Stratosphere-Troposphere Coupling and its Role in Surface Weather Predictability

extreme weather. Incorporating stratospheric variability realistically in models can help improve both weather forecasts and climate predictions. This special issue brings together a total of 8 papers studying the underlying dynamic/thermodynamic processes of stratosphere-troposphere couplings, their impacts on surface weather, their contribution to climate change, and their influences on temporo-spatial variations of trace gases important for environmental health. An improved understanding of these processes is essential to maximizing the value of the upper-boundary in the operational subseasonal forecasts, (complimentary to the low-frequency low-boundary forcing from ocean and land), and in future climate change projections. The findings of the studies published in this special issue provide ample evidence supporting the importance of stratosphere.The stratospheric "downward impact" is found not to be the same or even present in all cases. The quasi-simultaneous relationship between stratospheric anomalies and weather regimes is one of the focuses of this special issue. The best-known phenomenon, Sudden stratospheric warmings (SSWs), and its weather impact have been revisited by Choi et al. Building on the traditional two-type classification (i.e., displacement and split types), they put forward a new threetype classification that yields a clearer picture of the relationship between weak vortex states and tropospheric cold events. Specifically, they classified the conventional split type into displacement-split (DS) and split-split (SS) types based on the temporal evolution of the stratospheric polar vortex structure and wavenumber-1 and -2 wave activities. The conventional displacement type is classified as a displacement-displacement (DD) type. It is found that the surface air temperature drops notably over central to eastern North America within 20 days following a DS-type SSW. Hence, distinguishing among the three types of SSW can help improve forecasts of cold weather over North America, although the mechanism for the development of the North Pacific ridge remains to be investigated. In addition to the cold temperatures associated with a weakened stratospheric polar vortex, abnormally warm temperatures can be partly explained by the stratosphere as well. Zhang et al. reported that the abnormal warm weather in 2019/2020 winter and spring was highly related to the continuously positive Arctic Oscillation. One of the critical dynamical processes responsible for the continuously positive phase of Arctic Oscillation is the strong upward propagation of planetary waves that causes the subsequent downward propagation of the zonal circulation anomalies from the stratosphere to the troposphere.On interannual timescales, the stratospheric polar vortex also exhibits a close linkage to the winter climate in the extratropics. The anomalous weakening of the stratospheric polar vortex in winter can be closely related to the joint effect of a La Niña event, the west phase of the Quasi- HCl in recent decades might be elusive. The trend no longer exists after removing the period of 2010-2011 when there was a significant increase of Northern Hemisphere stratospheric HCl due to a strong polar vortex and a weakened residual circulation. He et al. found that the CO mixing ratio within the Asian Summer Monsoon Anticyclone (ASMA) region tends to be higher in the east phase of the QBO and the investigated potential dynamical mechanisms behind it.