The diffusion characteristics of thermal discharge from coastal power plants were studied by analyzing the Ningde Nuclear Power Plant and Kemen Power Plant, which are located in different geographical regions in China. The former is in the open sea, and the latter is in a well-sheltered bay. In the vicinity of the outfall areas of the two power plants, large-area surface temperature observations and tidal current observations were conducted. The results indicate that the thermal discharge diffusion characteristics of coastal power plants located in different geographical environments are significantly different. In the well-sheltered sea area of the Kemen Power Plant, the water temperature diffuses faster along the coast, in line with the direction of tidal movement, and slower in the offshore direction under the influence of rectilinear tidal currents within the bay, resulting in a significantly greater longitudinal diffusion distance of thermal discharge along the shore than the transverse diffusion distance offshore. In the area surrounding the Ningde Nuclear Power Plant, rotational currents diffuse the thermal discharge in various directions, causing the range of temperature rise to expand toward the outer sea. Dominant tidal currents within the tidal cycle in the sea area can influence the distribution of high-temperature rise zones near outfalls. The distribution of high-temperature rise zones predominantly occurs on the side with the higher average tidal velocity, either the ebb tide side if its velocity is greater than that of the flood tide or the flood tide side otherwise.

Utilizing data from the four survey voyages in the Beibu Gulf from 2021 to 2022, this study investigates the variations in water temperature and salinity at different survey periods across its central and eastern regions, analyzing their horizontal and vertical distribution patterns. The research further explores the relationship between these distributions and the gulf’s circulation dynamics, elucidating their underlying formation mechanisms. The results indicate that the variations of water temperature and salinity are obviously different in different periods. Horizontally, spring and summer survey voyages exhibit a transition from high-temperature (max 31.90 °C), low-salinity (min 29.54 PSU) waters in the north to low-temperature (min 21.10 °C), high-salinity (max 34.19 PSU) waters in the south, while autumn and winter survey voyages show low-temperature (min 18.57 °C), low-salinity (min 29.54 PSU) waters in the north shifting to medium-to-low-temperature (21.52–28.20 °C), high-salinity (max 34.19 PSU) waters in the south. Vertically, the spring survey period displays distinct stratification with multiple thermoclines and haloclines in different regions. The summer survey period exhibits the strongest thermohaline gradients, with intensified thermoclines and haloclines accompanied by regional up- and downwelling. In the autumn survey period, vertical mixing homogenizes temperature and salinity, while the winter survey period shows near-complete mixing, effectively eliminating stratification. These patterns are driven by a combination of oceanic dynamics and regional climatic factors, including monsoon forcing, solar radiation, coastal runoff, horizontal circulation, and up-/downwelling processes.