  A series of laboratory experiments has been carried out in a thermally driven
rotating annulus to study the onset of baroclinic instability, using horizontal
and uniformly sloping bottom topographies. Different wave flow regimes have
been identified and their phase boundaries -- expressed in terms of appropriate
non-dimensional parameters -- have been compared to the recent numerical
results of \citet{thomas_slope}. In the flat bottom case, the numerically
predicted alignment of the boundary between the axisymmetric and the regular
wave flow regime was found to be consistent with the experimental results.
However, once the sloping bottom end wall was introduced, the detected
behaviour was qualitatively different from that of the simulations. This
disagreement is thought to be the consequence of nonlinear wave-wave
interactions that could not be resolved in the framework of the numerical
study. This argument is supported by the observed development of interference
vacillation in the runs with sloping bottom, a mixed flow state in which
baroclinic wave modes exhibiting different drift rates and amplitudes can
co-exist.
