Swells are waves from other ocean areas or generated locally but do not absorb energy from the wind anymore. Swells have longer wavelength than wind waves and can propagate over a very long distance in the ocean. In this study, the in-situ data from NDBC buoys located openly to the southwest are used to determine the potential destination of swells propagating from the westerlies for southern hemisphere. Meanwhile, the CFOSAT SWIM data and ST6 reanalysis data are used to trace back the trajectories and the sources of these swells. Accordingly, we find 25 routes of swells originated from 4 series of ocean storms. To verify the accuracy of these paths, we check the variation of wave parameters in 48 hours before and after SWIM observation. It shows clearly that swells from the southwest have passed through and continue to travel northeastward. The one-dimensional wave spectra of SWIM data and NDBC buoy data are compared which indicates the attenuation of energy. It is shown that magnitude of decaying rate of swell energy increases with the spectral width of initial swell field. In addition, the general rate of increase for peak wavelength is an order of 0.01m/km, which is apparently the spectral width dependent. These are mainly due to the higher degree of dispersion and angular spreading for broader spectra. To quantity the energy that decays due to spherical spreading, the point source hypothesis is used between SWIM observation points and NDBC buoys. Besides, the ST6 reanalysis dataset without considering swell dissipation and negative wind input is compared to the real observation data to help obtain the spherical spreading values from sources to SWIM and from sources to buoys. Linear and nonlinear dissipation rates are calculated according to the air-sea interaction theory and wave-turbulence interaction theory. The result shows that the intensity of dissipation is stronger near the sources (the linear dissipation rate is about 10^(-7) m^(-1)) and decreases in the subsequent propagation.