SAR Observation of Eddy-Induced Mode-2 Internal Solitary Waves in the South China Sea

Two cases of mode-2 internal solitary waves (ISWs) induced by an anticyclonic eddy (AE) were clearly present in two synthetic aperture radar images acquired in the South China Sea (SCS) in April 2001. Similar ISWs were repeatedly observed in the same area one month later in May, but in that case, the ISW patterns indicated that they were regular mode-1 ISWs. To confirm that those ISWs in April are of mode-2 type, we analyze the in situ and other remote sensing data and propose two possible mode-2 ISW generation processes: 1) an eddy-induced change of the water stratification, which results in favorable hydrographic conditions for internal wave generation, and 2) the resonance between mode-1 internal tides and AE excites mode-2 internal tides, and the mode-2 internal tides disintegrate into mode-2 ISWs. The sea level anomaly and sea surface temperature data in April 2001 show that an AE existed in this area in April. Argo profile data within AEs from 1990 to 2014 in the SCS are used to show how an AE affects the vertical water properties and creates favorable conditions for mode-2 ISWs, and these conditions did exist in the study area in April 2001. The observed mode-2 ISW cases provide the first evidence of the existence of eddy-induced mode-2 ISWs in the SCS.