Externally-Forced Changes to Southern Hemisphere Stratosphere-Troposphere Coupling Variability

Stratosphere-troposphere coupling (STC) in the Southern Hemisphere (SH) occurs primarily from austral spring to summer, and the dominant mode of variability in this vertical coupling (the “STC mode”) represents the timing of the seasonal transition of the polar vortex and its subsequent coupling to the Southern Annular Mode (SAM). Because it represents downward coupling over a season, stratospheric winds projected onto the STC mode are useful as a seasonal predictor of spring and summer SH surface climate. However, it is not clear how the STC mode and its connection to seasonal predictability will evolve in response to changes in stratospheric ozone and increases in greenhouse gases. Here, using four large ensemble simulations from 1950 to 2100, we apply a “snapshot EOF” technique in order to track the evolution of the mode in response to prescribed ozone and RCP8.5 greenhouse gas changes. We find that for all large ensembles, the STC mode in late spring strengthens (weakens) during the period of ozone depletion (recovery), and these responses are largest in the model that includes interactive ozone. A majority of the large ensembles further suggest a significant weakening of the mode by 2075 in an extreme RCP8.5 climate, including a decrease in the fraction of variance explained by the mode over time and a decrease in the correlation of the mode with the SAM. The key result is that as ozone recovers and greenhouse gases increase, there is a projected decline in the seasonal predictability of austral surface climate associated with the STC mode.