Analysis of Interannual Variability of the Yellow Sea Bottom Cold Water Using Reanalysis Data

The Yellow Sea Bottom Cold Water (YSBCW), a cold water mass in the bottom layer of the Yellow Sea, contributes significantly to maintaining high nutrient concentrations, supporting phytoplankton growth, and enhancing primary productivity, which are essential for the productivity and structure of regional marine ecosystems. This study produced a 13-year reanalysis dataset (2010–2022) to analyze the seasonal and interannual variability of the YSBCW. Numerical modeling was conducted using the Regional Ocean Modeling System (ROMS) with a horizontal resolution of 1/20° and 41 vertical layers. The model domain covered 117–150°E and 21–54°N. Atmospheric forcing was provided by the European Centre for Medium-Range Weather Forecasts (ECMWF) ERA5, initial and boundary conditions by the Hybrid Coordinate Ocean Model (HYCOM), and tidal forcing by TPXO9. Data assimilation was performed daily using the Ensemble Kalman Filter (EnKF), assimilating sea surface temperature from the Operational Sea Surface Temperature and Ice Analysis (OSTIA) and temperature-salinity profiles collected by the Korea Institute of Ocean Science & Technology (KIOST), the National Institute of Fisheries Science (NIFS), the Korea Hydrographic and Oceanographic Agency (KHOA), and Argo floats. The reanalysis dataset reproduced the seasonal and interannual variability of the YSBCW and demonstrated its reliability through validation against observational data. Surface temperature analysis showed seasonal biases from -0.45°C in winter to 0.13°C in summer. The root mean square error (RMSE) values were 0.79°C in winter, the highest among all seasons, and 0.49°C in autumn, the lowest. The model captured the vertical distribution of the YSBCW well, although tidal effects and mixing were overestimated, resulting in stronger vertical mixing than that observed in the coastal regions. The annual YSBCW volume was calculated by determining the volume of water deeper than 30 m with a temperature below 10°C to analyze interannual variability. In August, the volume was largest in 2013 at 4 km³ and smallest in 2020 at nearly 0 km³. The average volume is around 3.5 km³, decreasing from 2019 to 2020 and then increasing from 2021 onwards. Spatially, the northern and eastern Yellow Sea showed relatively small changes in cold water distribution while the southern and western regions exhibited greater variability, with cold water below 10°C widely distributed in 2013 but almost absent in 2020. This reanalysis dataset reproduces the formation, dissipation, and interannual variability of the YSBCW. Future research will analyze how climate variability and atmospheric forcing influence the formation mechanisms and variability of the YSBCW.