A nonlinear baroclinic wave-mean oscillation with multiple normal mode instabilities

An unstable, nonlinear baroclinic wave-mean oscillation is found in a strongly supercritical quasigeostrophic f-plane numerical channel model with 3840 Fourier components. The growth of linear disturbances to this time-periodic oscillation is analyzed by computing time-dependent normal modes (Floquet vectors). Two different Newton-Picard methods are used to compute the unstable solution, the first based on direct computation of a large set of Floquet vectors, and the second based on an efficient iterative solver. Three different growing normal modes are found, which modify the wave structure of the wave-mean oscillation in two essentially different ways. The dynamics of the instabilities are qualitatively similar to the baroclinic dynamics of the wave-mean oscillation. The results provide an example of time-dependent normal mode instability of a strongly nonlinear time-dependent baroclinic flow.