AUTHOR=Kim Baekjin , Kim Seonghyeon , Kwon Soonyeol , Lee Sunjong , Kim Eung 

TITLE=Seasonal frontal variability and stratification coupling from 2022 CTD observations in the southern coastal waters of Korea

JOURNAL=Frontiers in Marine Science

VOLUME=Volume 12 - 2025

YEAR=2025

URL=https://www.frontiersin.org/journals/marine-science/articles/10.3389/fmars.2025.1723724

DOI=10.3389/fmars.2025.1723724

ISSN=2296-7745

ABSTRACT=IntroductionUsing May–November 2022 CTD observations from the central–eastern southern coastal waters of Korea, we quantified the variability of oceanic fronts and their coupling with water-column stability.MethodsWe used conventional |∇T|, |∇S| thresholds and a composite Integrated Frontal Index (IFI) of |∇T|, |∇S|, and N²; the stability effect is ΔIFI= IFITSN2 - IFITS​.ResultsFronts 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 (ΔTHD ): 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. ΔIFI 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.DiscussionThe framework captures transitions between salinity- and temperature-dominated regimes and quantifies the amplification–weakening mechanism associated with ΔTHD , providing an observation-based diagnostic for front–stratification coupling.