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

[AAS10-10] Driving mechanisms of the 2021 Sudden Stratospheric Warming: tropospheric precursors and stratospheric preconditioning

*Hyeong-Oh Cho¹, Min-Jee Kang¹, Young-Ha Kim¹, Seok-Woo Son¹, Dong-Chan Hong¹, Joonsuk Miguel Kang² (1.School of Earth and Environmental Sciences, Seoul National University, Seoul, Republic of Korea, 2.Department of the Geophysical Sciences, The University of Chicago )

Keywords:sudden stratospheric warming, tropospheric precursor, stratospheric precondition

The driving mechanisms of January 2021 sudden stratospheric warming (SSW) are examined by performing a series of numerical model experiments. In the aspect of tropospheric precursors, the role of the rapidly developing North Pacific cyclones in the 2021 SSW is examined. A set of numerical model experiments shows that the 2021 SSW is not reproduced without the cyclones due to the reduced upward propagation of the wavenumber-one wave, thereby reducing the breaking of the planetary-scale waves in the polar stratosphere. The role of the stratospheric precondition, upper stratospheric mean flow in the 2021 SSW is also investigated. It is found that the standard model prediction, initialized 16 days before the onset of the SSW, fails to predict this event. However, when the mean flow above 3 hPa (~ 40 km above the ground) is taken from the observations, the model successfully predicts the 2021 SSW by focusing the wave propagation toward the polar region. These results suggest that the combined effect of rapidly developing North Pacific cyclones and the preconditioned stratospheric mean state is attributable to the development of the 2021 SSW. This study suggests that both tropospheric precursors and stratospheric preconditioning should be considered for a better understanding of SSW and to improve its predictability.