Gravity wave drag
From AMS Glossary
gravity wave drag
A generally zonal acceleration produced by upward propagating gravity waves at levels where the waves break.
Gravity waves grow in amplitude as they propagate upward from their surface source regions into lower-density regions. When large amplitudes are attained, the waves break, leading to turbulence and dissipation. This produces a zonal force per unit mass on the zonal mean flow. Gravity wave drag plays an important role in explaining the zonal mean flow and thermal structure at higher atmospheric levels, particularly in the mesosphere.