The seasonal and interannual variations of the gravity waves (GWs) in the Antarctic mesosphere were examined using horizontal wind data at altitudes of 50-100 km above the medium frequency (MF) radar at Syowa Station (69.0°S, 39.6°E). The climatology of the GW variance reached a maximum in winter over a wide altitude range of 64-90 km, and the interannual variability was large below 80 km in spring and autumn. These features are consistent with observations from previous studies. In addition, we detected a weak but significant maximum in the GW variance in summer from 70-78 km as well as large interannual variability. Moreover, we examined three possible mechanisms underlying the GW interannual variability observed in summer. The first mechanism was modulation by stratospheric sudden warmings in the Arctic through inter-hemispheric coupling, and it was not clearly observed at Syowa Station. The second mechanism was modulation of the vertical filtering of GWs in association with the breakdown of the polar vortex in the Southern Hemisphere, and it was identified as a potential mechanism. The third mechanism was tropical convection and propagation to the Antarctic region, and it was identified as another likely mechanism of interannual variability in the GWs. This result was supported by the consistency between years with strong tropical precipitation and years with large GW variances at Syowa Station.
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