April 1, 2020 | Published by |

The past few months have seen some fascinating research coming out of the Heatwaves and Cold Air Outbreaks program. Some research has explored what were thought to be well-understood climate phenomena and found that perhaps our understanding may not be as clear-cut as previously assumed.

A powerful, but relatively rare, influence on Australia’s climate are sudden stratospheric warmings (SSW) that can slow or reverse the wind direction around the Antarctic. These happen rarely in the Southern Hemisphere but when they do occur they have predictable and severe weather impacts. It has long been assumed that the sudden upper stratosphere warming is the primary contributor to these predictable extremes. However, CLEX researchers revealed that it wasn’t the SSW itself that led to these impacts but the reversals of winds around the Antarctic that mattered. This suggests that any slowing or reversal of Antarctic winds may have similar impacts regardless of the cause.

SSWs are far more common in the Northern Hemisphere than the Southern Hemisphere but they occur more frequently at both poles in winter than any other season. The most recent example of a Northern Hemisphere event occurred in 2018. This SSW event was linked to a severe system nicknamed the “Beast from the East”, which brought very low temperatures and heavy snowfall to western Europe. Surprisingly, this link and its impacts had only previously been made through climate model studies. Our researchers working with international colleagues used, for the first time, observations to study the connection and analyse the impacts. The analysis confirmed the link between Northern Hemisphere SSWs and extreme events but the pattern was particularly unusual. The observations showed below‐average temperatures tended to precede SSW events, but the intensity of cold extremes, such as the coldest night of the month, were strongest after the SSW event.

Observational analyses have also long been an important part of determining how well our climate models reproduce real-world events. The fundamental research being carried out by our marine heatwaves program recently brought together observations of these oceanic heating events and compared them to marine heatwaves produced by climate models at low, medium and high resolutions. They found that, regardless of their resolution, all simulations have weaker, longer, and less-frequent marine heatwaves, when compared with the real world. Despite these differences, they found that simulations with medium and high resolutions realistically represented global spatial patterns of these heatwaves. However, the ocean simulations with high resolution were preferable when studying regional patterns of these events. This research will help us to better understand computer simulations of future oceans, under climate change conditions.

While we still have much to learn about marine heatwaves, heatwaves over land are better represented by climate models. This allowed our researchers to investigate if there was a difference in the number of heatwaves over land for exactly the same global average temperature in a world that was still warming compared to a world where the warming had plateaued. Using a novel methodology applied to CMIP5 projections, CLEX researchers found that the local temperatures experienced by 90% of people would be substantially higher in a transient (still warming) climate than an equilibrium climate where the temperatures have plateaued, for the same global temperature. The study demonstrates that it is vital that the use of transient or stabilised climate simulations is explicit in future projections, so decision-makers can best prepare for future warming.

Heatwaves over land are also influenced by local conditions, particularly in Australia. Dry soils have been shown to amplify heatwaves in the Northern Hemisphere but surprisingly little work has been done on this effect in Australia. In exploring these knowledge gaps, CLEX researchers found that regions with a larger drying trend tend to be more sensitive to land water availability and have more heatwave days. However, they found that the effect of dry soils before a heatwave varies considerably across Australia. The results of this study may require classifying the land into regions where soil water variability affects surface temperatures and where it doesn’t. This could be extended to other atmospheric processes to differentiate between local and remote influences.

Our models are also useful for immediate policy considerations as shown by a new cross-scale modelling framework for urban environments that have been applied to calculate how electricity and gas demand will change under future climate change and air conditioner (AC) ownership scenarios. Our researchers used Melbourne as a case study capturing interactions across building, urban and atmosphere scales at a higher temporal resolution than any location worldwide. The framework developed with this research was also able to undertake century-scale simulations, an order of magnitude longer than previous coupled building-urban-atmosphere studies. The building energy demand, urban climate and global climate modelling systems resulting from this study are open-source and model outputs are also available across the century at half-hour timesteps. This is important and timely research as our energy systems transform.

Adding to the depth of our research contingent, we welcomed  Kate Bongiovanni, a new Masters student at the University of Melbourne node, working with Linden Ashcroft and Jo Brown. Kate will be looking at the climatic effects of volcanoes with a case study of the 1883 Krakatoa eruption and its effects on the Australian climate. Finally, we must congratulate Joelle Gergis for her latest award, adding to what must already be a trophy cabinet already groaning under the weight of awards and acknowledgments from the past year. Joelle can now add the AMOS Science Outreach Award to the collection after a spectacular 2019 that saw her promote her book, Sunburnt Country: The History and Future of Climate Change in Australia, play a large role in the public conversation around climate and take on numerous public talks. This award follows on from Andrew King, who picked up the inaugural outreach award in 2018. That certainly suggests this is an outward-looking program.