Picture: Storm at Sea. Credit: Ray Bilcliff (Pexels).

Welcome to the first Weather and Climate Interactions RP report. The new program name is simply a result of rationalising CLEX’s continuing research program under new headings that more clearly delineate the focus of the work we do. The Weather and Climate Interactions research program encompasses a wide variety of phenomena that can vary between localised extreme events to weather and climate events that are global in nature or have been instigated many thousands of miles away.

As an example of the latter, CLEX researchers were part of an international team that explored how two ocean basins, the Atlantic and Pacific, even though they are separated by a landmass can directly influence slow fluctuations that could potentially be exploited to predict the climate over the next year to a decade. Led by NCAR scientist and CLEX PI Jerry Meehl, the team proposed that these ocean basins are mutually interactive, with each basin influencing and responding to processes in the other basin. They found that the Pacific and Atlantic are indeed mutually interactive through both mid-latitude teleconnections and the atmospheric Walker Circulation.

Perhaps the most well-known of the distant phenomena that directly affects Australia is the El Niño Southern Oscillation. Research has shown that it not only has an impact around the Pacific Basin but also reaches across the Australian continent to have an impact on the southwest Indian Ocean. Our researchers investigated whether atmospheric or ocean disturbances caused by the El Niño led to these disturbances in an effort to understand how far ahead they could be forecast. They found the atmospherically‐driven mechanism was responsible for most of the regional south‐western Indian Ocean climate impacts, while simultaneously disrupting the westward transmission of signals from Western Australia. This suggests that timescales for El Niño‐associated predictability are considerably shorter than if purely oceanic mechanisms were involved.

El Nino events also play an important role in rainfall over one of our most important agricultural regions, the Murray Darling Basin, and it turns out even the location of an El Nino event can make a difference. By separating El Niño into central Pacific and eastern Pacific events, CLEX researchers showed that the strength of a central Pacific event controls the rainfall amount for southeastern Australia. The stronger a central Pacific event is, the drier it is over Australia during the onset phase from April to September. But after October during the mature phase of El Niño, the strongest central Pacific events lead to more rainfall than normal over the southeast Australian river catchment known as the Murray Darling Basin, whereas the weakest central Pacific events lead to less rainfall than normal. This relationship is strongest in January to March around the time that the central Pacific event is fully developed. For the strongest central Pacific events, this can be explained by a change in the circulation from drier, more westerly flow during the onset phase to moister, more easterly onshore flow during the mature phase. This finding is important for agricultural and water resources planning efforts in the Murray Darling Basin region and may help with seasonal prediction efforts to predict drought‐breaking rain such as occurred in early 2020.

But oftentimes the origin of a localised climate impact is not so clear. The Amundsen Sea Low, which plays an important role in future of sea‐ice, and ice shelves in the West Antarctica region, is the deepest climatological low-pressure system on the planet. It is located offshore in western Antarctica but there are conflicting hypotheses about what causes its presence. Early studies proposed an important role of the shape and/or the height (mountains) of the Antarctic continent, more recent studies have suggested an important role may be played by the tropics and /or the mid-latitude cyclones in generating this climatological low. CLEX researchers, using climate models and aircraft aerodynamics software found the low is generated by local processes occurring close to Antarctica with little to no role played by the tropics.

Another ocean mystery that has perplexed climate scientists is the spatial and temporal structure of oceanic heat uptake in the cold tongue of the eastern Pacific Ocean. This is important because it plays a crucial part in setting the global climate system’s average state as well as influencing modes of variability such as the El Nino/Southern Oscillation. CLEX researchers and colleagues used a high-resolution numerical simulation of the cold tongue region to show that strong turbulent mixing occurs not only on the Equator but also off the Equator on the edge of the cold tongue associated with passing energetic oceanic waves with periods of 15-40 days known as Tropical Instability Waves. These results strongly suggest the need for observational campaigns that can extend current equatorial measurements of mixing to off-equatorial regions.

Further south in the cold Southern Ocean, the atmospheric winds, which play a major role in its capacity to take up heat, have been changing. These winds affect the overturning circulation but the details of the Southern Ocean’s response to these changing winds remain uncertain. CLEX researchers used a novel methodology using numerical simulations with a global ocean‐sea ice model suite that spans a hierarchy of spatial resolutions and was driven by realistic atmospheric forcing conditions, to reveal robust circulation changes in the short term. However, understanding the long-term response will be dependent the representation of eddies in the model.

While ocean circulation plays a major role in capturing heat in the ocean, atmospheric circulation can lead to historically warm temperatures during the Austral spring. Understanding what drives these high temperatures may lead to better forecasts of extreme heat in the future. CLEX researchers looked at three extreme heat events – September 2013, October-November 2014, and October 2015 – in reanalysis and in a seasonal prediction model. They compared the atmospheric circulation during each of the events to find circulation features to understand how the heat formed. Cyclonic circulation southwest of Australia and an atmospheric wave train with anticyclonic circulation over southern Australia were important features in these events. The model ensemble members that forecast the highest Australian maximum temperatures also best forecast these atmospheric circulation features. However, the model over-represented the relationship with the Pacific Ocean at the cost of the relationship with the Indian Ocean. This poor relationship may mean that the model might underestimate future extreme spring heat events, a factor that should be assessed in future seasonal prediction models.

In a further study of heatwaves over Australia, CLEX researchers took a new approach. CLEX researchers introduced a way of investigating heatwaves that allowed for the heatwave region to move and used this tool to look at summer heatwaves in south-east Australia. They found that heatwaves in south-east Australia are part of an area of hot weather that moves from west to east along with the surface and upper-tropospheric weather patterns, and many of the heatwaves seen in south-eastern Australia are part of moving weather system that has led earlier to hot weather across southwestern Australia. This suggests that majority of heatwaves affecting south-eastern Australia are part of large and strong weather systems propagating across Australia, and not due to stationary or blocked weather systems as seen in some other regions of the world.

So often, following a heatwave, come thunderstorms. In what is perhaps one of the most easily accessible and conversational research briefs, Todd Lane has explained that his research offers a new explanation for thunderstorm formation. He showed the way the wind varies with altitude above the clouds can change the way thunderstorms grow and how long they live. The gravity waves are reflected back down by these variations in the stratosphere, which can then affect the temperature and winds around the storms and make them grow, last longer, or die earlier than they otherwise would. If this mechanism is important, it will have implications for predicting weather and climate.

Another dramatic atmospheric phenomena that can lead to floods and is important to predict are atmospheric rivers. New research by Kim Reid and colleagues found that nine out of 10 of the costliest floods in New Zealand occurred during atmospheric river events. To quantify the impacts the researchers used an Atmospheric River Identification algorithm developed at the University of Melbourne to study Atmospheric River impacts (Reid et al. 2020); station rainfall data provided as a part of a collaboration with the National Institute for Water and Atmospheric Research, New Zealand; and flooding impact estimates from the Insurance Council of New Zealand. Since atmospheric rivers are easier to predict than extreme rainfall and can be forecasted up to five weeks in advance, this result may lead to improved forecasts of extreme rainfall and flooding events in New Zealand.

Observations also played a key role in research that looked at the air quality of Sydney and Melbourne during the Black Summer bushfires and then later as a result of COVID-19. CLEX researchers and colleagues quantified the air quality impact of these contrasting periods in the south-eastern states of Victoria and New South Wales (NSW) using a meteorological normalisation approach. Across Victorian sites, large increases in CO (188%), PM2.5 (322%) and ozone (22%) were observed over the RF prediction in January 2020. In NSW, smaller pollutant increases above the RF prediction were seen (CO 58%, PM2.5 80%, ozone 19%). This can be partly explained by the RF predictions being high compared to the mean of previous months, due to high temperatures and strong wind speeds, highlighting the importance of meteorological normalisation in attributing pollution changes to specific events. From the daily observation-RF prediction differences we estimated 249.8 (95% CI: 156.6-343.) excess deaths and 3490.0 (95% CI 1325.9-5653.5) additional hospitalisations were likely as a result of PM2.5 and O3 exposure in Victoria and NSW. In contrast overall, the air quality change during the COVID-19 lockdown had a negligible impact on the calculated health outcomes.

Aerosols also play a role in climate change and are not only produced by bushfires and industry but also by coral reefs. Currently, this source of aerosols produced by coral reefs is unaccounted for in climate science and hence the impact of coral reef extinction on aerosols and climate is unknown. In this study, CLEX researchers addressed this problem for the first time by using a climate model that can simulate complex chemistry, aerosol and cloud processes. The found the extinction of coral reefs could lead to small decreases in the number and size of aerosols. However, these decreases were found to be too small to influence how much sunlight reaches the ground or to change cloud properties.

Another piece of research using observations and climate models, examined reports of gaseous elemental mercury observations from Churchill, in the heart of the Latrobe Valley’s coal power generation fleet from June of 2013. Mercury values both day and night were significantly higher than the Southern Hemispheric average values. With an externalized annual health cost (2015-2016 values) of $88 million mercury emissions represent a significant health burden, though a small fraction of the annual total Latrobe valley’s health cost due to all air pollutants, which is ~$9.08B. Using WRF-Chem modelling we were only able to reconcile the temporal variation of the mercury observations if we assumed high soil contamination, and therefore emission fluxes, in a 15km radius around each generator. Such high soil contaminations have been observed in Spain and China, but no such soil survey for mercury has been conducted in Australia surrounding coal-fired power generators.

This extensive range of research is only part of what has been a, surprisingly, busy few months that also saw some of the RPs researchers take key positions in the climate science community. Andrea Taschetto accepted an invitation to join the CLIVAR Pacific Region Panel while Agus Santoso and Dietmar Dommenget were made editors for the American Meteorological Society’s Journal of Climate.  

Our researchers also received accolades and promotions for their work. Robyn Schofield was promoted to Associate Professor. Margot Bador who was awarded a European Marie-Curie fellowship (Marie Sklodowska-Curie Actions fellowship). This 2-year fellowship will allow Margot to conduct research at the Cerfacs institute in Toulouse (France) with a secondment at Météo-France, where she will investigate the future changes in extreme rainfall over Europe. Meanwhile, PhD student Kim Reid won an Outstanding Student Presentation Award at the most recent AGU meeting for her oral presentation on the sensitivity of atmospheric river detection to resolution, regridding and thresholds, work which she published in October last year.

Congratulations must also go to Yohanna Villalobos Cortes who recently handed in her PhD thesis, a major milestone in any scientific career. 

But as well as being awarded by our peers, the Weather and Climate Interactions team has been sharing our research with the public and stakeholders at a number of events. Todd Lane presented Telling the future: climate science and climate modelling at the Victorian Physics Teachers Conference and was a panellist for the Building Resilience session at the launch of Melbourne Climate Futures. Ailie Gallant and Agus Santoso presented a webinar briefing to audience including government Climate variability and change – past, present and future as part of the NESP Earth Systems and Climate Change Hub. In short, the newly named research program has hit the ground running and should COVID restrictions ease over the coming year as hoped, then 2021 could shape up to be an impressive year for us all.