Predictability of the 2020 Strong Vortex in the Antarctic Stratosphere and the Role of Ozone.

The Antarctic vortex of October–December 2020 was the strongest on record in the satellite era for the season 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 understand the potential role of ozone in the event and its associated anomalies of the Southern Annular Mode (SAM) and rainfall over south–eastern Australia and western Patagonia. Here, we show that the 2020 strong vortex event did not follow the canonical dynamical evolution seen in previous strong vortex events in spring but suddenly appeared as a result of the record‐low upward propagating wave activity in September 2020. The ACCESS‐S2 forecasts significantly underestimated the negative wave forcing in September even at zero lead time, irrespective of the ozone configuration, therefore falling short in predicting the record strength of the polar vortex in late spring 2020. Nevertheless, ACCESS‐S2 with prescribed realistic ozone that had large anomalies in the Antarctic stratosphere significantly better predicted the strong vortex and the subsequent positive SAM and related rainfall anomalies over south–eastern Australia and western Patagonia in the austral summer of 2020–21. This highlights the potentially important role of ozone variations for seasonal climate forecasting as a source of long‐lead predictability. Plain Language Summary: The Antarctic vortex of October–December 2020 was the strongest on record in the satellite observation era for that season when monitored at 60°S in the mid‐ to lower stratosphere (altitudes of 15–30 km). However, this super vortex event was poorly predicted by the Australian Bureau of Meteorology (BoM)'s seasonal climate forecast system even at 1‐month lead time. We argue that the 2020 strong vortex was likely caused by an abrupt reduction in the upward propagating wave activity from the troposphere in September 2020, which left the stratospheric vortex undisturbed and strong. The BoM seasonal forecast system substantially underpredicted the negative wave forcing in September 2020, resulting in a poor forecast performance for the vortex strength in the following season. Forecast experiments prescribed with observed versus climatological ozone concentrations further show that using the observed ozone, characterized by a significant loss over Antarctica in spring 2020, improved the ensemble mean forecasts for the 2020 vortex strength by ∼10%–20% at different levels of the stratosphere and the associated surface climate features such as the poleward shift of the Southern Hemisphere midlatitude jet and anomalously high rainfall over south–eastern Australia and low rainfall over western Patagonia in the following summer. Key Points: The Antarctic vortex of 2020 was the strongest event in the satellite era for the October–December mean in the mid‐ to lower stratosphereSignificant lack of tropospheric wave forcing in September 2020 was responsible for the sudden appearance of the record strong polar vortexImposing realistic ozone concentrations significantly improved forecasts of the 2020 polar vortex strength and its downward impact [ABSTRACT FROM AUTHOR]