The eastern Mediterranean boundary current: seasonality, stability, and spiral formation

Seasonal variability and the effect of bottom interaction on the dynamics of the along-slope boundary current flowing around the Levantine basin are investigated using nested high-resolution simulations of the Eastern Mediterranean Sea. The numerical solutions show a persistent boundary current year-round that is ≈ 60 km wide and ≈ 200 m deep. An enstrophy balance diagnostic reveals significant bottom-drag influence on the boundary current, leading to anticyclonic vorticity generation in thin regions along the coast, which in turn become unstable and roll into surface intensified anticyclonic spirals characterized by O(1) Rossby numbers. An eddy kinetic energy generation analysis suggests that a mix of baroclinic and barotropic instabilities are likely responsible for the spiral formation. The boundary current and spirals play a crucial role in the cross-shore transport of materials. In winter, the anticyclonic spirals frequently interact and exchange material with the energetic offshore submesoscale flow field. In summer, when the offshore flow structures are relatively less energetic, the spirals remain confined to the boundary current region as they are advected by the boundary current and undergo an upscale kinetic energy (KE) cascade that is manifested in spiral merging, and growth up to 100 km in diameter. In both seasons, a coarse-graining analysis demonstrates that the cross-scale KE fluxes are spatially localized in coherent structures. The upscale KE fluxes typically occur within the spirals, while the downscale KE fluxes are confined to fronts and filaments at spiral peripheries.