Is the Midlatitude Zonal Flow Absolutely Unstable?

Abstract

An analysis is performed of the growth and propagation of unstable baroclinic wave packets in relatively realistic midlatitude zonal currents. The absolute growth rates are calculated, incorporating the effects of both Ekman friction and barotropic shear. It is found that these effects do not alter earlier conclusions (based on simpler models)that a finite easterly low-level wind is required for absolute instability in models with continuous shear. Ekman friction exacerbates the situation, shifting the absolute instability threshold from essentially zero surface wind to negative surface wind. While absolute instability of the midlatitude terrestrial jets thus 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-growing 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. Consequences of the results for the nature of the storm tracks are discussed.