Horseshoe‐Shaped Vortices Accompanied With Breaking of Upward Propagating Gravity Waves and Their Relation to Mean‐Flow Acceleration

Abstract Breaking gravity waves (GWs) are a major source of turbulence and momentum deposition in the middle atmosphere. In spite of the importance of this phenomenon, the small‐scale vortex structures associated with wave breaking remain poorly documented under realistic conditions. We perform a three‐dimensional compressible simulation of orographic GWs observed above Syowa Station, Antarctica, using 125 m horizontal grid spacing from the ground to the mesosphere. In the 75–80 km region upward‐propagating waves break and exhibit numerous horseshoe‐shaped vortices whose bends extend obliquely downward. By examining the time evolution of a representative horseshoe‐shaped vortex, we identify the mechanism responsible for the downward elongation of the vortex tube: (a) horizontal vorticity aligned with the GW phase lines is amplified as the wave approaches the overturning condition; (b) a convectively driven downward flow displaces the vortex tube downward, creating the initial horseshoe shape; (c) vertical shear associated with the GW tilts and stretches the tube, reinforcing the downward flow. The direction of the initial vortex tube is determined largely by the baroclinic effect of the GW. The present study also addresses the relationship between the vortex tube structure and the mean flow acceleration (0.3–0.4 m s −1 min −1 ). The vortex tube deformation associated with wave breaking can be interpreted as a manifestation of the irreversible cascade that transfers the GW momentum to the mean flow.