Numerical Simulation and Observational Data Analysis of Mesoscale Eddy Effects on Surface Waves in the South China Sea

Abstract

Surface current velocities of mesoscale eddies have a unique annular structure, which can inevitably influence surface wave properties and energy distribution. Sensitivity experiments of ideal mesoscale eddies on waves were carried out by the Simulating WAves Nearshore (SWAN) wave model to investigate these influences. In addition, China–France Oceanography SATellite Surface Wave Investigation and Monitoring (CFOSAT-SWIM) observational data of a large warm-cored eddy in the South China Sea (SCS) during the period of October–November 2019 were used to validate the influence of mesoscale eddies on waves. The results illustrated that mesoscale eddies can alter wave properties (wave height, period, and steepness) by 20–30%. Moreover, wave direction could also be modified by 30°–40°. The current effect on waves (CEW) was more noticeable with strong currents and weak winds, and was governed by wave age and the ratio of wave group velocity to current velocity. Wave spectra clearly indicated that current-induced variability in wave energy distribution happens on a spatial scale of 5–90 km (i.e., the sub- and mesoscales). Through comparing the difference of wave energy on both sides of an eddy perpendicular to the wave propagation direction in an eddy, a simple way to trace the footprints of waves on eddies was devised.

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

“…For the variation in wave direction, our result of 23.3°–28.9° is larger than the 9° deflection caused by a typical eddy in the Antarctic Circumpolar Current region (Gallet & Young, 2014) and 10° in the Equatorial Current region (Echevarria et al., 2021), but smaller than 30°–45° at the strong frontal jet (Romero et al., 2017). Compared to the variations in SWH and wave direction induced by strong warm eddies near the Vietnam coast (Wang et al., 2022), the statistical results of wave modulation derived from the 33 eddies distributed in the whole SCS in our study are one‐half weak. The current vorticity and wave‐eddy angle are found to be factors affecting the wave direction, while eddy‐induced wind direction variation also plays an important role in the eddy area.…”

“…Wang et al. (2022) found the wave heights of eastward propagation waves decrease (increase) in the northern (southern) two quadrants across a warm eddy. Here, we choose the long axis of the eddy in the zonal direction as the x ‐axis, and divide the eddy into four quadrants in the clockwise direction, that is, quadrants 1 and 4 on the northern side and quadrants 2 and 3 on the southern side.…”

“…The SWH and wave direction variations seem to be less dependent on the eddy quadrants but depend mainly on the wave-current angle. The dependence of the wave height variation on the eddy quadrant shown by Wang et al (2022) is implicitly associated with the wave-eddy angle.…”

“…As the sea surface wave encounters a background current, exchanges of momentum and energy may occur between the wave and current, resulting in significant variations in wave features (Holthuijsen & Tolman, 1991; Romero et al., 2020). Thus, wave‐current interactions have become an area of high research interest in ocean dynamics (Cheng et al., 2022; Echevarria et al., 2021; Tamura et al., 2008; Wang et al., 2022; Wolf & Prandle, 1999; Xie et al., 2001).…”

“…Unfortunately, such important research has not yet been actively and thoroughly pursued. Using a numerical model, Wang et al (2022) investigated the effects of larger warm eddies on surface waves in the western SCS near Vietnam and found the wave height varied by 20%-30% as that derived from two cases of CFOSAT data. However, there is still a lack of studies on wave modulation by eddies in the whole SCS based on observations.…”