Water Structure in the Utrish Nature Reserve (Black Sea) during 2020-2021 According to Thermistor Chain Data

This paper reports the water temperature structure and associated coastal processes in the NE part of the Black Sea. In situ temperature was measured in the water area of the Utrish Nature Reserve. The thermistor chain was moored in 2020 and included 6-10 temperature sensors with an accuracy of +/- 0.025 degrees C and time step of one minute. The seasonal variations in the water temperature, upwelling events, internal waves and diurnal cycle were analyzed. The maximum value of SST (28.6 degrees C) was registered in the subsurface layer in August 2021; the minimum (7.7 degrees C) was registered in March 2022. Estimates of the diurnal temperature cycle were obtained according to spectral analysis. Summer months show the diurnal cycle more than 60% of the time, and the cold period shows it less than 10% of the time. Internal waves appeared in thermocline with periods from 5 min to 20 h. The strongest Ekman upwelling was registered in September 2021. The water temperature dropped from 26 degrees C to 16 degrees C in 10 h. Additionally, quality assessments of two hydrodynamic models were made. The models showed a good correlation (0.9) with water temperature measurements, but RMSE could reach 1-1.8 degrees C for subsurface layers. Temperature variability and its characteristics are an important basis for future coastal ecosystem studies in the Utrish.