Storm tracks and large-scale variability in the Southern Hemisphere: observations and CMIP6 models

Storm tracks are a key component of global atmospheric circulation. Their influence ranges from macro- to mesoscale dynamics; from large-scale movement of heat and momentum to extreme weather events. The scale of their impact makes understanding storm track dynamics critical to forecasting and climate projections. In this study, we investigate observations of the Southern Hemisphere storm track, its interactions with large-scale circulation patterns, and how these interactions affect Southern Hemisphere variability. We assess CMIP6 model fidelity to observations, with the aim of providing confidence for projections of future climatology under predetermined scenarios. Finally, we determine a storm track climatology at the end of the 21st century under high- and low-emissions scenarios.

We use a range of multivariate statistical techniques to investigate connections between storm activity and low-frequency variability. We find consistent relationships, with recurring spatial structures clearly linked to large-scale variability patterns, although responses exhibit non-linearities and interference from other influences can obscure linkages. Notably, the Southern Annular Mode emerges as the organising principle governing co-variability. We find CMIP6 models successfully represent low frequency variability, but highlight persistent systematic biases that undermine fidelity to general storm track characteristics – in particular, underestimated storm activity, especially in the Indian Ocean. We find considerable model consensus in patterns of change, but which is exaggerated in the high-emissions scenario: intensified storm activity borne of sharpening meridional temperature gradients. Consensus on how large-scale variability might change is more elusive; however, results support the El Niño-Southern Oscillation teleconnection intensifying, along with an eastward moving wave source.