The Motion of a Point Vortex near Large-Amplitude Topography in a Two-Layer Fluid

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 surfaceintensified 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.

Section: A Backgroundmentioning

confidence: 99%

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

Sutyrin

Stegner

Taupier‐Letage

et al. 2009

“…He examined the formation of topographic cyclones, the cross-topography transport, and the generation as well as the propagation of topographic Rossby waves in a one-layer ocean. More recently, White and McDonald (2004) studied interactions of point vortices in a two-layer fluid with a large-amplitude steplike topography. Although the topography is higher than the interface, the depth variation over the topography in the upper layer is small compared with the total layer thickness.…”

Section: Introductionmentioning

confidence: 99%

Shelf Circulation and Cross-Shelf Transport out of a Bay Driven by Eddies from an Open-Ocean Current. Part I: Interaction between a Barotropic Vortex and a Steplike Topography

Pedlosky

Flierl

2011

This paper examines interaction between a barotropic point vortex and a steplike topography with a bayshaped shelf. The interaction is governed by two mechanisms: propagation of topographic Rossby waves and advection by the forcing vortex. Topographic waves are supported by the potential vorticity (PV) jump across the topography and propagate along the step only in one direction, having higher PV on the right. Near one side boundary of the bay, which is in the wave propagation direction and has a narrow shelf, waves are blocked by the boundary, inducing strong out-of-bay transport in the form of detached crests. The wave-boundary interaction as well as out-of-bay transport is strengthened as the minimum shelf width is decreased. The two control mechanisms are related differently in anticyclone-and cyclone-induced interactions. In anticycloneinduced interactions, the PV front deformations are moved in opposite directions by the point vortex and topographic waves; a topographic cyclone forms out of the balance between the two opposing mechanisms and is advected by the forcing vortex into the deep ocean. In cyclone-induced interactions, the PV front deformations are moved in the same direction by the two mechanisms; a topographic cyclone forms out of the wave-boundary interaction but is confined to the coast. Therefore, anticyclonic vortices are more capable of driving water off the topography. The anticyclone-induced transport is enhanced for smaller vortex-step distance or smaller topography when the vortex advection is relatively strong compared to the wave propagation mechanism.

“…Various interaction processes can be explored thoroughly by varying the eddy's strength, size, or initial location as well as the topography's profile, but the response to the intermittent forcing of an oceanic current cannot be inferred from the single-eddy-topography interaction in any simple way. What is also neglected in the prior literature is the effect of the shelf geometry, which was mostly assumed to be either infinite (Wang 1992;White and McDonald 2004) or bounded by a straight coast (Frolov et al 2004). The effect of a straight coast is straightforward: it makes the eddy translate along the wall as required by the no-normal-flow condition, but a curved coast is more complex.…”

Section: Introductionmentioning

confidence: 99%

Cross-Shelf and Out-of-Bay Transport Driven by an Open-Ocean Current

Pedlosky

Flierl

2011

This paper studies the interaction of an Antarctic Circumpolar Current (ACC)-like wind-driven channel flow with a continental slope and a flat-bottomed bay-shaped shelf near the channel's southern boundary. Interaction between the model ACC and the topography in the second layer induces local changes of the potential vorticity (PV) flux, which further causes the formation of a first-layer PV front near the base of the topography. Located between the ACC and the first-layer slope, the newly formed PV front is constantly perturbed by the ACC and in turn forces the first-layer slope with its own variability in an intermittent but persistent way. The volume transport of the slope water across the first-layer slope edge is mostly directly driven by eddies and meanders of the new front, and its magnitude is similar to the maximum Ekman transport in the channel. Near the bay's opening, the effect of the topographic waves, excited by offshore variability, dominates the cross-isobath exchange and induces a mean clockwise shelf circulation. The waves' propagation is only toward the west and tends to be blocked by the bay's western boundary in the narrow-shelf region. The ensuing wave-coast interaction amplifies the wave amplitude and the cross-shelf transport. Because the interaction only occurs near the western boundary, the shelf water in the west of the bay is more readily carried offshore than that in the east and the mean shelf circulation is also intensified along the bay's western boundary.