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

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 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline" id="im1"> <mml:mrow> <mml:mrow> <mml:mo>|</mml:mo> <mml:mrow> <mml:mo>∇</mml:mo> <mml:mi>T</mml:mi> </mml:mrow> <mml:mo>|</mml:mo> </mml:mrow> </mml:mrow> </mml:math> , <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline" id="im2"> <mml:mrow> <mml:mrow> <mml:mo>|</mml:mo> <mml:mrow> <mml:mo>∇</mml:mo> <mml:mi>S</mml:mi> </mml:mrow> <mml:mo>|</mml:mo> </mml:mrow> </mml:mrow> </mml:math> thresholds and a composite Integrated Frontal Index (IFI) of <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline" id="im3"> <mml:mrow> <mml:mrow> <mml:mo>|</mml:mo> <mml:mrow> <mml:mo>∇</mml:mo> <mml:mi>T</mml:mi> </mml:mrow> <mml:mo>|</mml:mo> </mml:mrow> </mml:mrow> </mml:math> , <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline" id="im4"> <mml:mrow> <mml:mrow> <mml:mo>|</mml:mo> <mml:mrow> <mml:mo>∇</mml:mo> <mml:mi>S</mml:mi> </mml:mrow> <mml:mo>|</mml:mo> </mml:mrow> </mml:mrow> </mml:math> , and N²; the stability effect is <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline" id="im5"> <mml:mrow> <mml:mi>Δ</mml:mi> <mml:mtext>IFI</mml:mtext> <mml:mo>=</mml:mo> <mml:mtext> </mml:mtext> <mml:mi>I</mml:mi> <mml:mi>F</mml:mi> <mml:msub> <mml:mi>I</mml:mi> <mml:mrow> <mml:mi>T</mml:mi> <mml:mi>S</mml:mi> <mml:msup> <mml:mi>N</mml:mi> <mml:mn>2</mml:mn> </mml:msup> </mml:mrow> </mml:msub> </mml:mrow> </mml:math> - <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline" id="im6"> <mml:mrow> <mml:mi>I</mml:mi> <mml:mi>F</mml:mi> <mml:msub> <mml:mi>I</mml:mi> <mml:mrow> <mml:mi>T</mml:mi> <mml:mi>S</mml:mi> </mml:mrow> </mml:msub> </mml:mrow> </mml:math> ​. 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.