Symbiotic Relation between Planetary and Synoptic-Scale Waves

Abstract

It is hypothesized that the low-frequency planetary scale waves and the high-frequency cyclone-scale waves in an equilibrated state of the atmosphere are symbiotically dependent upon one another. This is demonstrated with an analysis of a dissipative atmospheric model driven by a zonally symmetric forcing. Under a geophysically relevant parameter condition, the synoptic scale waves in the equilibrated state of this system intermittently extract a sufficient amount of energy from the modified instantaneous zonal flow to compensate not only for their own dissipative loss of energy but also for a net supply of energy to the planetary scale waves through the upscale energy cascade process. The planetary scale waves gain this energy in the barotropic form. The planetary scale waves, in turn, create localized strong baroclinic regions whereby the synoptic scale waves may preferentially intensify downstream of the model planetary jet streams. Such cyclone scale eddies collectively give rise to two model stormtracks that have a coherent statistical relation with the zonally traveling planetary scale waves. The zonal propagation of the planetary waves is slowed down due to interaction with the synoptic scale waves. These findings are deduced from a multifacet diagnosis of a long record of the model evolution. First, some salient statistical characteristics and the energetics of the equilibrated flow are analysed. Then, the dominant planetary scale wave in the equilibrated state is used as a reference to construct a phase-shifted composite flow and a corresponding record of the synoptic scale anomaly flow. Using a complete local energetics analysis of the synoptic scale anomalies, we delineate how the planetary scale jet streams statistically organize the synoptic scale eddies downstream of the jet cores. The phase-shifted composite flow is finally analysed for its linear instability properties. Evidence is shown to relate the nonlinear synoptic scale anomaly flow to an unstable local normal mode with similar structural and energetic properties.