Drying of Taganrog Bay during wind-driven setdowns from satellite and ADCIRC model data

Abstract

Intense wind-driven setdowns can cause extensive drying of the shallow Taganrog Bay in the Azov Sea, significantly impacting the marine infrastructure and ecosystem. Satellite measurements of MSI Sentinel-2 and OLI Landsat 8 during 2015–2021 were used to identify 28 intense cases of such processes. Using a simple difference method based on near-infrared measurements, we calculated the area of a drying zone (S) and investigated its relation with storm intensity and duration. Our findings indicate that S in the estuarine part of the bay can exceed 300 km². Setdowns most often occurred in the autumn during intense and persistent easterly winds with wind speed exceeding 8 m/s. The largest areas of drying zone (S > 100 km²) were observed during continuous winds lasting more than 10 days. The correlation between the integral action of wind stress and S was 0.74, and for prolonged events (more than 5 days) it was 0.92.

Further, satellite data was used to validate the results of ADCIRC numerical modelling. Based on the modeling data, we investigated the influence of wind speed on the area of bottom drying in order to obtain a simple parameterization of this process. At the initial moment of setdown, the sea level decreases as t^1/2 and linearly depends on the square root of the integral wind stress, while the drying zone increases as t^1/4. The relation between sea level and wind speed was universal for different wind amplitude until an equilibrium state is reached. Interestingly, during most intense winds (>20 m/s), the drying zone decreases, which is related to the drying out of the banks located at the entrance of the Taganrog Bay limiting water outflow from the bay.