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

• Published in: Geophysical research letters 2021-05, Vol.48 (10), p.n/a
• Main Authors: Gupta, Aman, Birner, Thomas, Dörnbrack, Andreas, Polichtchouk, Inna
• Format: Article
• Language: English
• Subjects: Antarctic vortex breakdown; ERA5 reanalysis; gravity waves; planetary waves–gravity waves interaction; stratospheric circulation and variability; zonal momentum budget
• ISSN: 0094-8276; 1944-8007
• DOI: 10.1029/2021GL092762

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. Plain Language Summary Strong flow over mountains during winters and instabilities in the southern hemisphere troposphere can excite gravity waves that propagate from near‐surface all the way to atmospheric heights of 50–80 km. At these heights, they dissipate momentum and decelerate the strong eastward winds. Knowing the structure and extent of the forcing by these waves can help to better understand their role in driving the stratospheric and mesospheric circulation. However, the net forcing by these waves is not accurately known on account of limited observations and because global climate models cannot sufficiently resolve them. This study illustrates that the high‐resolution ERA5 reanalysis, which forms a natural bridge between observations and free running climate models, can be used to estimate the mean forcing due to such gravity waves and can help assess their role in springtime deceleration of polar vortex. Such an analysis was not possible using previous reanalysis datasets due to low resolution. The findings show that, indeed, gravity wave forcing can provide a large fraction of the deceleration needed to slow down the strong westerly winds in late winters. Key Points ERA5 partially resolves gravity waves allowing evaluation of their contribution to the springtime polar vortex breakdown Climatology of the gravity wave forcing highlights relevance of gravity wave refraction and oblique propagation and suggests deceleration of up t