  Two-dimensional (2D) turbulence, despite being an idealization of real flows,
is of fundamental interest as a model of the spontaneous emergence of order
from chaotic flows. The emergence of order often displays critical behavior,
whose study is hindered by the long spatial and temporal scales involved. Here,
we experimentally study turbulence in periodically driven nanofluidic channels
with a high aspect ratio using superfluid helium. We find a multistable
transition behavior resulting from cascading bifurcations of large-scale
vorticity and critical behavior at the transition to quasi-2D turbulence
consistent with phase transitions in periodically driven many-body systems. We
demonstrate that quasi-2D turbulent systems can undergo an abrupt change in
response to a small change in a control parameter, consistent with predictions
for large-scale atmospheric or oceanic flows.
