Strong hurricanes aren’t as effective as weak ones at transmitting wind energy to the ocean’s surface, a counterintuitive finding that may enable researchers to better estimate the size of storm surges.
When winds blow across the ocean, aerodynamic drag—friction where air and water meet—transfers some of the wind’s momentum to the water. That transfer creates waves and currents. Scientists have long presumed that higher wind speeds result in higher drag and larger momentum transfers, says William J. Teague, an oceanographer at the Naval Research Laboratory in Bay St. Louis, Miss. However, he and his colleagues now report field data that contradict that notion.
Six current-measuring instruments were fortuitously sitting on the seafloor in the path of Hurricane Ivan, which struck the Gulf Coast in mid-September 2004 (SN: 6/11/05, p. 382: . . . and churn up big waves, too). From the speed and direction of subsurface currents, Teague and his colleagues estimated the hurricane’s wind speed and a parameter called the coefficient of drag, which describes how efficiently the wind’s momentum transfers to surface waters.
The coefficient of drag reached its peak when wind speeds measured around 115 kilometers per hour—a pace just shy of the threshold for a category 1 hurricane, the team reports in the March 23 Science. Beyond that peak, the drag coefficient steadily dropped as wind speeds increased. That might have been because hurricane-force winds caused large waves to break, dissipating energy through the prodigious generation of ocean spray, bubbles, and foam, Teague says.