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