Abstract
The relationship between steady states of the large-scale flow regimes revealed by multimodality in phase space and quasi-resonant axes of a linearized atmospheric model (neutral vectors) is investigated by means of a hierarchy of three hemispheric quasigeostrophic(QG) models, in which the flow is relaxed toward a zonally symmetric equilibrium state and orography provides the only source of asymmetric forcing. Two of these models describe only the interaction between the mean zonal flow and planetary waves of zonal wavenumber 3, including one or two meridional wavenumbers. The third model also has a zonal wavenumber-3 symmetry, but includes waves up to total wavenumber 21, and therefore is able to properly represent the interactions between planetary waves and baroclinically unstable synoptic-scale waves.
Three stationary solutions are found for the highly truncated models, two of which represent states with large wave amplitude but opposite phase. These two steady states produce two well-defined flow regimes in the higher-resolution hemispheric model. This result confirms that flow regimes found in highly truncated models are not merely an artifact of the excessive truncation and can still be found when a sufficiently large number of degrees of freedom is included. The agreement between large-scale steady states and regime centroids is improved if the average effect of high-frequency transients on the planetary-scale flow is parameterized by adjusting the sources of zonal available potential energy and kinetic energy in the highly truncated models.
Using these hemispheric models, it is shown that the leading dynamical and generalized neutral vectors correctly identify the axis linking the centroids of the two regimes as the axis with the smallest linear time derivative. Although anomalies in transient eddies are important in determining the spatial pattern of these centroids, the ultimate source of the two regimes of the hemispheric QG model is the existence of multiple stationary solutions of the large-scale flow. There is no inconsistency between the energetically fundamental role played by high-frequency transients and the capacity of nonlinear large-scale dynamics to determine the multimodal structure of the model's phase space. The neutral vector analysis also confirms that tropical diabatic heating could effectively generate an extratropical response along an axis corresponding to the difference between opposite regime anomalies.
