Kinematic Characteristics of Mesoscale Eddies in the Northwestern Pacific and South China Sea

Mesoscale eddies are a widely occurring "weather-scale" oceanic dynamic phenomenon. Identifying mesoscale eddies and analyzing their kinematic characteristics are of great significance for oceanographic research, fisheries, navigation, and military strategy. This study utilizes satellite altimeter data from 1993 to 2022, combined with an optimized mesoscale eddy identification and tracking algorithm, to identify and track mesoscale eddies in the Northwest Pacific and the South China Sea, obtaining key kinematic parameters of eddies in these regions. A total of 16 240 eddies were identified, with cyclonic and anticyclonic eddies occurring in nearly equal numbers. Long term eddies (lasting 28 d or more) accounted for 6 458 cases, while short-term eddies (10~27 d) totaled 9 782, making up 60.2% of all eddies and contributing 21.1% of the total eddy kinetic energy. This underscores the critical role of short-term eddies in mesoscale eddy research and oceanic energy balance. Further analysis revealed that the subtropical countercurrent region, the western Luzon Strait, and eastern Vietnam are the primary generation areas for long-term eddies. Meanwhile, short-term eddies are mainly generated in the western boundary current region of the Northwest Pacific, the subtropical countercurrent region, the western Luzon Strait, the western side of Luzon Island, and southeastern Vietnam. The spatial distribution of short-term eddies' mean radius, amplitude, relative vorticity, propagation speed, and kinetic energy resembles that of long-term eddies, but their intensity is significantly lower, suggesting that eddy lifespan is influenced by eddy strength. Regardless of lifespan, eddies in the Northwest Pacific exhibit greater numbers, larger mean radius, higher amplitudes, and faster propagation speeds than those in the South China Sea, while their mean vorticity is lower. The mean kinetic energy of eddies in both regions is comparable. Among the key eddy formation areas, southeastern Vietnam has the highest relative vorticity, the subtropical countercurrent region has the greatest number and amplitude of eddies, and the tropical region features the largest radius and kinetic energy. Meanwhile, the western Luzon Strait shows intermediate values across all parameters. This study delineates the fundamental kinematic characteristics of mesoscale eddies in the Northwest Pacific and the South China Sea, highlights the importance of short-term eddies, and enhances the understanding of mesoscale eddies in the studied regions.