  Recent experiments have reported a novel transition to elasto-inertial
turbulence in the Taylor--Couette flow of a dilute polymer solution. Unlike
previously reported transitions, this newly discovered scenario, dubbed vortex
merging and splitting (VMS) transition, occurs in the centrifugally unstable
regime and the mechanisms underlying it are two-dimensional: the flow becomes
chaotic due to the proliferation of events where axisymmetric vortex pairs may
be either created (vortex splitting) or annihilated (vortex merging). In this
paper, we present direct numerical simulations, using the FENE-P constitutive
equation to model polymer dynamics, which reproduce the experimental
observations with great accuracy and elucidate the reasons for the onset of
this surprising dynamics. Starting from the Newtonian limit and increasing
progressively the fluid's elasticity, we demonstrate that the VMS dynamics is
not associated with the well-known Taylor vortices, but with a steady pattern
of elastically induced axisymmetric vortex pairs known as diwhirls. The amount
of angular momentum carried by these elastic vortices becomes increasingly
small as the fluid's elasticity increases and it eventually reaches a marginal
level. When this occurs, the diwhirls become dynamically disconnected from the
rest of the system and move independently from each other in the axial
direction. It is shown that vortex merging and splitting events, along with
local transient chaotic dynamics, result from the interactions among diwhirls,
and that this complex spatio-temporal dynamics persists even at elasticity
levels twice as large as those investigated experimentally.
