Some science behind the scenes

Storm waves and surf

Nonlinear ocean wave interactions in ocean storms produce pervasive infrasound vibrations around 0.2 Hz, known as microbaroms.

The evidence that severe waves produce infrasound seems to be fairly well established and proven.

Researchers from the USArray Earthscope, for example,  have tracked down a series of infrasonic humming noises produced by waves crashing together and thence into the ocean floor, off the North-West coast of the USA. Potentially, sound from these collisions could travel to many parts of the globe.

Microbaroms are caused by standing sea waves in marine storms.  The  low-level natural-infrasound lies in the frequency range from 0.02 to 10 Hz and they have a period of about five seconds.

They are extremely interesting signals, because they are ‘pulsed’.  The pressure versus time traces of microbaroms appear as an amplitude modulated string of “pearls” or wave packets of 0.2 Hz oscillations with a packet length of from 2 to 5 cycles. An example can be seen in Figure 1 below.

Figure 1. Pressure trace for Microbarom signals at I53US for a 400 second time period from 23:30:00 to 23:36: 40 UT on Oct. 21, 2005.

Microbaroms are radiated in all directions from a marine storm centre.  At infrasonic arrays all over the Earth during all seasons, in periods of no local wind-noise, microbaroms can be detected in the infrasound signal observations even though the microphone arrays may be thousands of kilometers from the source storms. The main thing which can disrupt them is upper level head winds, thus infrasound may not be observed at an infrasonic array if the strength and direction of the upper level winds along the propagation path between a marine storm and an infrasonic array disrupts it.  The microbarom signal strength observed at an infrasonic array is greatest in the direction that is down-wind from the source.

An example of a microbarom-producing marine storm on October 21, 2005 in the Gulf of Alaska is shown in Figure 2 as a map of the surface- pressure contours. The long black arrow in Figure 2 from the centre of the storm shows the direction of propagation of the microbarom signals to I53US at Fairbanks. During the day the location of the storm changed, drifting toward the south from an azimuth of 220 deg. to 180 deg over a 24 hour period. This motion of the source storm is consistent with the change in the direction of the microbarom signals received at I53US as plotted in Figure 3 below. The azimuth of the received microbarom signals drifted at about 1.7 degrees per hour following the storm’s motion.

Figure 2. Surface pressure contours for the marine storm of Oct 21,2005 in the Gulf of Alaska from the U. S. Weather Bureau at Fairbanks. The black arrowhead shows the direction of propagation of microbaroms from the storm to I53US.

 

Figure 3. Drift with time of the microbarom azimuth of arrival on Oct. 21, 2005 at a rate of about 1.7 deg/hour as the storm center moved from SSW to SSE.

If the propagation conditions are suitable then it is sometimes possible to triangulate on the source region of a microbarom storm using several highly separated infrasonic arrays.

This was done for the tropical storm “Barbara” in the Pacific Ocean of June 20, 2001 by using microbarom azimuth data from Hawaii, Fairbanks and data from an array near Los Angeles. The intersection of the three azimuth directions from the three infrasonic arrays is shown in Figure 4 to be in the Pacific where tropical storm “Barbara” was located.

 

Figure 4. Triangulation on microbaroms from the tropical storm Barbara on June 20, 2001 from Hawaii, Fairbanks and Los Angeles.