Recent research on climate projection using chemistry–climate models suggests that the trend in upwelling in the tropical lower stratosphere can be largely affected by the location of the gravity wave drag through the downward control mechanism (e.g., McLandress and Shepherd 2009 Okamoto et al. The momentum deposition is important for maintaining weak wind layers in the lower stratosphere and in the upper mesosphere, and for simultaneously driving meridional circulations. Gravity waves are an essential component of the earth’s climate because of their ability to transport momentum mainly upward from the lower atmosphere (e.g., Fritts and Alexander 2003). Partial reflection from a rapid increase in static stability around 10 hPa and/or gravity wave generation through nonlinear processes are possible mechanisms to explain the downward energy flux. The frequency of the downward energy flux is positively correlated with the gravity wave energy over the southern Andes. Another interesting feature is a significant downward energy flux in winter, which is observed in the lower stratosphere to the south of the southern Andes. This result indicates that orographic gravity waves can affect the mean winds at horizontal locations that are far distant from the source mountains. The leeward energy propagation extends to several thousand kilometers, which explains part of the gravity wave distribution around the polar vortex in winter. Results of ray-tracing and cross-correlation analyses are consistent with this theoretical expectation. Linear theory in the three-dimensional framework indicates that orographic gravity waves are advected leeward significantly by the mean wind component perpendicular to the wavenumber vector. The wave energy distribution in winter is not zonally uniform, but it is large leeward of the southern Andes and Antarctic Peninsula. Gravity waves have large amplitudes in winter and are mainly distributed in the region surrounding the polar vortex in the middle and upper stratosphere, while the gravity wave energy is generally weak in summer. Gravity wave characteristics in the middle- to high-latitude Southern Hemisphere are analyzed using simulation data over 3 yr from a high-resolution middle-atmosphere general circulation model without using any gravity wave parameterizations.
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