Observational evidence of the interaction of ocean wind-sea with swell

Abstract

More than 10 000 spectra of ocean wave data were acquired from a series of buoys moored in the Southern Ocean off the west coast of Tasmania for a period of seven years. Spectra were grouped according to the wind speed and whether the wind direction was onshore or offshore and the mean spectrum found for each group. The frequencies of the low frequency cut-offs and of the spectral peaks of the resulting mean spectra were found to be independent of the wind speed in contrast to self-similar standard spectra such as JONSWAP. This property is attributed to the presence of a swell background which controls the evolution of wind-seas in the open ocean. During offshore winds, the spectral variance between 0.04 Hz and 0.16 Hz was found to be negatively correlated with wind speed indicating that swell is in turn shaped either by the wind-sea or by the wind itself. Relationships between various spectral and time domain parameters and the wind speed were investigated empirically. The slope/acceleration measures such as m4, the spectral fourth moment, were found to be highly correlated with wind speed.