Nonlinear Processes in Geophysics www.nonlin-processes-geophys.net 1023-5809 1607-7946 9 3/4 2002 10.5194/npg-9-289-2002 http://www.nonlin-processes-geophys.net/9/289/2002/ http://www.nonlin-processes-geophys.net/9/289/2002/npg-9-289-2002.html http://www.nonlin-processes-geophys.net/9/289/2002/npg-9-289-2002.pdf 289 309 0000-00-00 Bifurcations and instabilities in rotating, two-layer fluids: II. β-plane A. F. Lovegrove I. M. Moroz P. L. Read Atmospheric, Oceanic and Planetary Physics, University of Oxford, United Kingdom Mathematical Institute, University of Oxford, United Kingdom Present address: Detica Ltd., Guildford, United Kingdom In this paper, we show that the behavior of weakly nonlinear waves in a 2-layer model of baroclinic instability on a <font face="Symbol">b</font>-plane with varying viscosity is determined by a single, degenerate codimension three bifurcation. In the process, we show how previous studies, using the method of multiple scales to derive evolution equations for the slowly varying amplitude of the growing wave, arise as special limits of the general evolution description.