The strongest and most persistent upward propagation of zonal wavenumber 1 (WN1) Rossby waves from the troposphere on record led to the rare Antarctic sudden stratospheric warming (SSW) in September 2019. The dynamical contribution from instantaneous anomalous WN1 and its linear interference with the climatological WN1 contributed equally to the event. The unprecedented WN1 planetary wave behavior is further attributed to a long-lived midlatitude circumpolar Rossby wave train in the troposphere that was sustained by anomalous convection, first over the subtropical Pacific Ocean east of Australia and then over the eastern South Pacific. Besides the tropospheric wave forcing, the phase of the quasi-biennial oscillation in the upper stratosphere also facilitated the weakening of polar vortex. Moreover, this SSW strongly influenced the tropospheric circulation via the Southern annular mode, favoring conditions linked to the 2019 bushfires in eastern Australia. Plain Language Summary A sudden stratospheric warming (SSW) is an extreme event associated with rapid warming high up in the stratosphere, which can influence weather conditions at the surface. In September 2019, a rare strong SSW occurred in the Southern Hemisphere, providing further insight into these events. Here, we show that this SSW was caused by exceptionally strong and long-lasting atmospheric waves propagating upward from the troposphere into the stratosphere. Anomalous tropical convection is a possible source for these large-scale atmospheric waves. Besides, the zonal winds in the tropical upper stratosphere set a favorable background condition for the occurrence of the SSW. Our results also link the SSW to subsequent anomalous tropospheric circulation and the late 2019 bushfires in eastern Australia.

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Changes in the stratospheric polar vortex (SPV) can remarkably impact tropospheric circulation. Based on the diagnosis of reanalysis data, this study finds that the location shift rather than the strength change dominates the intraseasonal variability of SPV. Further analysis suggests that it couples well with the tropospheric circulation, forming an intraseasonal stratosphere-troposphere oscillation (STO). The STO shows periodic westward propagation throughout its life cycle and has a deep structure extending from the troposphere to the stratosphere. It reflects the movement of the SPV towards North America, then North Pacific, Eurasia, and the North Atlantic, and causes significant changes in surface air temperature over North America and East Asia. The mechanism of the STO involves Rossby wave propagation between the troposphere and stratosphere and cross-scale interactions in the troposphere. Upward Rossby wave propagation from the troposphere over East Asia maintains the STO's stratospheric component, and the reflection of these waves back to the troposphere contributes substantially to the STO's tropospheric center over North America. Meanwhile, the linear and nonlinear processes explain the STO's westward propagation in the troposphere, which facilities vertical wave propagation changes. The STO unifies the SPV shifts, the retrograding tropospheric disturbances, and the wave coupling processes into one framework and provides a holistic view for a better understanding of the intraseasonal stratosphere-troposphere coupling. Given its oscillating nature, timescale, and widespread surface response, the STO may be a potential source of predictability for the subseasonal-to-seasonal prediction.