Oceanic winds estimated from underwater ambient noise observations in SWADE

Abstract

The WOTAN technique to estimate oceanic winds from underwater ambient sound developed in the FASINEX experiment is evaluated using measurements from the Surface Wave Dynamics Experiment (SWADE). The WOTAN was deployed from the Minimet buoy, MET-3, which was part of the SWADE array, and carried sensors for measuring the surface meteorology with sufficient accuracy. The WOTAN sensor was located at a depth of 513 m and recorded ambient noise levels at 11 frequencies in the range 3-25 kHz every 7.5 minutes for over six months. These noise sound spectrum levels are compared with the observed buoy wind speeds and other computed wind variables to establish frequency and wind speed dependencies. The ambient sound timeseries are first screened to eliminate contamination from shipping and precipitation (rain) noise. This quiet data set is then standardized to 1 m depth using site and frequency-dependent correction factors. The variation with wind speed of the sound spectrum level at 1 kHz and the spectral slope is further examined. Linear and quadratic relationships between noise level and wind variables are derived. These relationships are then inverted for estimating the wind variables using the noise spectrum and pressure levels. Statistical comparisons are made between the WOTAN derived estimates and the buoy observations. Differences between the FASINEX and SWADE algorithm are also examined.