1. Where does background noise in the ocean come from? ORE 654 Guest Lecture Fred Duennebier fred@soest.hawaii.edu October 21, 2011

2. Outline Ocean Background Acoustic Levels What has been observed? Microseisms Kibblewhite, Wake, Cessaro, Webb, H2O, Bromirski What’s missing? New Measurements ALOHA Cabled Observatory (ACO) Correlation with Wind Correlation with Waves Seismic Correlation Importance of Spreading Function Conclusions

3. E A: Ship B: Rain C: Humpback D: Blue E: Minke

4. play sounds

7. Webb, 1992 Noise levels near 0.2 Hz are ~18 dB higher in the Pacific than the N. Atlantic and ~42 dB higher than in the ice-covered Arctic.

8. What’s missing? What would make the data more useful? LONG time series deep-ocean large fetch concurrent wind, wave, seismic measurements

10. WHOTS OCEAN BUOY at Station ALOHA, 2010

11. Waimea Bay Waverider Buoy

16. ALOHA PROOF ACOUSTIC DATA 20 – month acoustic time series 0.02- 10 KHz Bandwidth 24- bit samples 24,000 samples/sec pre-whitening filter >150 dB dynamic range

20. ACO Installed and Operating as of June, 2011

26. How does energy from the WIND get to the ocean floor?

27. WAVE GENERATION

28. How does energy from the WIND get to the ocean floor? WAVE GENERATION Longuet-Higgins mechanism

29. How does energy from the WIND get to the ocean floor? WAVE GENERATION Longuet-Higgins mechanism Lateral transport as seismic waves

30. Wave Amplitude Wavenumber 2π/ Wavenumber at peak amplitude

31. frequency at peak wave amplitude Wind speed at 10 m elevation Nondimensional frequency Wave frequency

33. Longuet-Higgins Pressure at the ocean floor Angular acoustic Frequency Density/sound velocity Overlap Integral Spreading Function Angle from downwind direction HUGHES’ Equation

39. Why is this important? For wave energy to propagate to the ocean floor requires opposing wave trains of the same frequency. The sound then observed in the deep- ocean will have TWICE THE FREQUENCY of the waves and amplitude proportional to the product of the amplitudes of the opposing waves.

41. 0.1 1 10 0.1 1 10 0.1 0.5 5 10 15

45. Using Nondimensional Frequency

46. Summer

47. Winter

48. Sound Propagation by the Longuet-Higgins mechanism requires OPPOSING waves. Most long-period seas have only a very small opposing component, and do not contribute to the sound field.

49. Kedar et al., 2008