Is the midlatitude zonal flow absolutely unstable?

Abstract

While absolute instability of the midlatitude terrestrial jets seems unlikely, the conclusion stands that the atmosphere is at least near the boundary of absolute instability, especially under conditions prevailing in the oceanic storm tracks. In consequence, slow-moving short waves with growth rates below the maximum normal-mode growth rate typically contribute more to the amplification of wave packets as they cross a finite length baroclinic zone than the faster-growing but faster-moving most-unstable mode. For oceanic storm track conditions, a disturbance can amplify by a factor of several hundred during its time in a typical zonally localized baroclinic zone, even though the flow is not absolutely unstable. A measure of linear growth that is more revealing than normal-mode growth rate is proposed, which could be suitable for diagnostic studies. -from Authors