Figure 4
From: Observation of a giant nonlinear wave-packet on the surface of the ocean
Left column: the signal recorded by the buoy is plotted in red line. The blue lines represent the modulus of the envelope computed by using Hilbert transform method. The blue shaded area indicates the region considered for the nonlinear spectral analysis. In the three cases reported in the left column, the width of the time window used for IST analysis is 180 s. Central column: discrete IST spectra computed using the experimental signal plotted in the blue shaded regions shown in the left column: zero boundary conditions (ZBC) are applied. Two discrete eigenvalues are found well above the real axis only for the large wave packet (see upper line). Right column: the spectra computed using the experimental signal plotted in the blue shaded regions shown in the left column: periodic boundary conditions (PBC) are applied. With these boundary conditions, spines crossing the real horizontal axis are obtained instead of discrete eigenvalues that are obtained with ZBC. Using PBC spines are spectral signatures of nonlinear modes. In (c) two spines of large amplitude are obtained when the giant wave packet is analyzed. Those two spines are not found when the window of analysis does not overlap the giant wavepacket.
