane to slip over the sea."We should be able to improve storm surge predictions based on this finding," said William Teague, an oceanographer with the Naval Research Laboratory at the Stennis Space Center in Mississippi.

( this study appears in the journal of science)