An experimental study of regime transitions in a differentially heated baroclinic annulus with flat and sloping bottom topographies

TL;DR

This study experimentally investigates how different bottom topographies in a rotating annulus affect the onset and behavior of baroclinic instability, revealing differences from numerical predictions and highlighting nonlinear wave interactions.

Contribution

It provides experimental validation and comparison of flow regimes in a rotating annulus with flat and sloping bottoms, highlighting nonlinear effects not captured in previous simulations.

Findings

Flat bottom results agree with numerical predictions.

Sloping bottom experiments show qualitative differences from simulations.

Interference vacillation observed with sloping bottom, indicating nonlinear interactions.

Abstract

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.