Q. J. R. Meteorol. Soc. (2005), 131, pp. 545–564 doi: 10.1256/qj.03.91 Upward and downward injection of Rossby wave activity across the tropopause: A new aspect of the troposphere–stratosphere dynamical linkage By KAZUAKI NISHII 1 and HISASHI NAKAMURA 1,2∗ 1 University of Tokyo, Japan 2 Frontier Research Center for Global Change, Yokohama, Japan (Received 4 June 2003; revised 19 July 2004) SUMMARY Propagation of zonally confined Rossby wave activity between the troposphere and lower stratosphere of the southern hemisphere during austral winter and spring of 1997 is studied, by using a wave-activity flux and refractive index, both defined for stationary Rossby waves on a zonally asymmetric time-mean flow. A particular event of large-scale quasi-stationary cyclogenesis in the troposphere is discussed, to which downward wave- activity injection from anticyclonic anomalies upstream that had developed in the exit region of the lower- stratospheric polar-night jet (PNJ) contributed substantially. Consistent with that downward injection, phase lines of observed stream-function anomalies exhibited a distinct eastward tilt with height. The development of the anticyclonic anomalies occurred at the leading edge of a quasi-stationary Rossby wave train propagating along the PNJ, that had originated from a tropospheric blocking ridge farther upstream. Though less common than upward wave-activity injection across the tropopause, similar events of downward wave-activity injection occurred several times during that season, primarily in the regions south of Australia and over the central South Pacific, over each of which the PNJ exit overlapped with a tropospheric subpolar jet to form a vertical waveguide locally. It is argued that the downward wave-activity propagation is essentially due to refraction in the vertically sheared westerlies, and the zonal asymmetries in the time-mean flow are a likely factor for the observed geographical preference of the downward wave-activity injection. KEYWORDS: Blocking Cyclogenesis Low-frequency variability Polar-night jet Refractive index Wave-activity flux 1. I NTRODUCTION The planetary-scale dynamical linkage between the stratosphere and troposphere has been discussed with respect mainly to the upward influence of the tropospheric processes, in recognition of the fact that the source of upward-propagating planetary waves is in the troposphere. For example, a stratospheric sudden warming (SSW), which is one of the prominent features in the wintertime stratosphere, is characterized by the marked deceleration of a westerly polar-night jet (PNJ), in association with the break- down of the non-acceleration condition for planetary waves propagating upwards from the troposphere (e.g. Matsuno 1971). More generally, modulations of the stratospheric zonal-mean westerlies are caused by their interaction with upward-propagating planetary waves (e.g. Hirota and Sato 1969; Shiotani and Hirota 1985). Some studies, however, have examined the downward dynamical influence from the stratosphere to the troposphere. For example, Geller and Alpert (1980) argued on the basis of their numerical experiment that variations in the stratospheric PNJ induced by anomalous solar ultraviolet radiation could influence the tropospheric circulation by modifying vertically propagating planetary waves. Likewise, Boville (1984) showed through his model experiments that changes in the stratospheric PNJ could alter the tropospheric circulation characteristics by modulating planetary wave propagation. Kodera et al. (1990) found that the enhanced upper-stratospheric PNJ in December tends to be followed by the stronger westerlies over the tropospheric polar region in February. In that process, weak anomalies in the zonal-mean zonal wind first appear in ∗ Corresponding author: Department of Earth and Planetary Science, Graduate School of Science, University of Tokyo, Tokyo 113-0033, Japan. e-mail: hisashi@eps.s.u-tokyo.ac.jp c  Royal Meteorological Society, 2005. 545