Wintertime upper-layer circulation in the yellow sea: SOM-based pattern analysis and synoptic event modulation

The wintertime upper-layer circulation in the Yellow Sea is influenced by both the southern compensatory inflow and the shelf-break jet. Using high-resolution ocean reanalysis data for five winters during 2020–2025, this study applies the Self-Organizing Map (SOM) technique to classify the dominant circulation states and combines these results with eddy kinetic energy (EKE) diagnostics to examine circulation variability during synoptic-scale atmospheric events. The SOM analysis identifies two primary circulation pathways whose relative prominence differs across the analyzed five winters. The shelf-break jet was strongest in winter 2021, shifted eastward toward the central trough in 2023, and appeared weaker during 2022 and 2024, indicating substantial differences in circulation intensity and structure within the sample period. Composite analysis suggests a three-phase evolution of synoptic-scale disturbances, including preconditioning, entrance intensification, and post-peak frontal maintenance. These phase-dependent changes are statistically consistent with variations in northerly wind forcing, the density front along the continental slope, and the topographic constraint of the Yellow Sea trough. During strong northerly wind events, enhanced wind stress and cross-shelf pressure gradients are associated with stronger compensatory inflow from the southern entrance, which in some cases coincides with a more organized shelf-break jet along the 50–80 m isobaths. By relating event-scale variability to winter circulation states within the analyzed five winters, this study provides a statistically supported, process-based interpretation of how transient atmospheric forcing may modulate Yellow Sea winter circulation. The results contribute to understanding short-term circulation variability and may also be relevant for marine operations under energetic conditions and for the assessment of cross-shelf transport processes affecting regional ecosystems.