Anthropogenic and natural stressors strongly influence the coastal zone. Observations and measurements are needed to forecast the bio-productivity of the coastal zone. Based on measurements from the Winchi profiler, the fine-scale thermohaline and bio-optical structure of the coastal water was observed in the northwestern Sea of Japan in July, 2025. The Winchi profiler was equipped with the EXO2 probe, which measured temperature, conductivity, pressure, and bio-geo-optical parameters, including fDOM and turbidity of the seawater. During the survey, a quasi-two-layer thermohaline structure was observed. The quasi-inertial oscillations modulated the density layer structure. We observed manifestations of several events. At first, when the upper mixed layer was observed near 5 m, isopycnals showed higher-amplitude variations up to 15 m at the full depth of 30 m. Then, the event of the coastal upwelling was observed.  In the first event, we observed slow, upward isopycnals reaching the sea surface from depths of 20 m to 7 m, when the wind was favourable for upwelling. The rate of the isopycnal uplifting was about 5 m per day. Moreover, we observed high vertical mixing induced by the abrupt intermingling of the upwelling. The quasi-homogeneous vertical distributions of temperature and salinity were confirmed. High-amplitude disturbances and coastal upwelling significantly altered the thermohaline stratification, shifting it from a quasi-layer with a thin mixed layer to a quasi-homogeneous state with the pycnocline near the bottom. Moreover, during the survey, quasi-inertial oscillations predominated in the density stratification variability.

fine-scale measurements,near-inertial oscillations,coastal upwelling