Idealized Life Cycles of Planetary-Scale Barotropic Waves in the Middle Atmosphere

Abstract

A nonlinear nondivergent barotropic model is used to investigate the mature and decay stages of waves arising from barotropic instability of planetary-scale eastward jets. Two simulations were carried out in weakly diffusive and unforced conditions, corresponding to two types of linear instabilities. One simulation was initialized with a high-latitude polar night jet and the other with a double-peaked jet, which has a maximum at subtropical latitudes and a subsidiary maximum at high latitudes. The polar night jet is chosen to be unstable on its poleward side; an unstable wave 1 (the polar mode) developed into a persistent dipolar vorticity pattern traveling coherently around the globe in slightly more than 3 days. The dipolar feature decays with a timescale of about three weeks, before the anticyclonic anomaly is destroyed in a region of strong cyclonic shear. For the case of an instability associated with the double-peaked jet, which has a region of negative vorticity gradient between the two jets, one finds a regime of dispersive waves of higher wavenumber, the wave 4 being the most unstable and the wave 3 the most long lived. The eddy energy decays more slowly in this case. Synoptic maps of vorticity illustrate the rearrangement of vorticity during the wave life cycles, and the time evolution of material contours is traced by computing the trajectories of a large number of fluid parcels. Large-scale stirring is taking place inside the vortex during the growing and mature stage of the polar-mode life cycle. Extensive mixing between the inside and the edge of the vortex follows during the decay of the wave. as the vorticity recovers zonal symmetry. For the double-peaked jet, filaments of fluid are being drawn out of the high-vorticity latitudinal bands to the north and the south and quickly sheared in the anticyclonic shear zone.