To investigate the behavioral characteristics of the water temperature inversion layer (TIL), we used data (KODC) from areas in the Northern East China Sea from 1995 to 2016. Water temperature and salinity surveys were conducted 8820 casts over 22 years. Of these, 1589 water temperature inversion layers were found, and the probability of occurrence was 18.0%. In the Gageo island, probability of TIL occurrence in winter was 25 times higher than in summer. On the other hand, in the south of Jeju Island, summer values were 3.7 times higher than winter values. A T–S diagram analysis shows the components of the water temperature inversion layers. Yellow Sea Cold Water was mainly found in the winter, while Jeju Warm Currents and Tsushima Warm Currents were found in summer. The correlation between the probability of the occurrence of a monthly water temperature inversion layer and the amount of seawater volume transported into the study area was analyzed. The correlation coefficient was higher than r = 0.8 in parts of southern Jeju Island. On the other hand, the correlation coefficient was r = −0.6 in the Gageo Island. The spatial correlation index for the seawater volume transport and the water temperature inversion layer is presented.

In this study, we investigated the effect of a water temperature inversion layer on the propagation of acoustic waves in the western coastal sea of Jeju Island in April 2015. When the acoustic source and receiver are simultaneously located within the water temperature inversion layer depth, the long-range propagation of acoustic waves is confirmed by numerical modeling. This is caused by the duct effect due to the water temperature inversion phenomenon. For the experimental area without the water temperature inversion layer, when the acoustic source and receiver are simultaneously located below thermocline depth, the long-range propagation of acoustic waves is also confirmed. This is generally caused by the seasonal water temperature profile.

Introduction: Using 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. Methods: We used conventional | ∇ T | , | ∇ S | thresholds and a composite Integrated Frontal Index (IFI) of | ∇ T | , | ∇ S | , and N²; the stability effect is Δ IFI = I F I T S N 2 - I F I T S ​. Results: 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 (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. Discussion: The 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.

To analyze variations in the vertical distribution of water temperatures and the impact of advection in the central Yellow Sea, multi-layer water temperature and current observations were conducted from 31 May to 8 June 2021. Water temperatures exhibited a typical three-layer summer structure, with a uniform deep-layer temperature averaging 8.23 ± 0.05 °C. The current field was dominated by northeast–southwest tidal currents, but residual current characteristics indicated that non-tidal components significantly influenced circulation. Water temperature changes lagged tidal changes by about 3 h, with strong correlations (R > 0.7), especially in deep layers. Residual currents showed significant correlations with water temperature variations, which were attributed to advective displacement or baroclinic currents. Empirical orthogonal function (EOF) and complex EOF analyses revealed that thermocline variations (T1, explaining approximately 75% of total variance) were driven by strong northward (C1, approximately 34%) and cyclonic (C2, approximately 32%) advection. In deep layers, slight temperature changes were caused by southward Yellow Sea Cold Water Mass (C1) and northward Yellow Sea Warm Current Water (C2) propagation. This study confirms that vertical water temperature variations result from a complex interaction between various advection patterns, with southward tide-induced residual currents (C3, approximately 12%) playing a key dynamic role.