Resonant Excitation of Hemispheric Barotropic Instability in the Winter Mesosphere

Abstract

The subtropical mesospheric jet observed by the Nimbus 7 Limb Infrared Monitor of the Stratosphere in late 1978 was flanked to the north and south by regions of reversed potential vorticity gradient. In mid-December, enhanced planetary wave activity propagating upward into the mesosphere led to visible overreflection from the low-latitude reversed gradient region and rapid deceleration of the jet. It is argued, first, that the overreflection visible in the geopotential height field was probably genuine, and not likely to have been due to Rossby waves incident on an inertially unstable region. Nor was it due to the opposing mean meridional circulation. Second, the observed dominance of wave 1 in the overreflected flux may have been attributable to hemispheric barotropic instability: a low-wavenumber type of instability on the sphere related to the midlatitude modes discovered by Hartmann. In comparison to the barotropically unstable eigenmodes for higher zonal wavenumbers, the wave 1 mode has a slower growth rate but larger spatial extent. For practical purposes, it is a radiating mode excitable by sources in the far field. Equally important, the phase speed of the eigenmodes can be made exactly zero when the mean flow vanishes just within this region, as observed in mid-December 1978. Resonant excitation is therefore possible. Realistic opposing mean meridional advection has only a slight effect on the barotropic eigenmode, provided that high-wavenumber oscillations are filtered out of the calculation, acting to reduce the growth rate and shift the subtropical secondary amplitude maximum a few degrees towards the pole.