A thermal parametric model has been developed for analyzing observed regional sea temperature profiles based on a layered structure of temperature fields (mixed layer, thermocline, and deep layers). It contains three major components: (1) a first-guess parametric model, (2) high-resolution profiles interpolated from observed profiles, and (3) fitting of high-resolution profiles to the parametric model. The output of this parametric model is a set of major characteristics of each profile: sea surface temperature, mixed-layer depth, thermocline depth, thermocline temperature gradient, and deep layer stratification. Analyzing nearly 15,000 Yellow Sea historical (1950--1988) temperature profiles (conductivity-temperature-depth station, 4825; expendable bathythermograph, 3213; bathythermograph, 6965) from the Naval Oceanographic Office's Master Oceanographic Observation Data Set by this parametric model, the Yellow Sea thermal field reveals dual structure: one layer (vertically uniform) during winter and multilayer (mixed layer, thermocline, sublayer) during summer. Strong seasonal variations were also found in mixed-layer depth, thermocline depth, and thermocline strength.