The Korea/Tsushima Strait (KTS) has exhibited a significant increase in temperature stratification over the past four decades, yet its underlying cause remains unclear. A Cyclostationary Empirical Orthogonal Function analysis is applied to identify the dominant modes of temperature variability using in-situ observations obtained from the Korea Oceanographic Data Center (1982–2023). Two major modes are identified aside from the seasonal cycle: one capturing the interannual variability of the Korea Strait Bottom Cold Water, and another representing the long-term strengthening of temperature stratification. While the former mode shows no significant trend in its principal component time series, the latter mode serves as a key contributor to the long-term trend of the observed temperature variability. Further analysis shows that this mode is linked to variations in the strength and position of the axis of the Tsushima Warm Current (TWC), which is the primary inflow through the KTS. While the enhanced inflow transports more heat through the upper layer, an offshore shift of the TWC allows coastal subsurface cold water along the east coast of Korea to reach the bottom layer of the KTS more effectively in summer, thereby reinforcing stratification. This mode is also negatively correlated with the Pacific Decadal Oscillation (PDO) index at a lag of 1–2 years, suggesting that the large-scale wind stress curl anomalies associated with the PDO may play a role in modulating the variability of the TWC. These findings underscore the need for continued monitoring of temperature variability in the KTS, given its implications for heat transport through the KTS and its impact on the ecosystem in the East/Japan Sea.

Korea/Tsushima Strait,Tsushima Warm Current,Temperature Stratification