Abstract
The instability characteristics of three-dimensional flows typical of the Northern Hemisphere winter stratosphere and mesosphere are examined in a multi-level spherical quasi-geostrophic model. The basic-state distorted polar night jets are obtained from a numerical simulation of the sudden warming in a nonlinear spectral model. The instability is studied at days 10 (Flow I) and 15 (Flow II), prior to the onset of the model sudden warming, in addition, a parameter study is carried out in which the basic state zonal flow, static stability, wave amplitude, Rayleigh friction and Newtonian cooling are changed. It is found that the zonally averaged jets are stable in the presence of Rayleigh friction and Newtonian cooling. However, the three-dimensional distorted flows are unstable and the fastest growing modes have growth rates between 0.0664 and 0.144 day−1 (doubling times between 10.44 and 4.80 days). In the presence of damping, the disturbance streamfunctions for Flows I and II have largest amplitude at 61.5 km in the polar regions and fall off above and below.
A mechanism of competition between the tendency toward zonal mean flow instability with eastward traveling disturbances and wave instability with stationary disturbances is identified. It is suggested that differences between warmings in the Northern and Southern Hemispheres may in part be influenced by this mechanism.
The zonally averaged zonal winds u of the perturbations are centered at the same locations as the disturbance streamfunctions; with the direction of the largest u taken to be easterly, the zonally averaged temperature of the disturbances corresponds to heating of the polar stratosphere and cooling of the polar mesosphere.
The results indicate that wave instability may be an important contributing factor in the sudden warming, which complements Matsuno's mechanism involving vertical wave propagation and wave-zonal flow interaction.
