The evolution of surface quasi-geostrophic modons on sloping topography

TL;DR

This paper investigates the behavior of modons on sloping topography, revealing steady solutions in one regime and decay due to wave wake interactions in another, with implications for related vortex phenomena.

Contribution

It introduces a semi-analytical method for steady modon solutions and analyzes decay mechanisms due to wave wakes, extending understanding of vortex dynamics on sloping topography.

Findings

Steady solutions exist when modons move opposite to Rossby waves.

Wave wakes cause slow decay of modons when moving with Rossby waves.

Decaying vortices break down over time due to wave-induced asymmetry.

Abstract

This work discusses modons, or dipolar vortices, propagating along sloping topography. Two different regimes exist which are studied separately using the surface quasi-geostrophic equations. First, when the modon propagates in the opposite direction to topographic Rossby waves, steady solutions exist and a semi-analytical method is presented for calculating these solutions. Second, when the modon propagates in the same direction of the Rossby waves, a wave wake is generated. This wake removes energy from the modon causing it to decay slowly. Asymptotic predictions are presented for this decay and found to agree closely with numerical simulations. Over long times, decaying vortices are found to break down due to an asymmetry resulting from the generation of waves inside the vortex. A monopolar vortex moving along a wall is shown to behave in a similar way to a dipole, though the presence of the wall is found to stabilise the vortex and prevent the long-time breakdown. The problem is mathematically equivalent to a dipolar vortex moving along a density front hence our results apply directly to this case.