Seasonal Evolution of Stratosphere-Troposphere Coupling in the Southern Hemisphere and Implications for the Predictability of Surface Climate

摘要

Stratosphere-troposphere coupling in the Southern Hemisphere (SH) polar vortex is an important dynamical process that provides predictability of the tropospheric Southern Annular Mode (SAM) and its associated surface impacts. SH stratosphere-troposphere coupling is explored by height-time domain empirical orthogonal function (EOF) analysis applied to the zonal mean-zonal wind anomalies averaged over the Antarctic circumpolar region (55-65 degrees S; U55-65 degrees S). The leading EOF explains 42 of the height-time variance of U55-65 degrees S and depicts the variations of the vortex that is tightly tied to the seasonal breakdown of the vortex during late spring. The leading EOF pattern, defined here as the stratosphere-troposphere coupled mode, is characterized by variations in U55-65 degrees S that develop in early winter near the stratopause, change sign from late winter to early spring, gain maximum amplitude during October in the upper stratosphere, and then extend downward to the surface from October to January. This stratosphere-troposphere coupling during the spring months appears to be preconditioned by anomalies in upward propagating planetary wave activity and a meridional shift of the vortex as high as the stratopause and as early as June. Interannual variations of the stratosphere-troposphere coupled mode are highly correlated with those of the tropospheric SAM, Antarctic stratospheric ozone concentration, Antarctic sea ice concentrations in the South Pacific and the Weddell Sea, and SH regional climate during late spring-early summer. Anomalies in the upper stratospheric flow as early as June are thus a potentially important source of predictability for the tropospheric SAM and its associated impacts on surface climate in spring and summer. Plain Language Summary We have explored the seasonal evolution of the strength of the Southern Hemisphere (SH) stratospheric polar vortex and its coupling to the lower atmosphere in 1979-2017, using atmospheric reanalyses that assimilate observational data. Variations in the strength of the Antarctic circumpolar flow in the troposphere during austral late spring to early summer are found to be coupled to variations in position and strength of the stratospheric vortex as high as the stratopause and as early as the preceding June, notably higher and earlier than indicated in previous studies. This suggests that variations in the winter upper stratospheric jet can serve as a potentially important source of predictability for SH surface climate in the late spring-early summer. We explore the physical processes associated with the seasonal evolution of SH stratosphere-troposphere coupling and its impacts on surface climate.