A document by Tiffany A Shaw. Click on the document to view its contents.

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.

Recent work has suggested that tropical Pacific decadal variability and external forcings have had a comparable influence on the observed changes in the Southern Hemisphere (SH) summertime eddy-driven jet over the satellite era. Here we contrast the zonally asymmetric response of the SH eddy-driven jet to tropical Pacific decadal variability using ensembles of the Community Earth System Model version 1 (CESM1) in a pacemaker framework, where sea surface temperatures (SSTA) in the tropical Pacific are nudged to observations. In both coupled and uncoupled experiments, the observed South Pacific jet intensification is found in all seasons, indicating the tropical Pacific SST cooling anomaly impacts the South Pacific jet mainly via direct atmospheric processes. By contrast, only the coupled pacemaker reproduces the South Atlantic-Indian jet poleward shift in the summertime, suggesting that the air-sea coupling is essential in driving the teleconnections between tropical Pacific SSTA and South Atlantic-Indian jet variations.

Outputs from new state-of-the-art climate models under the Coupled Model Inter-comparison Project phase 6 (CMIP6) promise improvement and enhancement of climate change projections information for Australia. Here we focus on three key aspects of CMIP6: what is new in these models, how the available CMIP6 models evaluate compared to CMIP5, and their projections of the future Australian climate compared to CMIP5 focussing on the highest emissions scenario. The CMIP6 ensemble has several new features of relevance to policy-makers and others, for example the integrated matrix of socio-economic and concentration pathways. The CMIP6 models show incremental improvements in the simulation of the climate in the Australian region, including a reduced equatorial Pacific cold-tongue bias, slightly improved rainfall teleconnections with regional climate drivers, improved representation of atmosphere and ocean extreme heat events, as well as dynamic sea level. However, important regional biases remain, evident in the excessive precipitation over the Maritime Continent and precipitation pattern biases in the nearby tropical convergence zones. Projections of temperature and rainfall from the available CMIP6 ensemble broadly agree with those from CMIP5, except for a group of CMIP6 models with higher climate sensitivity and greater warming and increase in some extremes after 2050. CMIP6 rainfall projections are similar to CMIP5, but the ensemble examined has a narrower range of rainfall change in summer in the north and winter in the south. Overall, future national projections are likely to be similar to previous versions but perhaps with some areas of improved confidence and clarity.