In this study, variability of the vertical temperature gradient around the Korean marginal seas were investigated using serial oceanographic observation data from the Korea Hydrographic and Oceanographic Agency (KHOA) and the National Institute of Fisheries Science (NIFS), along with ARGO data for 2020~2024. Due to recent variations in atmospheric and oceanic conditions over the past five years, the ocean stratification structure has changed, and the vertical temperature gradient exhibited significant interannual variability in the range of approximately for 0.7~4.5 ℃/m. This range differs substantially from the commonly used range for 0.7~1.5 ℃/m suggested in the World Ocean Dataset (WOD) and previous studies.
 When conventional quality control (QC) criteria for vertical temperature gradients, such as those used in WOD profile data, are directly applied to the Korean marginal seas, there is a risk that normal data within the range of 1.5 to 4.5 ℃/m may be misclassified as outliers, leading to excessive data loss. By applying the newly derived threshold values from this study, the recovery rate of valid observations improved by more than 40% compared to existing methods. Therefore, it is necessary to establish an optimized QC framework that reflects the regional characteristics of the Korean Peninsula—where shallow and deep seas coexist—as well as oceanographic changes driven by climate variability.