Nonlinear Processes in Geophysics www.nonlin-processes-geophys.net 1023-5809 1607-7946 12 6 2005 10.5194/npg-12-1033-2005 http://www.nonlin-processes-geophys.net/12/1033/2005/ http://www.nonlin-processes-geophys.net/12/1033/2005/npg-12-1033-2005.html http://www.nonlin-processes-geophys.net/12/1033/2005/npg-12-1033-2005.pdf 1033 1041 2005-12-20 Experiments on transitions of baroclinic waves in a differentially heated rotating annulus Th. von Larcher larcher@las.tu-cottbus.de C. Egbers Department of Aerodynamics and Fluid Mechanics, Brandenburg University of Technology Cottbus, 03046 Cottbus, Germany Experiments of baroclinic waves in a rotating, baroclinic annulus of fluid are presented for two gap widths. The apparatus is a differentially heated cylindrical gap, rotated around its vertical axis of symmetry, cooled from within, with a free surface, and filled with de-ionised water as working fluid. The surface flow was observed with visualisation technique while thermographic measurements gave a detailed understanding of the temperature distribution and its time-dependent behaviour. We focus in particular on transitions between different flow regimes. Using a wide gap, the first transition from axisymmetric flow to the regular wave regime was characterised by complex flows. The transition to irregular flows was smooth, where a coexistence of the large-scale jet-stream and small-scale vortices was observed. Furthermore, temperature measurements showed a repetitive separation of cold vortices from the inner wall. Experiments using a narrow gap showed no complex flows but strong hysteresis in the steady wave regime, with up to five different azimuthal wave modes as potential steady and stable solutions.