[A-OS12] Ocean Science Revealed by Global Observation System

[AOS12-06] Anticyclonic Eddy Formation from the Kuroshio Loop Current in the Northern South China Sea: Insights from 24 Years of Altimetry Data

*Yu-Chun Su¹, Ming-Huei Chang^1,2, Yu-Hsin Cheng³ (1.Institute of Oceanography, National Taiwan University , 2.Ocean Center, National Taiwan University, 3.Department of Marine Environmental Informatics, National Taiwan Ocean University)

Keywords:Kuroshio, Kuroshio loop current, anticyclonic eddy, northeast monsoon

The Kuroshio, a western boundary current in the North Pacific, passes through the Luzon Strait—a gateway between Taiwan and Philippines—and can enter the northern South China Sea (SCS), forming a loop current southwest of Taiwan, particularly during the winter. The loop current can evolve into anticyclonic eddies (AEs) through shedding processes. We computed the Rossby number as Ro=z/f, where z and f are the relative vorticity and planetary vorticity, respectively, using satellite altimetry data collected from 2000 to 2024. Time series of Ro, obtained by taking the average value of Ro within a 200 × 200 km region where the loop current and AE are generally formed, reveals the intense negative vorticity events (<-0.35f) associated with the loop current or AE (1) primarily occurs in November-March and (2) has a significant increase in long-term trends in the past 24 years. As a result, more intense AEs have been observed over the past decade. The method of the Okubo-Weiss parameter extracts six AE events occurring in 2011–2012, 2013–2014, 2016–2017, 2019–2020, 2020–2021, and 2022–2023, which are further examined. The six AEs reveal an average propagation speed of 0.08 m s^-1, an average eddy kinetic energy of 0.1 m² s^-2, an average diameter of ~160 km, and a 200 to 400 days lifespan. The lifespan of these eddies is categorized into three stages based on dynamic evolution: developing (~30%), mature (~20%), and propagating (~50%) stages. All six AEs occurred during the northeast monsoon burst. The peak wind stress leads Ro value 15-60 days, suggesting winter wind stress (> 0.22 N m^-2) is essential for the AE formation. Other dynamic factors, including the duration of the northeast monsoon, shifts in the Kuroshio entry patterns into the South China Sea, and the eddies off the eastern coast of Taiwan, may lead to variations in lag time. Interestingly, four of the six identified events occurred during the La Niña years, which are known to trigger stronger northeast monsoons.