Seasonal variations of gravity wave activity in the lower stratosphere over an Antarctic Peninsula station

An 8 year series of 965 high-resolution radiosonde soundings over Rothera (67 degrees S, 68 degrees W) on the Antarctic Peninsula are used to study gravity wave characteristics in the lower stratosphere. The gravity wave energy is shown to have a seasonal variation with peaks at the equinoxes; the largest peak is around the spring equinox. During the winter months and extending into the spring, there is both an enhancement in the downward propagating wave activity and a reduction in the amount of critical-level filtering of upward propagating mountain waves. The horizontal propagation directions of the gravity waves were determined using hodographs. It was found that there is a predisposition toward northward and westward propagating waves above Rothera. This is in agreement with previous observations of gravity wave momentum flux in the wintertime mesosphere over Rothera. These results are consistent with a scenario whereby the stratospheric gravity wavefield above Rothera is determined by a combination of wind flow over topography-generating waves from below, and sources such as the edge of the polar stratospheric vortex-generating waves from above, especially during winter and spring.