Downward coupling mechanism of Sudden Stratospheric Warming: A Mass Flux Perspective

Sudden Stratospheric Warming (SSW) is an important source of subseasonal-toseasonal predictability due to its significant and long-lasting impacts on surface climate. However, the mechanism of its downward coupling has not yet been fully elucidated. In this study, we investigate the downward coupling mechanism of SSW in terms of mass redistribution. Out of the 65-year dataset of the Japanese 55-year Reanalysis (JRA-55), 40 SSW events are identified. Their composite shows a significant increase of tropospheric geopotential and pressure anomalies over the Arctic (60-90°N) after the onset with a prominent surface amplification. The decomposition of tropospheric anomalies into surface pressure and air temperature components reveals that the downward coupling mainly results from surface pressure change. It is further found that surface pressure change during the SSW onset is primarily caused by the poleward mass flux near the tropopause, which is mainly driven by momentum flux change in the upper troposphere. This momentum flux change is consistent with the poleward propagation of wave, and it is may associated with the warm temperature anomalies during the SSW. These findings provide a new insight on SSW downward coupling and surface amplification.