Non-orographic gravity waves in the lower stratosphere are analyzed using numerical models (WRF and ECMWF) and long-duration balloons, over the Southern Ocean during austral spring (October to December). The probability distribution functions (PDF) of the gravity wave momentum fluxes are found to vary in a simple and robust way relative to the background local wind speed. PDFs of momentum fluxes conditional on the local background wind speed have their tails systematically sorted, with larger frequency of occurence in stronger winds. The PDFs are well approximated as lognormal distributions. The median is found to increase linearly with the background wind speed. This relation is also investigated in a state-of-the-art parameterization of non-orographic gravity waves. The relation is found to be essentially absent, unless phase velocities are dramatically changed.

Mechanisms that can contribute to this relation are discussed: colocation of the stratospheric jet with tropospheric sources, filtering by critical levels, lateral propagation into regions of stronger winds. Analysis of this relation at different heights in model output and in the offline parameterization suggest that lateral propagation and the importance of wind and shear in producing waves play an important role.

This approach provides a novel, compact way to describe the gravity wave field and its relation to the large-scale flow, and an additional constraint to tune parameterizations.

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*email: riwal.plougonven@lmd.polytechnique.fr
*Preference: Invited