Trends of Eddy Kinetic Energy in the South China Sea Observed from Three Decades of Satellite Altimetry Records

The long-term trends over the past three decades of surface eddy kinetic energy (EKE) in the South China Sea (SCS) were investigated using satellite altimetry datasets. Here, the linear and nonlinear trends derived from the ordinary least-squares and multidimensional ensemble empirical mode decomposition methods, respectively, are presented and compared. The comparison indicated that EKE has undergone a nonlinear evolutionary process during 1993–2023. In the first decade, the SCS exhibited a dominant decline in EKE, while there was an accelerating upward trend in the last decade. The long-term trends were inhomogeneous in spatial structure, and the accelerated growth of EKE primarily occurred north of 10°N. Eady theory suggested that the enhanced baroclinic instability over the past three decades may have served as the primary trigger for the boosted mesoscale variability. In addition, the trends of EKE in the SCS exhibited noteworthy seasonality, showing a prominent increase in winter and a decrease in the other three seasons. Although the nonlinear trends of the annual mean EKE failed the 95% significance test, statistically significant nonlinear trends still existed in seasonal series. Among the four seasons, the nonlinear trends in summer were statistically significant across all time points, and significant trends were observed in winter prior to 2003.