[A-AS10] Stratosphere-troposphere Processes And their Role in Climate

[AAS10-P01] Seasonality of Sudden Stratospheric Warming in the Southern Hemisphere

*Koki Iwao¹, Shingo Watanabe², Toshihiko Hirooka³ (1.National Institute of Technology, Kumamoto College, 2.Japan Agency for Marine-Earth Science and Technology, 3.Department of Earth and Planetary Science, Kyushu University)

Keywords:sudden stratospheric warming, planetary wave, ozone heating

In the Northern Hemisphere, sudden stratospheric warmings (SSWs) frequently occur mainly in mid-winter (December-February) and have been extensively investigated by many authors. On the other hand, SSWs in the Southern Hemisphere (SH) are very rare and have only occurred two times in early spring of September in 2002 and 2019, owing to the strong wave activity in this period. This seasonal difference of their occurrence is considered to be partly due to too strong westerlies during winter for planetary waves to propagate into the stratosphere. However, mechanisms causing the seasonal difference associated with the wave activity change in the stratosphere are still unknown, because it has been rarely observed in the SH and observational studies are insufficient. Hence, this study carefully investigates plausible mechanism of the strong wave activity in early spring, leading to the SSW occurrence in the SH, by using reanalysis datasets ERA5 and MERRA-2.
Our analysis reveals that Eliassen-Palm (E-P) fluxes of stationary planetary waves (STPWs) diverged in the lower stratosphere and converged in the upper stratosphere, when SSWs occurred in the SH in September 2019. This result suggests that STPWs were generated in the lower stratosphere, and propagated into and dissipated in the upper stratosphere. The latitudinal gradient of ozone concentration was found to increases during early spring in the lower stratosphere, as ozone depletion progresses in the polar vortex in the SH. So, when the STPWs were amplified in the lower stratosphere, ozone-waves would be also amplified in this season. The amplified ozone-waves could enhance new STPWs in the lower stratosphere by zonally asymmetric heating. Resultantly, the enhanced STPWs in the lower stratosphere might contribute to cause SSWs in the SH and to specify the timing of their occurrence.