Trapped waves in the edge region of stratospheric polar vortices

Abstract

A global distribution and seasonal variability of short‐period (<2 days) disturbances in the lower and middle stratosphere are investigated using 6‐hourly European Centre for Medium‐Range Weather Forecasts reanalysis data over 15 years (1979–1993). Two‐dimensional spectral analysis and a recently developed Lagrangian‐mean diagnostic are used for the analysis. It is shown that the short‐period disturbances are most active around the polar‐night jet in each winter hemisphere. They have a wavelike structure in the longitudinal direction with typical zonal wavelengths of 1,700–2,000 km (i.e., zonal wave number 10–12) and a latitudinally evanescent structure with a half width of about 1,500 km, suggesting that they are waves trapped in the edge region of the polar vortex, where the latitudinal gradient of isentropic potential vorticity is maximized. The trapped waves have a nearly barotropic structure over a depth greater than 10 km. Because the polar vortex in the Northern Hemisphere is much deformed and not symmetric around the pole, dynamical characteristics of the trapped waves in the Southern Hemisphere alone are investigated by applying a lag‐correlation analysis to the reanalysis data. The characteristics of trapped waves are fairly dependent on the state of the polar vortex, such as zonal wind speed and its vertical shear. They have a significant westward intrinsic phase velocity from June through October, when the polar vortex is most stable. This indicates that the trapped waves during this period have their own dynamics, contrary to the filaments being passively advected by the large‐scale flow.