Depth-dependent ocean warming and marine heatwaves through two decades at Ieodo Ocean Research Station in the East China Sea

Recently, substantial effort has been made to understand the fundamental characteristics of warm ocean temperature trend (ocean warming) and extremes (marine heatwaves, MHWs). However, most research focused on surface signatures of these events, relying on satellite and reanalysis data. While surface ocean warming and MHWs can have dramatic impacts on climate change and marine ecosystems, these events along the seafloor can also significantly affect climate and ecosystems. Monitoring these changes requires long-term, fixed-point observations across multiple depth layers. In this study, we investigate long-term temperature trends and MHW characteristics by analyzing 20 years (2004–2023) of temperature data at three depths—surface (3 m), middle (20 m), and bottom (38 m)—collected at the Ieodo Ocean Research Station in the northern East China Sea near the Korean coast. We find that the air temperature increased by 0.70°C per decade. Correspondingly, ocean warming trends were 0.64°C/decade at the surface, 0.66°C/decade in the middle layer, and 0.88°C/decade at the bottom, with the greatest warming observed in the bottom layer. As MHW frequency and intensity increased across all three layers, bottom MHWs (BMHWs) were found to be more intense and persistent than surface MHWs (SMHWs). While BMHWs, middle MHWs, and SMHWs often co-occur, BMHWs can also exist independently of SMHWs. This study provides direct evidence of distinct warming trends and MHW characteristics across ocean layers based on long-term in situ observations, contributing a valuable dataset for understanding climate-driven changes in the marine environment and supporting efforts to predict and mitigate their ecological and environmental consequences.