Amplification of a Surface-Intensified Eddy Drift along a Steep Shelf in the Eastern Mediterranean Sea

摘要

The datasets of the Eddies and Gyre Path Tracking (EGYPT)/EGITTO program in the eastern Mediterranean Sea reveal a large mesoscale anticyclone traveling along the Libyan shelf. Surface drifter trajectories combined with a CTD transect accurately quantify the horizontal velocity and the vertical structure of this surface-intensified anticyclone. The observed westward drift speed is significantly higher than expected from the beta effect alone. To study the impact of a steep shelf topography on the propagation of compact surface-intensified vortices, the authors used a two-layer beta-plane model with steep continental slope and nearly zonal boundary. A perturbation theory derived by G. Sutyrin for a circular vortex in the upper layer with the lower layer at rest as a basic state is generalized for nonuniform slope in the presence of the image effect. An integral momentum balance is used to derive the drifting velocity of an upper-layer vortex with the main assumption that a stable and steady drifting solution of the two-layer system exists. The interface is described by a steady drifting circular dome at the leading order. This approach allows the problem to be reduced to the calculation of the deep-flow pattern, depending on the interface shape and topography. When the topographic slope beneath the eddy changes rapidly from a steep continental slope to a gentle continental rise, most of the deep-flow pattern is shifted offshore. The corresponding anticyclonic deep-flow feedback provides an additional along-slope propagation, which is proportional to the basic drift speed and the steepness parameter.