Surface Wave Development and Ambient Sound in the Ocean

Wind, wave, and acoustic observations are used to test a scaling for ambient sound levels in the ocean that is based on wind speed and the degree of surface wave development (at a given wind speed). The focus of this study is acoustic frequencies in the range 1–20 kHz, for which sound is generated by the bubbles injected during surface wave breaking. Traditionally, ambient sound spectra in this frequency range are scaled by wind speed alone. In this study, we investigate a secondary dependence on surface wave development. For any given wind‐speed, ambient sound levels are separated into conditions in which waves are 1) actively developing or 2) fully developed. Wave development is quantified using the non‐dimensional wave height, a metric commonly used to analyze fetch or duration limitations in wave growth. This simple metric is applicable in both coastal and open ocean environments. Use of the wave development metric to scale sound spectra is first motivated with observations from a brief case study near the island of Jan Mayen (Norwegian Sea), then robustly tested with long time‐series observations of winds and waves at Ocean Station Papa (North Pacific Ocean). When waves are actively developing, ambient sound levels are elevated 2–3 dB across the 1–20 kHz frequency range. This result is discussed in the context of sound generation during wave breaking and sound attenuation by persistent bubble layers. Plain Language Summary Recordings of sound in the open ocean are usually louder when it is windy. This is because winds cause breaking waves at the surface of the ocean (whitecaps), which inject bubbles into the upper ocean. This study introduces wave measurements as a secondary dependence on ambient sound levels. We show that including wave measurements can improve interpretation of sound in the ocean, relative to using wind measurements alone. In particular, sound is loudest when there are a lot of small waves breaking and no larger waves present. Key Points Underwater sound from 1 to 20 kHz is usually associated with wind forcing and the bubbles generated by ocean surface waves Considering surface waves in addition to surface winds can improve quantification of underwater sound Ambient sound levels are elevated 2–3 dB when waves are developing, relative to fully developed waves at given wind speed