Impact of Water Column Stratification on Oil spill dispersion – Experimental simulations coupling glider data in the Thracian Sea

Current research on oil spills focuses on predicting oil spill dispersion trajectories affected by forcing data, such as winds, currents, waves, and oil type. In parallel, high-resolution ensembles in forcings address the forecasting uncertainty in oil spill simulations. Since the oil spill remains mostly at the sea surface, the water column dynamics, i.e., stratification vs. homogeneous mixing conditions, have been greatly overlooked in oil spill modeling.

This experiment attempts to enlighten the impact of water column dynamics, directly linked to seasonality, and examine the oil spill behavior when all other external factors are unchanged. Data from two underwater glider surveys along the deeper North Aegean Trough (Thracian Sea) were used to determine the high-resolution profiles of sea temperature and salinity. These glider missions collected CTD data for over 30 days, in March 2024 (winter profiles) and July 2024 (summer profiles). Two hypothetical 5-day oil spill scenarios were conducted using the OpenOil model to simulate the accidental oil release at sea surface. Both scenarios applied identical initial conditions and forcings in terms of hydrodynamics, waves, and winds, as collected from CMEMS and NOAA GFS, respectively.

Results revealed notable differences between the scenarios. The winter profile, characterized by a well-mixed water column (temperature: 14–14.6°C, salinity: 38–39 ppt from 0–150 m), exhibited a higher percentage of beaching, with a difference of 11%. Conversely, the summer profile, marked by strong stratification (temperature: 15–25°C, salinity: 34–39 ppt from 150–0 m), resulted in a higher percentage, increased by 11%,  of surface particles remaining at sea after 5 simulation days. Furthermore, oil biodegradation was more pronounced in the summer scenario, as higher temperatures enhanced microbial activity.

Thus, this study demonstrates that water column stratification significantly influences oil dispersion and biodegradation. The findings underscore the importance of incorporating well-defined temperature and salinity profiles into oil spill modeling improving the predictive capacity and response strategies to mitigate environmental disasters. Underwater glider data may be used in this direction.

Keywords: Stratification; oil spill simulations; OPENOIL; glider; seasonal vertical profile; temperature; salinity