Stationarity and separation of the Sevastopol eddies in the Black Sea: The role of eddy-topographic interaction and submesoscale dynamics

Abstract

One of the eddy “hotspots” in the Black Sea is the area of stationary Sevastopol anticyclone. Here this eddy can reside for >3–6 months significantly affecting the cross-shelf exchange and nutrient fluxes in the Black Sea. Detaching from its stationary location, the eddy moves westward, where it finally dissipates. In this article, we use altimetry measurements and a high-resolution (1 km) Nemo numerical model to investigate in detail the physical processes responsible for the observed evolution of the Sevastopol eddies. The main reason for the stationarity of such eddies is the sharp turn of the continental slope, which plays the role of the meridional wall blocking the vortex propagation in the cyclonic direction. Such a wall is situated at depths of 200–500 m and acts only on sufficiently intense eddies with large vertical extensions, while weak eddies with a small thickness do not feel it. The eddy trajectory near the obstacle represents a cyclonic spiral, which displaces to the southeast before the eddy separation. The process of vortex separation from the wall can prolong for several months and can be divided into 4 stages: 1) Movement to the north after the collision with the meridional wall due to the action of the reactive forces; 2) The formation of an attached submesoscale cyclonic lens-like eddy (SCE) at 200–400 m caused by the rise of horizontal shear due to the interaction of currents with the local uplift of the bathymetry near the continental slope; 3) The movement of the formed vortex dipole to the southwest due to self-propelling force; 4) Displacement of the dipole to the south beyond the meridional wall and further movement of the Sevastopol eddy to the west.During the separation process, Sevastopol AE loses a large amount of energy due to leakage of the mass during its interaction with the wall and transfer of a large part of its energy to the attached SCE. This process explains the observed decrease in the intensity of the Black Sea eddies to the west of the places of their stationarity.