The Stratosphere–Troposphere Oscillation as the Dominant Intraseasonal Coupling Mode between the Stratosphere and Troposphere.

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 toward North America, then the 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, time scale, and widespread surface response, the STO may be a potential source of predictability for the subseasonal-to-seasonal prediction. Significance Statement: Stratospheric circulation plays a vital role in influencing tropospheric weather and climate, but its variability and coupling with the troposphere have not been fully understood for the intraseasonal time scale. This study finds that the Northern Annular Mode is the leading mode of variability in the extratropical Northern Hemisphere stratosphere on time scales longer than 60 days, which reflects the changes in the intensity of the stratospheric polar vortex. In contrast, the shift of the stratospheric polar vortex excels as the leading mode on time scales shorter than 60 days and is identified as a stratosphere–troposphere oscillation (STO) phenomenon. In the stratosphere, the STO is characterized by the shift of the polar vortex and rotates clockwise with time. In the troposphere, the STO is manifested as a large-scale westward-propagating circulation in the midlatitudes, with significant near-surface temperature anomalies across the continents. The formation of the STO is further attributed to the vertical and horizontal Rossby wave propagation. As STO is a periodic oscillation, it may serve as a potential predictability source for subseasonal-to-seasonal climate prediction. [ABSTRACT FROM AUTHOR]