An Event-Based Framework Reveals Regionally Distinct Coastal Cold-Water Regimes Along the East Coast of Korea

Abstract

Coastal cold-water events (CWEs) reduce nearshore sea surface temperature (SST) and can cause ecological and socioeconomic impacts; however, objective detection and process-based diagnostics remain limited in marginal seas influenced by western boundary currents. We developed a station-based framework that identifies CWEs by isolating event-scale SST variability after removing diurnal and seasonal signals, and applied it to hourly SST observations from 15 coastal stations along the east coast of Korea during 2004–2023. Spatial coherence revealed three distinct regimes (southern, central, and northern). CWEs in the northern and central regimes occurred earlier in the warm season, intensified more rapidly, and exhibited pronounced cooling-recovery asymmetry, whereas CWEs in the southern regime occurred later, persisted longer, and evolved more gradually. Lagged composites of cross-shore Ekman transport and CWE-targeted SST showed that southern CWE development was consistent with sustained wind forcing, whereas intense cooling in the central and northern regimes occurred despite weaker, shorter wind impulses, implicating oceanic preconditioning. We infer that boundary-current preconditioning sharpens the coastal thermocline, enabling moderate wind to uplift cold subsurface water. Cooling durations matched the theoretical two-layer upwelling timescale in the central and northern regimes but not in the south, where the low Burger number places upwelling under bottom boundary layer control, so cooling is gated by sustained wind. Applying the metric to a global ocean reanalysis revealed state-dependent skill, with systematic warm bias and larger errors during CWEs. The framework provides a transferable basis for CWE diagnostics, event-focused model evaluation, and coastal-hazard prediction.