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Stationary longitudinal vortical rolls emerge in katabatic and anabaticPrandtl slope flows due to the dominance of the normal component of thebuoyancy force over flow shear. Here, we further identify self pairing of theselongitudinal rolls as a unique flow structure. The topology of thecounter-rotating vortex pair bears a striking resemblance to speaker-wires andtheir interaction with each other is a precursor to further destabilization andbreakdown of the flow field into smaller structures. On its own, a speaker-wirevortex retains its unique topology without any vortex reconnection or breakup.For a fixed slope angle $\alpha=3^{\circ}$ and at a constant Prandtl number, weanalyse the saturated state of speaker-wire vortices and perform a bi-globallinear stability analysis based on their stationary state. We establish theexistence of both fundamental and subharmonic secondary instabilities dependingon the circulation and transverse wavelength of the base state of speaker-wirevortices. The dominance of subharmonic modes relative to the fundamental modehelps explain the relative stability of a single vortex pair compared to thevortex dynamics in presence of two or an even number of pairs.These instabilitymodes are essential for the bending and merging of multiple speaker-wirevortices, which break up and lead to more dynamically unstable states,eventually paving the way for transition towards turbulence. This process isdemonstrated via direct numerical simulations with which we are able to trackthe nonlinear temporal evolution of these instabilities.

Speaker-wire vortices in stratified anabatic Prandtl slope flows and their secondary instabilities