In the austral spring seasons of 2020-2022, the Antarctic stratosphere experienced three consecutive strong vortex events. In particular, the Antarctic vortex of October-December 2020 was the strongest on record in the satellite era for that season at 60°S in the mid- to lower stratosphere. However, it was poorly predicted by the Australian Bureau of Meteorology’s operational seasonal climate forecast system of that time, ACCESS-S1, even at a short lead time of a month. Using the current operational forecast system, ACCESS-S2, we have, therefore, tried to find a primary cause of the limited predictability of this event and conducted forecast sensitivity experiments to climatological versus observation-based ozone to understand the potential role of the ozone forcing in the strong vortex event and associated anomalies of the Southern Annular Mode (SAM) and south-eastern Australian rainfall. Here, we show that the 2020 strong vortex event did not follow the canonical dynamical evolution seen in previous strong vortex events in spring, whereas the ACCESS-S2 control forecasts with the climatological ozone did, which likely accounts for the inaccurate forecasts of ACCESS-S1/S2 at 1-month lead time. Forcing ACCESS-S2 with observed ozone significantly improved the skill in predicting the strong vortex in October-December 2020 and the subsequent positive SAM and related rainfall increase over south-eastern Australia in the summer of December 2020 to February 2021. These results highlight an important role of ozone variations in seasonal climate forecasting as a source of long-lead predictability, and therefore, a need for improved ozone forcing in future ACCESS-S development.

The factors driving variability in rainfall stable water isotopes (specifically δ¹⁸O and deuterium excess, d = δ²H - 8 δ¹⁸O) were studied in a 13-year dataset of daily rainfall samples from coastal southwestern Western Australia (SWWA). Backwards dispersion modelling, automatic synoptic type classification, and a statistical model were used to establish causes of variability on a daily scale; and predictions from the model were aggregated to longer temporal scales to discover the cause of variability on multiple timescales. Factors differ between δ¹⁸O and d and differ according to temporal scale. Rainfall intensity, both at the observation site and upwind, was most important for determining δ¹⁸O and this relationship was robust across all time scales (daily, seasonal, and interannual) as well as generalizing to a second observation site. The sensitivity of δ¹⁸O to rainfall intensity makes annual mean values particularly sensitive to the year’s largest events. Projecting the rainfall intensity relationship back through ∼ 100 years of precipitation observations can explain ∼ 0.2-0.4‰ shifts in rainfall δ¹⁸O. Twentieth century speleothem records from the region exhibit signals of a similar magnitude, indicating that rainfall intensity should be taken into account during the interpretation of regional climate archives. For d, humidity during evaporation from the ocean was the most important driver of variability at the daily scale, as well as explaining the seasonal cycle, but source humidity failed to explain the longer-term interannual variability.