Effects of Horizontal Wind Structure on a Gravity Wave Event in the Middle Atmosphere Over Syowa (69°S, 40°E), the Antarctic

Nightly mean potential energy of gravity waves (GWs) per unit mass (E-p) over Syowa Station (69 degrees S, 40 degrees E) was calculated from temperature profiles observed by the Rayleigh/Raman lidar from 2011 to 2015. The E-p values in the upper stratosphere and lower mesosphere were significantly enhanced on 8-21 August 2014, except on 12 August. A ray-tracing analysis showed that large-scale GWs emitted from various latitudes could be refracted and forced to converge above Syowa due to the poleward tilting of the polar night jet with altitude. It should be noted that E-p on 12 August was smaller than the other values during the enhancement, despite similar polar night jet conditions. A synoptic-scale disturbance, which passed on 12 August, could have blocked the GWs from propagating upward through critical level filtering. These results suggest that convergence of the wave should be considered as a part of the intermittency of the GWs. Plain Language Summary Atmospheric waves of short horizontal scale, known as gravity waves (GW), transport momentum through the atmosphere from the Earth's surface, and drive north-south circulations. These airflows profoundly influence the temperature structure at heights corresponding to the ozone layer. However, the small scale of GWs makes it necessary to artificially represent them in atmospheric models. GW activity is quite variable in space and time, and the representation of this variation is a key to improve long-term climate forecasts. Previous observational studies mainly discussed variations of GW sources and wind filtering, but such variations do not explain the observations in this study. We suggest that the GWs generated at various latitudes converged to our observation area enhancing the local GW activity. Moreover, the convergence was found to be related to the structure of the polar night jet. This result suggests that the lack of wave horizontal propagation typical in model wave parameterizations could contribute to their cold bias and unrealistic representation of ozone hole over the Antarctic in spring.