In September 2019, a minor but strong sudden stratospheric warming (SSW) event occurred in the Southern Hemisphere. We examine the dynamical characteristics of the gravity waves (GWs) and Rossby waves (RWs), especially quasi-6-day waves (Q6DWs), during this event based on Program of the Antarctic Syowa (PANSY) radar observations and high-resolution Japanese Atmospheric General circulation model for Upper Atmosphere Research (JAGUAR) simulations. For the GWs, strongly negative vertical fluxes of zonal momentum in the stratosphere were observed around the edge of the polar vortex during the SSW event. In the mesosphere, strongly positive momentum fluxes were observed in the Eastern Hemisphere, where westward winds were dominant associated with the SSW event. For the RWs, two types of Q6DWs appeared during the SSW event: one with eastward phase velocity (Q6DW-E) and one with westward phase velocity (Q6DW-W). These waves had a baroclinic structure in vertical, differing from normal-mode 5-day Rossby waves. It is shown that Q6DW-E, which was observed prior to the SSW onset, was an unstable wave owing to the baroclinic instability in the high-latitude mesosphere. Conversely, Q6DW-W was observed after the onset and had characteristics of an upward-propagating internal RW. It is considered to be generated by barotropic/baroclinic instability in the upper stratosphere. This instability was likely caused by forcings resulting from the in situ generated Q6DW-E and RWs originating from the mid- and high-latitude troposphere, as well as the GW forcings, which were positive in the mesosphere and negative in the stratosphere associated with the SSW event.

We conducted two 10-day observational campaigns in 2019 targeting turbulence in the troposphere and lower stratosphere by adopting a frequency domain radar interferometric imaging technique using Program of the Antarctic Syowa (PANSY) radar and radiosonde observations obtained at Syowa Station in the Antarctic. Overall, 73 cases of Kelvin-Helmholtz (K-H) billows were detected, and 2 characteristic cases were examined in detail. In the first case with the longest duration of ~6.5 h, the K-H billows had thickness of ~800 m and horizontal wavelength of ~2500 m. According to a numerical simulation of the environmental conditions, continuously existing orographic gravity waves maintained strong vertical shear of the horizontal winds sufficient to cause the K-H instability. In the second case with the deepest thickness of ~1600m, the K-H billows had duration of ~1.5 h and horizontal wavelength of ~4320m. Numerical simulation suggested that an enhanced upper-tropospheric jet associated with a well-developed synoptic-scale cyclone caused the K-H instability. Such background conditions, frequently observed in the Antarctic coastal region, are typical mechanisms for K-H excitation. Linear stability analysis also indicated that the characteristics of the observed K-H billows were consistent with the most unstable modes. Furthermore, statical analysis was performed using data of all 73 observed cases. The characteristics of K-H billows observed at Syowa Station are similar to those observed over Japan. However, the weaker vertical shear and longer wave period of the K-H billows over Syowa Station reflect that the tropospheric jet over the Antarctica is not as strong as that over Japan.