The strong winds of a tropical cyclone whip up the sea surface, driving ocean waves a dozen meters (about 40 feet) high. When one such ocean wave runs into another wave that has an equal period but is traveling in the opposite direction, the interaction produces low-frequency sound waves that can be detected thousands of kilometers away. Known as microbarom, the infrasound signals produced by interacting ocean surface waves have typical frequencies around 0.2 hertz.
Researchers previously determined that as a hurricane travels along its track, waves generated by the storm earlier in time will interact with those generated later on, producing a strong microbarom signal in the storm's wake. Researchers also found, however, that microbarom signals are produced by regular surface ocean behavior, including swell, surface waves, or non-tropical cyclone storms.
To identify how tropical cyclone-produced waves interact with ambient surface ocean waves, and to determine whether the tropical cyclone microbarom signal could be isolated from the background noise, Stopa et al. examined the infrasound signals detected by an International Monitoring System infrasound sensor array in Hawaii during the passage of Hurricanes Neki and Felicia in 2009.
The authors used modeled wind speeds to simulate the wave conditions during the hurricanes, then used these estimates to drive an acoustic model that enabled them to calculate the microbarom infrasound activity. They find that the microbarom signals observed by the Hawaiian sensor array aligned with their modeled estimates. The authors note that the infrasound signal of the cyclone-generated waves tended to swamp the detectors, drowning out the much weaker signals of the ambient wave interactions. They suggest that given further refinements, measuring the infrasound signal of microbarom waves could be a good way to detect and measure the wave conditions near a tropical cyclone.
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More information: Atmospheric infrasound from nonlinear wave interactions during Hurricanes Felicia and Neki of 2009, Journal of Geophysical Research - Oceans, doi: 10.1029/2012JC008257 , 2012