Winter marine cold spells driven by intrusion of cold, less saline water on the western coast of the East/Japan Sea

Abstract Global ocean warming has led to a projected decrease in the frequency and intensity of marine cold spells (MCSs). However, the East/Japan Sea presents a climatic paradox, as winter MCSs continue to occur despite a clear, long-term warming trend. This study investigates the mechanisms underlying the sustained occurrence of these events. Quantitative analysis of the mixed-layer heat budget indicated that extreme coastal cold anomalies are closely linked to the intensified southward flow of cold and less saline North Korea Cold Water from East Korea Bay. The combined dynamics of horizontal advection and vertical entrainment demonstrate a synergistic mechanism: this low-salinity intrusion establishes a robust haline stratification that restricts vertical mixing, traps cold water at the surface to sustain continuous atmospheric heat loss, and enhances severe cooling. Further time-lagged correlation analyses indicated that the low-salinity coastal conditions associated with these winter MCSs originated from freshwater inflow through the Korea Strait (KS) during the preceding summer and autumn. Considering that the CMIP6 climate change scenario projections indicate an increase in the Changjiang River discharge, which is the primary freshwater source through the KS, these findings suggest that salinity-induced density changes can act as a critical precondition for sustaining the potential for coastal MCSs in a warming ocean.