Lower‐stratospheric Rossby wave trains in the southern hemisphere: A case‐study for late winter of 1997

Abstract

AbstractBehaviour of quasi‐stationary circulation anomalies observed in the lower stratosphere of the extratropical southern hemisphere during austral late winter of 1997 is studied. The anomalies are defined as daily low‐pass‐filtered departures from the circulation varying slowly with the seasonal cycle. A wave‐activity flux and refractive index for stationary Rossby waves are utilized in the analysis, each of which is defined locally for the zonally varying westerlies. Subseasonal fluctuations in the lower stratosphere were often associated with zonally confined wave trains emanating upward from localized, quasi‐stationary anomalies in the troposphere, including blocking ridges. The three‐dimensional propagation of the waves was found sensitive to the structure of a local waveguide. Upward injection of Rossby‐wave activity into the stratosphere tended to occur slightly upstream or just beneath of a lower‐stratospheric waveguide associated with the developed polar‐night jet (PNJ), which led to the subsequent formation of a well‐defined wave train downstream along that jet. The distribution of the lower‐stratospheric subseasonal variability thus exhibits significant zonal asymmetries, reflecting those in the PNJ structure and the distribution of tropospheric disturbances. Seasonal evolution of the PNJ and that of the tropospheric intraseasonal variability substantially modulated the lower‐stratospheric activity of subseasonal fluctuations. Copyright © 2004 Royal Meteorological Society