Modelling the shear edge eddies of the southern Agulhas Current

Observations have revealed the presence of cyclonic eddies embedded in the landward border of the southern Agulhas Current. These shear edge eddies are most prevalent in the Agulhas Bank shelf bight, have a diameter of about 50–100 km and are represented by a thermal dome and a surface warm plume inshore. In an attempt to better understand their generation and behavior, we have used a high-resolution model designed for the general region. The ocean model employed is the Regional Ocean Modeling System that solves the free surface, hydrostatic, primitive equations over variable topography using stretched, terrain-following coordinates in the vertical, orthogonal curvilinear coordinates in the horizontal, and featuring high-order schemes. The model simulates the creation of cyclonic shear edge eddies and their attendant surface plumes of warm water along the edge of the Agulhas Bank with a high degree of verisimilitude. The dimensions of the model features, their hydrographic structure and their velocities bear a strong resemblance to what has been observed. The simulation suggests that shear edge eddies remain trapped in the Agulhas Bank shelf bight and that eddies that subsequently travel downstream represent leakages from the resident shear eddy. This intermittent but frequent leakage of cyclonic motion downstream propagates at roughly 8 km day−1 at intervals of roughly 20 days, and may trigger the detachment of a cyclonic eddy at the tip of the Agulhas Bank.