Weak Polar Vortex Events and QBO Modulation: Pathways Linking Stratospheric Variability to Australian Heat and Fire Risk

The Southern Hemisphere polar vortex provides a key pathway for stratosphere–troposphere coupling and can influence Australian spring and summer climate, including extreme heat, drought, and fire weather. However, the extent to which this coupling depends on the phase of the Quasi-Biennial Oscillation (QBO) remains unclear. Here, we assess how the QBO modulates the influence of polar-vortex variability on Australian spring and summer climate.

Using ERA5 reanalysis, we define a weak-vortex index based on polar-cap temperature at 100 hPa (Temp100; 65–90°S), where anomalously warm Temp100 indicates weak-vortex conditions. Associated circulation and surface anomalies are diagnosed using regression and composite analyses, conducted separately for easterly (EQBO) and westerly (WQBO) QBO phases. Downward propagation of stratospheric anomalies is examined using height–time regressions of polar-cap geopotential height and temperature. Tropospheric coupling is quantified through correlations with the Southern Annular Mode (SAM) index and Australian near-surface temperature.

Weak-vortex events are characterised by anomalous polar-cap warming and coherent stratospheric height anomalies that descend toward the lower stratosphere. The timing of this downward influence exhibits a pronounced dependence on the QBO phase. During EQBO, the tropospheric response is delayed, emerging in November and persisting into mid-December, whereas under WQBO the surface response is largely confined to October. Temp100 is negatively correlated with the SAM index in both QBO phases, but peak coupling occurs in November–December during EQBO and in October during WQBO. Australian near-surface temperature shows corresponding seasonality and distinct spatial patterns. Under EQBO, warming is strongest over southeastern Australia in November and shifts toward northeastern regions in December. Under WQBO, warming emerges over northern Australia in October, while cooling dominates southern regions.

These results highlight the QBO as a key modulator of Southern Hemisphere polar-vortex variability and its downward influence, identifying a potential source of extended-range predictability for regional Australian climate. Ongoing work will quantify impacts on heat and fire-weather extremes, test sensitivity to event definitions, and assess whether subseasonal forecast systems reproduce the observed QBO dependence.