Seasonal Frontal Variability and Stratification Coupling from 2022 CTD Observations in the 1 Southern Coastal Waters of Korea

Baekjin Kim¹Seonghyeon Kim¹Soonyeol Kwon¹Sunjong Lee²Eung Kim^1*

• ¹Sea Power Reinforcement·Security Researcher Department, Korea Institute of Ocean Science and Technology (KIOST), Busan, Republic of Korea
• ²Vessel Observation Team, Korea Institute of Ocean Science and Technology (KIOST), Geoje, Republic of Korea

Using May–November 2022 CTD observations from the central–eastern southern Korean shelf, we quantified the variability of oceanic fronts and their coupling with water-column stability. We used conventional |∇T|, |∇S| thresholds and a composite Integrated Frontal Index (IFI) of |∇T|, |∇S|, and N²; the stability effect is ∆IFI= 〖IFI〗_(TSN^2 ) - 〖IFI〗_TS. Fronts were coherently organized along the coastal–offshore boundary zones near the 70–100 m isobaths. Their intensity and principal formation depths reflected seasonal stratification and thermocline–halocline alignment (∆THD): fronts were sharp and continuous when the clines co-occurred in summer, but diffuse and deeper when offset during autumnal cooling and mixing. Quantitatively, in the upper layer (10–30 m) the IFI closely co-locates with salinity fronts (≈99–100%) and also with temperature fronts (~87–93%), whereas agreement declines and becomes more variable below about 40–50 m. ∆IFI exhibited a depth-dependent pattern (mean −5.7 at 10 m, −1.3 at 60 m, −4.6 at 80 m), consistent with surface weakening, mid-layer mitigation, and renewed deep weakening. The framework captures transitions between salinity- and temperature-dominated regimes and quantifies the amplification–weakening mechanism associated with ∆THD, providing an observation-based diagnostic for front–stratification coupling.