The Periodic Cyclonic Eddy in Southwestern Taiwan and Its Interannual Variation Related to Large‐Scale Climate Variations.

Periodic eddies are a type of eddy that occur almost annually in fixed timeframes with similar patterns and trajectories. Nearly every year from April to June, under the combined effect of the barotropic instability of the mean flow and wind work, a cyclonic eddy (the Taiwan Cyclonic Eddy, TCE) forms in the southwest of Taiwan, then propagates westward, and finally dissipates near the Dongsha Islands. TCE exerts a significant impact on the Kuroshio intrusion into the South China Sea (SCS) and water exchange. Based on multi‐year in situ and satellite observations, this study reveals the thermohaline structure and evolutionary process of the TCE. The evolutions of the three‐dimensional structures of temperature, salinity, and geostrophic velocity of the TCE are analyzed based on reconstructed data. The TCE shows important interannual variations associated with El Niño–Southern Oscillation (ENSO), and the relationship between ENSO and the TCE is modulated by the Pacific Decadal Oscillation (PDO). In the negative phase of the PDO, the intensity of the TCE is significantly correlated with the Niño‐3.4 index. In contrast, in the positive phase, the ENSO–TCE relationship becomes weak and non‐significant. Further investigations indicate that these differences are related to the establishment of the low‐latitude Pacific–East Asian Teleconnection, influencing local wind stress curl in the region. This offers a new perspective on understanding the interannual variation of periodic mesoscale eddies in the SCS. Plain Language Summary: The Taiwan Cyclonic Eddy (TCE) represents a significant periodic mesoscale eddy in the South China Sea (SCS), demonstrating annual occurrences and profound impacts on regional oceanography and climatological dynamics. This study expands on multi‐year in situ and satellite observations to elaborate the three‐dimensional thermohaline structure and dynamic behavior of the TCE, emphasizing its seasonal consistency and interannual variability influenced by major climatic oscillations such as El Niño–Southern Oscillation (ENSO) and the Pacific Decadal Oscillation (PDO). The TCE's formation is primarily driven by the instability of the background current and significant wind forcing. Its interannual variation shows a non‐stationary relationship with ENSO, which is significantly modulated by the PDO. The study enhances our understanding of the interannual dynamics of the TCE by highlighting how shifts in large‐scale climatic conditions can directly alter the local structural and dynamic characteristics of the eddy. Understanding the TCE's formation and variation is vital as it affects water exchange in the SCS, and thus influences local marine ecosystems in the region. By studying this phenomenon, scientists can better predict the impacts of such eddies on marine and climatic environments. Key Points: The Taiwan Cyclonic Eddy (TCE) forms annually from April to June southwest of Taiwan due to current instability and wind forcingMulti‐year observations show the TCE has significant vertical extensions and temperature/salinity anomalies, impacting local conditionsThe TCE's evolution is tied to El Niño–Southern Oscillation and influenced by Pacific Decadal Oscillation phases, being stronger in negative phases due to enhanced teleconnections and wind adjustments [ABSTRACT FROM AUTHOR]