Annual Variability of Middle-Layer Water Temperature in the East Sea, Korea: Insights from Long-Term Observations (1995–2022)

The variability of middle-layer water temperatures in the East Sea of Korea is a critical indicator of oceanic and ecological changes, complementing the more widely studied surface temperature trends. While surface water temperatures have shown an annual increase of approximately 0.05℃/yr, this study focused on analyzing temperature variability at depths of 300 m, 400 m, and 500 m, where seasonal fluctuations are minimal. Using CTD data from 64 stations collected by the Korea Oceanographic Data Center between 1995 and 2022, the study revealed annual temperature increases of 0.0126℃/yr at 300 m, 0.0085℃/yr at 400 m, and 0.0093℃/yr at 500 m. Spatial analysis indicated that the south of Ulleungdo exhibited relatively high temperature variability, correlating with the formation zone of the Ulleung Warm Eddy, which plays a pivotal role in regional heat and material transport.

Significantly, the observed warming below 300 m depth—beneath the permanent thermocline—suggests that deep-sea regions, previously thought to exhibit minimal thermal variation, are increasingly affected. This warming trend has potential implications for deep-sea ecological dynamics, oxygen concentration, and biogeochemical processes. Furthermore, these changes align with global patterns of ocean warming driven by anthropogenic climate change.

To better understand the long-term impacts on the East Sea’s thermal structure and its ecosystem, it is essential to extend monitoring efforts to deeper layers below 500 m. Such observations will provide insights into the interplay between regional oceanographic phenomena, such as the Ulleung Warm Eddy, and broader climate change influences.