Modulation Effects of Mesoscale Eddies on Sea Surface Wave Fields in the South China Sea Derived From a Wave Spectrometer Onboard the China-France Ocean Satellite

This study aims to examine the modulation effects of mesoscale eddies on sea surface wave fields in the South China Sea using data measured by a wave spectrometer named Surface Wave Investigation and Monitoring onboard the China-France Ocean Satellite from July 2019 to August 2021. The statistical results show that the significant wave heights (SWHs) decrease (increase) from inside to outside of warm (cold) eddies, while the wavelengths have minimum values at the eddy edge in all cases. In cases of strong warm eddies, the maximum variations in SWH, wavelength, and wave direction caused by mesoscale eddy modulation reach up to 18%, 24%, and 28.4 degrees, respectively. The eddy modulation on wave parameters is stronger at low sea states. The correlation analysis reveals that the wave parameter variations across eddies result from the eddy current shear and the direction difference between the eddy current and the waves. The dynamic analysis indicates that the deformation term of the eddy current is a dominant term affecting the SWH at the eddy edge, and the eddy current divergence and its product with wave direction determine the wavelength variation. The eddy vorticity deflects the wave direction by approximately 2 degrees-3 degrees, and the wave-eddy angle and eddy-modulated wind variation also contribute to the wave direction variation. The background current-wave interaction is negligible in the eddy cases studied. Plain Language Summary Sea surface waves have a direct influence on all human activities that take place at sea. Therefore, scientists have developed technologies for obtaining data on sea surface wave fields from satellite observations. Among them, the wave spectrometer onboard the China-France Ocean Satellite is an updated instrument providing real-time wave height, wavelength, and wave direction. This study aims to clarify the modulation effects of ocean mesoscale eddies on sea surface wave fields using wave spectrometer data. The test area is the South China Sea deep basin, where energetic mesoscale eddies appear in satellite altimeter images year-round. Statistical analysis results of the data from July 2019 to August 2021 indicate that mesoscale eddies indeed modulate sea surface wave fields with maximum statistical mean variations of significant wave height, wavelength, and wave direction up to 18%, 24%, and 28.4 degrees. Physically, the wave parameter variations result from the eddy current shear and direction difference between the eddy current and waves. The effects of the background current and the eddy-induced wind variation are analyzed. Current-wave interaction theories generally support observations of wave parameter variability from wave spectrometers.