Characterizing cold surge induced storm surge in the northern East China Sea: A 60-year hindcast reveals paradoxical trends in surge heights and return levels

Recurrent cold surges represent a major hazardous weather phenomenon in the northern East China Sea. While their synoptic-scale meteorological processes are well-established, their impact on oceanic dynamics, particularly storm surge behavior, remains insufficiently understood. This study presents the first detailed hindcast and analysis of storm surges associated with 780 cold surge events over the past 60 years. The investigation focuses on the spatiotemporal characteristics and long-term variability of these storm surges, with particular attention to changes in storm surge return levels. The results reveal a general decline in the occurrence frequency, annual maxima, and spatial extent of cold surge induced storm surges. However, a paradoxical increase in surge height return levels is observed across most coastal regions. This counterintuitive trend is attributed to shifts in the surge height distribution, characterized by a rising proportion of high-percentile surge events and a decreasing occurrence of moderate-percentile surges. This redistribution alters the exceedance probability curve by flattening the upper tail of the probability density, subsequently leading to increased 20-year and 50-year return level estimates. On a monthly scale, February exhibits the most intense and widespread storm surges. Long-term trends are evident in transitional months associated with the onset and weakening of the East Asian winter monsoon. Upward trends in maximum storm surges are found in September and March, while a declining trend is observed in April. These findings provide new insights into the evolving nature of cold surge induced storm surges and their implications for coastal disaster management and mitigation strategies. • 60-year hindcast finds declined intensity of cold surge induced storm surge • Coastal storm surge return level increased despite reduced maximum surge height • Variations in return levels are linked to shift in surge height distribution • Long-term changes in surge height are concentrated in transitional months