Spectrogram analysis of the time-frequency characteristics of ocean wind waves

The spectrogram based on the short-time Fourier transform is proposed as a tool to study the time frequency evolution of the properties of ocean wind waves. After defining the method to determine the spectrogram of wind waves an example of application is given by analyzing the time series from three sea states concerning a swell, a wind sea and a mixed sea situation. It is shown how the power spectral density function changes with time and its nature when a wave group exists. The result suggests that the energy transfer from high to low frequencies does not occur continuously but in localized periods, exactly when the wave groups occur.