Abstract
We study the influence of bottom topography on the interaction of two identical vortices in a two-layer, quasi-geostrophic model. The two vortices have piecewise-uniform potential vorticity and are lying in the upper layer of the model. The topography is a smooth bottom slope. For two cyclones, topography modifies the merger critical distance and the merger efficiency: the topographic wave and vortices can advect the two cyclones along the shelf when they are initially far from it or towards the shelf when they are initially closer to it. They can also advect the two cyclones towards each other and thus favour merger. The cyclones deform, and the potential vorticity field undergoes filamentation. Regimes of partial vortex merger or of vortex splitting are then observed. The interaction of the vorticity poles in the two layers are analysed to explain the evolution of the two upper layer cyclones. For taller topography, two new regimes appear: vortex drift and splitting; and filamentation and asymmetric merger. They are due to the hetonic coupling of lower layer vorticity with the upper layer vortices (a heton is a baroclinic vortex dipole, carrying heat and momentum and propagating horizontally in the fluid), or to the strong shear that the former exerts on the latter. The interaction of two anticyclones shows regimes of co-rotation or merger, but specifically, it leads to the drift of the two vortices away from the slope, via a hetonic coupling with oppositely-signed vorticity in the lower layer. This vorticity originates in the breaking of the topographic wave. The analysis of passive tracer evolution confirms the inshore or offshore drift of the fluid, the formation of tracer fronts along filaments and its stirring in regions of vortex merger. The trajectories of particles indicate how the fluid initially in the vortices is finally partitioned.
Highlights
The initial conditions of this model are a pair of identical, circular vortices of radius R = 0.5, enclosing uniform quasi-geostrophic relative vorticity q1 = 1 or q1 = −1, lying at a distance d from each other and at a distance dc from the topography
As the upper layer vortices move across the slope, a topographic wave is formed in the lower layer
The particle evolution in the lower layer follows closely that of the tracer and of the lower layer potential vorticity, previously shown; There are exchanges of particles between the two cyclones; the merged vortex contains particles from both of them, but with a majority of particles from the westernmost cyclone
Summary
Vortices are prevalent and long-lived features of ocean dynamics. They play a key role in the transport of momentum, heat, salt, chemical tracers and biological species, across the ocean basins. The process of vortex merger has been studied in simple configurations; the evolution of two equal vortices, or of two unequal vortices, with initially axisymmetric velocities, in the absence of external currents, has been investigated in two-dimensional incompressible flows [8,9,10,11,12,13,14,15,16] and in rotating, stratified flows [17,18,19,20,21,22,23,24,25,26]. Zhang et al (2011) [30] studied the transport of shelf water, associated with the topographic wave-vortex interaction They found that the topographic vortex created by a cyclone remains near the shelf, while that created by an anticyclone couples with it and moves offshore; anticyclones are more capable of advecting water away from the shelf.
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