Tropical cyclones (TCs) can rapidly cool the upper ocean, but such cooling is often treated as a transient cold wake. This study examines a persistent cold event following Super Typhoon Krathon in the northeastern South China Sea using multi-platform SST observations, altimetry diagnostics, and model heat budgets. The observations identified a coherent regional marine cold spell lasting more than one week, with locally extreme intensity near Dongsha Island. Heat-budget diagnostics indicate that rapid initial cooling was dominated by entrainment and mixed-layer-base exchange, followed by slow thermal recovery associated with a cyclonic cold-core structure. This post storm mesoscale adjustment maintained subsurface cold water near the mixed-layer base and delayed surface warming. These results extend the interpretation of TC cold wakes from short-lived storm imprints to potential precursors of sub-seasonal marine cold extremes in dynamically active marginal seas.