We have observed in the past several episodes of southern swell using altimetry-derived Significant wave heights. Those are storm-generated swells, especially from storms occurring in the roaring forties. They propagate in the whole Indian Ocean from the Southern Ocean, including in towards areas like La Reunion Island, where low winds allow swell free propagation. On late August 2022 a warning was emitted by Météo France for such an episode threatening La Réunion on Sept. 1^st & 2^nd, 2022.

CFOSat has new capabilities with respect to the altimeters concerning wave measurements. The SWIM instrument onboard enables to estimate the directional wave spectrum - height, direction and wavelength (or period), thanks to its rotating beam with a varying angle. With such information available, the propagation of a Southern Swell in the Indian Ocean can be observed through the swell wavelength, which is linked to the energy released by the phenomenon (the longer the wavelength, the higher the released energy). SAR sensors such as the one onboard Sentinel-1 have been used to retrieve swell information, too, for a longer time.