Importance of Gravity Wave Forcing for Springtime Southern Polar Vortex Breakdown as Revealed by ERA5

Abstract

Planetary waves (PWs) and gravity waves (GWs) are the key drivers of middle atmospheric circulation. Insufficient observations and inaccurate model representation of GWs limit our understanding of their stratospheric contributions, especially during the Antarctic polar vortex breakdown. This study employs the strength of the high-resolution ERA5 reanalysis in resolving a broad spectrum of GWs in southern midlatitudes and its ability to estimate their forcing during the breakdown period. Most of the resolved southern hemisphere GWs deposit momentum around 60°S over the Southern Ocean. Further, a zonal momentum budget analysis during the breakdown period reveals that the resolved GW forcing in ERA5 provides as much as one-fourth of the necessary wind deceleration at 60°S, 10 hPa. The parameterized GW drag, mostly from non-orographic sources, provides more than half of the wind deceleration. Both findings highlight the key role of GWs in the vortex breakdown and discuss possibilities for further stratospheric GW analysis.