Picture: Inside a wave. Credit: Joshua Dewey Unsplash
It has been remarkable how much we have achieved in this extraordinarily difficult year. Research coming out of the Teleconnections and Variability program over the past four months has strongly focused on how influences in one part of the world can have direct impacts on another.
For Australia, one of those influences has come from the Indian Ocean. The ocean temperatures in this region are now 1°C warmer than they had been in the mid 20th century. Our researchers found that the warming in this single ocean basin has had global impacts. The warming ocean and increased evaporation led to higher rates of precipitation around the ocean basin. The heat dissipation caused by this increased rainfall has a direct impact on atmospheric circulation that has suppressed rainfall half a world away in the tropical Atlantic Ocean. In the extratropical regions, the most pronounced features include a meridional pressure gradient and strengthening of westerlies in the North Atlantic during austral summer and a zonal wave 3 pattern in the Southern Hemisphere during austral winter. These patterns and associated fields are similar in structure to the circulation trends observed in nature over the past 50 years. This finding suggests a need to take the global influence of the Indian Ocean into greater consideration in observations and climate model studies of the past few decades.
Intriguingly, while the surface waters of the Indian Ocean have warmed more than other ocean basins, the amount of heat stored in the top 700m between 1960-2000 did not exhibit such strong increases. It is only since 2000 that we have seen a rapid increase in heat storage. Our researchers used ocean model simulations to investigate this unusual behaviour, which revealed how winds can impact the upper-ocean temperature structure in the Indian Ocean, either through the atmosphere or via an oceanic connection from the Pacific. Understanding this process and whether it is likely to occur again gives us insight into with long-term changes in the Indian Ocean that directly affect the regional climate in surrounding countries, and how it may respond in the near future to a warming climate.
As we have already seen the direct impacts of a warming climate in our oceans through the rise of marine heatwaves. Working with the Heatwaves and Cold Outbreaks research program and an international group of researchers, we were able to investigate how marine heatwaves have changed and the distant influences that could enable us to better forecast them in the future. The team found that the worst marine heatwaves actually occurred before average ocean temperatures peaked in each hemisphere and that a key element that led to their creation were clear skies and a lack of wind. These conditions are often associated with stalled high-pressure systems. The researchers also investigated how this impacted algal blooms, which are the foundation of ocean ecosystems. In the tropics marine heatwaves restricted algal growth, while in the extra tropics closer to the poles — where nutrients are readily available — the additional sunlight reduced ocean mixing and lead to an increase in these blooms.
To get a better understanding of the future impacts of climate change it sometimes pays to look far in the past. A particularly useful period for this is the mid-Pliocene, around 3 million years ago when temperatures were 3°C higher than our preindustrial climate and carbon dioxide concentrations were at similar levels. Using climate model simulations of this period, our researchers examined how precipitation would change in such a world. The study found that the peak rainy season in the Southern Hemisphere saw a marked decline in precipitation due to a large-scale reorganisation of atmospheric circulation. Should this come to pass in the near future as a result of climate change it would lead to less rain in South America, South Africa, and South Pacific islands. In Australia the outcome is less clear but there are suggestions that the Australian monsoon season would become more intense.
A shift in El Nino events is another change that could be a consequence of global warming. Researchers have already noted changes in the El Niño events, with more occurring in the central Pacific than in the eastern Pacific. The location and intensity of these El Niño events brings differing impacts to Australia and many other countries around the world. Using a range of climate models our researchers found that the general pattern of warming across the Pacific with climate change was likely to have a profound effect on where El Niños formed and their likely intensity. Another important influence on how future El Niños may manifest was how the models represented Pacific decadal variability, suggesting that natural variability within the climate system may have as much bearing on future El Niños as global warming.
Natural variability within systems can also have an impact on regional climate. A nice example of this is research that investigated the winds that circle Antarctica and how they contract and expand. In general, these winds have been moving closer to Antarctica and this contraction has been described using a hemispheric index known as the Southern Annular Mode. Schematics of these general trends essentially show the winds in the form of a doughnut. However, the weather systems that influence the rainfall, temperature and wind conditions that we experience do not look anything like a doughnut. By grouping weather systems by similar patterns rather than averaging conditions over months, seasons or years, CLEX researchers found that between Australia and Antarctica, the ‘doughnut’ structure of SAM is split into multiple ‘flavours’ and is more likely to have ‘bite marks’ out of it than be a perfect ring. These different flavours of SAM mean a hemispheric index often fails to capture the regional variability in surface weather conditions over southern Australia and East Antarctica. This suggests more complex regional analyses will be required to improve our understanding of the impacts of these winds on Australian climate.
But is not just the winds around Antarctica that can influence our climate but as continuing research shows, Antarctica and the ice around it also influence Australia’s climate. This makes it important for us to be able to validate the data that does come from this data sparse region. A key data point is surface air temperature above Antarctica. This information is usually derived from reanalysis datasets that are best guess estimates made up from combining observations and models, similar to weather maps. However, multiple reanalysis products have very different surface trends in this region over the past 40 years of satellite observations, so which one should we use? CLEX researchers aimed to bring clarity to these trends through a study based on the well-established fact that sea ice cover is very closely related to surface air temperature. The idea was that researchers could use trends in Antarctic sea ice as an independent validation for the reanalysis trends. This simple approach worked beautifully, showing not only which reanalysis products had the ‘best’ trends overall (at least in terms of their agreement with sea ice), but also highlighting regions where the products are more or less reliable. This will be invaluable information for Antarctic and Southern Ocean researchers who come across the old problem of ‘what data should I use?
The oceans around Antarctica and the sea ice that encompasses the continent play an important role in deep ocean currents and carbon storage. The storage of carbon in the deep ocean is partly a reflection of the biological activity that occurs when the water is at the surface before it sinks. This is why it is important to evaluate how unexpected and dramatic changes at the surface will change biological and chemical cycles, so that we can understand their impact on a larger scale. In a recent paper, our researchers investigated how a major glacier tongue break in the Mertz polynya in Antarctica impacted phytoplankton blooms. Larger phytoplankton blooms increase the amount of carbon that can be stored in the deep ocean. The researchers found that after the glacier tongue break the bloom duration and ice-free period decreased; the start of the bloom and the retreat of sea-ice were delayed; and the intensity of the bloom and the sea-ice concentration increased. These findings show that natural changes can impact the timing of phytoplankton growth that may have consequences for the rest of the ecosystem, from Antarctic krill to baleen whales.
Our researchers also came together to produce a textbook that is likely to be of foundational importance in understanding the El Niño Southern Oscillation and its response to climate change. El Niño Southern Oscillation in a Changing Climate published by Wiley for the American Geophysical Union is a comprehensive and accessible exploration of ENSO and how it is altering with human caused climate change. Through the course of its pages the book tracks the historical development of ideas about ENSO, explores the underlying physical processes and how it has varied over decades and now centuries thanks to advances in paleo-reconstructions. The book also reveals the latest science on how ENSO responds to external factors. These external influences coupled with the chaotic nature of the phenomena itself are examined further in chapters that explain the barriers and potential areas of research that may help us forecast these events in advance. Most importantly, as we look to a future affected by global warming and changes to our climate, the book covers the extensive impacts on extreme ocean, weather, and climate events; fisheries; marine ecosystems; and the global carbon cycle.
Beyond the usual selection of research papers in high impact journals designed for our scientific peers, Amelie Meyer also published a paper, The future of the Arctic: what does it mean for sea ice and small creatures?, In the journal Frontiers for Young Minds. This is an open-access journal specifically written by scientists and reviewed by children and teens. It’s a great outlet to produce rigorous science articles that the scientists of tomorrow can read.
Amelie has also recently been appointed to The Scientific Committee on Oceanic Research funded Analysing ocean turbulence observations to quantify mixing (ATOMIX) working group. The SCOR working groups are funded over four years to deliberate on a narrowly focused topic and develop a peer-reviewed publication and other products that will advance their topic. Details about the ATOMIX WG can be found here.
Meanwhile, Zebedee Nicholls was part of a team that has developed a CMIP6 visualisation tool of large-scale averaged time series aggregated to global, hemispheric and land and ocean averages. If you want to make your own plots the team are developing examples of how more complex visualisations can be performed and are building a gallery of visualisations, which will all be available here. Find out more about the tool by visiting the website.
It is always rewarding to look back and summarise the past four months of the impressive work produced by CLEX and the members of this program, but it is equally enjoyable to see how our people have been acknowledged by their peers and others. And as this year draws to a close we have certainly seen those acknowledgements come in thick and fast.
Adele Morrison was recently announced as one of five winners of the $25,000 2020 L’Oreal-UNESCO For Women in Science Fellowship. You can find out more about it and read Adele’s interview here.
Navid Constantinou and Ryan Holmes were awarded DECRAS in this year’s round, continuing CLEX’s fine record of talented young researchers who have received this grant. Ryan received his DECRA through the University of Sydney for his research, Mixing and air-sea coupling in the Pacific: Toward better El Nino forecasts, while Navid will be researching Machine learning of subgrid ocean physics for global ocean models.
Congratulations also go to Andrew Hogg, Matthew England, Adele Morrison, Paul Spence, Ryan Holmes, William Hobbs, Callum Shakespeare, Ben Evans, Simon Marsland, and Stephen Griffies who are all part of a large grant to build Australia’s next-generation ocean-sea ice model
Meanwhile, Nathan Bindoff and Richard Matear are part of a Discovery Project that aims to quantify how the ocean’s biological pump – which exports newly formed organic matter into the ocean interior – responds to environmental change. Another Discovery Project grant has been awarded to Helen Phillips, Maxim Nikurashin, Bernadette Sloyan, and Susan Wijffels that expects to develop new knowledge of ocean-atmosphere interactions along the path of the ITF from the Pacific to the Indian Ocean.
At our recent workshop, Giovanni Liguori was awarded best paper for an ECR for A joint role for forced and internally-driven variability in the decadal modulation of global warming. A week later Matthew England, and Alex Sen Gupta appeared in this years Clarivate Analytics most cited researchers.
We have also seen a range of promotions with Peter Strutton promoted to full professor at the University of Tasmania and Callum Shakespeare promoted to level C. Callum is a true product of the Centre of Excellence system. He was initially an ARCCSS honours student, and after his PhD in Cambridge, returned back to ARCCSS as a postdoc. He is currently mid-way through his DECRA and a CLEX Associate Investigator.
And it gives us great pleasure to continue producing future researchers of great quality. Saurabh Rathore submitted his PhD, Investigating the Hemispheric Asymmetry in Global Ocean Warming and the Links Between Sea Surface Salinity and Australian Precipitation, at IMAS, UTAS recently. His supervisors were Nathan Bindoff, Helen Phillips and Ming Feng (CSIRO). A short time later Jiale Lou who submitted his thesis, South Pacific ocean climate dynamics and predictability. Jiale was supervised by Neil Holbrook and Terry O’Kane.
And now we welcome another student into our program, Felipe Silva who has joined CLEX as a PhD student via IMAS. Felipe will be working on dynamical oceanography of a standing meander in the Antarctic Circumpolar Current: A parallel investigation with observations and models.