Dependence of quasi-stationary Rossby waves on the background zonal flow

Near stationarity of large-scale Rossby waves can be associated with extreme seasons e.g. high seasonal rainfall or persistent hot or cold weather. Here we investigate the dependence of quasi-stationary waves on the structure the background zonal flow. Idealised experiments are performed using a global primitive equation model with a weak relaxation to a baroclinically unstable background zonal mean state in which the latitude and strength of the jet can controlled. This unstable background state generates sustained wave activity through repeated baroclinic lifecycles.

To link the structure of the jet the evolution of Rossby waves, modes of variability are extracted using the Empirical Normal Mode (ENM) technique. This technique extracts the dominant modes of variability which like dynamical modes are orthogonal with respect to a psuedo-momentum (wave activity) norm.  Due to the choice of norm these modes possess an intrinsic linear phase-speed determined by their structure in the same way it would be for dynamical modes. It is found that despite the non-linearity of the simulations the flow is dominated by a few ENMs whose depend systematically on the background state jet latitude and strength in a manner which can be well understood through the dynamics of modes. It is suggested that seasons with anomalously persistent mid-latitude Rossby wave activity may be related to the interannual variability in the background state zonal flow.