Two-particle dispersion in the Gulf of Gabès using a high resolution nested ocean model

Measuring relative dispersion in coastal ecosystems is important for both ocean health and society. Submesoscale dynamics interacting with mesoscale eddies influence mixing processes and phytoplankton blooms dispersion. Two-particle dispersion statistics over an initial spatial scale (0.7 - 1 km) are analysed in the Gulf of Gabès (central-southern Mediterranean Sea) using a high-resolution ocean model through a multiple nesting approach. The model is forced by ERA5 atmospheric fields, while the lateral boundary conditions and initial conditions are provided by daily fields from a Mediterranean Sea reanalysis. The analysis focuses on the turbulent fluid aspects of phytoplankton dispersion in coastal areas under bloom and non-bloom conditions. The results are presented in terms of kurtosis (normalized fourth moment of the pair separation distances), relative diffusivity (particles’ spreading velocity) and time scale-dependent pair separation rate (pair velocity scales normalized by separation distance). At the submesoscale (0.7 – 2km), the non-local exponential regime is absent in both bloom and non-bloom conditions, where the dispersion is locally driven by energetic submesoscale structures. For scales ranging 2-15 km, the two-particle statistics follow the theoretical Richardson regime, which is well detected in the case of a bloom. This regime implies the presence of an inverse energy cascade range where energy is transferred from small to large scales. The diffusive regime is absent for all scales and in both bloom and non-bloom conditions.