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

Over the past three decades, the long-term trends of surface eddy kinetic energy (EKE) in the South China Sea (SCS) were investigated using the satellite altimetry datasets. Here, the linear and nonlinear trends derived from the ordinary least squares and multidimensional ensemble empirical mode decomposition methods, respectively, were presented and compared. The comparison indicated that the EKE had undergone a nonlinear evolutionary process during 1993~2023. In the first decade, the SCS exhibited a dominant decline of EKE, while there was an accelerating upward trend in the last decade. The long-term trends were inhomogeneous in the 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 serve as the primary trigger for the boosted mesoscale variability. In addition, the trends of EKE in the SCS exhibited noteworthy seasonality, which showed 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 exist in seasonal series. Of 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.