2018: Year in review

Research

The Climate & Fluid Physics group conducts research into fluid physics and thermodynamic processes that are relevant to the Earth system. Our current research priorities include oceanic convection, ice-ocean interactions, the energy balance of the land surface and the large-scale circulation of the ocean. Our research profile includes funded contributions from the ARC Centre of Excellence for Climate Extremes and the Consortium for Ocean-Sea Ice Modelling in Australia (COSIMA).

This year saw the complete release of ACCESS-OM2 (the ocean-sea ice component of Australia's ACCESS climate model) which is coordinated by COSIMA. ACCESS-OM2 is currently available at 3 different resolutions (1°, 0.25° and 0.1°) with a consistent set of parameters. We have completed full spinup and evaluation simulations, with a model evaluation exercise underway.

We continued our work on the global water cycle and drought. Early in the year saw new collaborative research (with Tsinghua University and the Research School of Biology at ANU) on how to assess stationarity in rainfall data (Sun et al 2018). Recently we have been applying these new concepts to the Australian rainfall record with the surprising conclusion that rainfall has, for the most part, remained stationary over the last 120 years in many (but not all) parts of Australia. We intend to build on that work in the coming year and combine those analyses with new approaches to assess hydrologic variability over the global land surface. We anticipate a new framework that fully incorporates climate variability into assessments of change in the global water cycle.

Continuing with the theme of water, over the past decade, climate scientists have published numerous articles arguing that global warming will be accompanied by an overall increase in aridity. However, one of the great puzzles in climate science, now known as “aridity paradox”, is that inter-glacial periods with higher levels of atmospheric CO2 are generally thought to be warmer, wetter and greener while glacial periods with lower levels of atmospheric CO2 are colder, drier and browner. We first identified this important paradox and have been steadily working on the topic since 2015. At the end of 2018 we have finally been able to resolve the paradox. In brief, it turns out that the assessments of increasing aridity with warming were not actually based on climate model projections. Instead they were based on off-line assessments that ignored the critical response of vegetation to rising levels of atmospheric CO2. In particular, vegetation shows a direct biochemical response to rising atmospheric CO2 by an increase in the water use efficiency of photosynthesis. This water use efficiency effect had not been factored into previous assessments. When that effect is incorporated, we have been able to demonstrate that in terms of long-term averages, current climate models project a higher CO2 world to be warmer, wetter and greener (Yang et al 2018). This turns out to be consistent with the geological record and with modern observations over the last 50 years.

While developing ACCESS-OM2, we have continued to make scientific progress using existing modelling tools. We used a Lagrangian tracking approach to estimate the pathway of floating kelp around the Southern Ocean, showing for the first time that kelp is able to float to Antarctic beaches (Fraser et al. 2018). The critical step in this study was to recognise the role played by huge surface waves in the Southern Ocean in pushing the kelp southwards during storm events. This landmark study was published in Nature Climate Change and has received significant attention.

Simulations of kelp floating in the Southern Ocean. Particles were released from South Georgia, and travel around Antarctic, before a small percentage of the particles (those coloured orange) made it to the Antarctic coastline.

 

At the fundamental level, we have continued our world-leading research into the dynamics of internal waves and their role in forcing larger-scale, eddying ocean flows. Atmospheric scientists have long known that internal waves play a first-order role in sustaining the atmosphere's overturning circulation, known as the Brewer-Dobson circulation. Our recent ultra-high resolution "wave resolving" simulations show a Simulations of kelp floating in the Southern Ocean. Particles were released from South Georgia, and travel around Antarctic, before a small percentage of the particles (those coloured orange) made it to the Antarctic coastline. similar behaviour in the ocean, with tidally generated waves at the ocean bottom propagating up and driving enhanced eddy fields in the upper ocean (Shakespeare & Hogg, 2018). We are now looking to parameterise these effects in global ocean models (e.g. ACCESS-OM2) and evaluate their impact on global circulation. On a different note, in early 2018 data that came from spacecraft Juno revealed that the zonal jets in the atmosphere of Jupiter (i.e., Jupiter's coloured stripes) continue for 3,000km deep beneath the cloud tops. This is about 5% of the gaseous giant's radius, but there was no explanation for why the jets terminate at that particular depth. In collaboration with Lawrence Livermore National Laboratory (USA), we developed a theory that describes how zonal jets, turbulence, and magnetic fields interact. Using principles from statistical physics of turbulent systems, they devised a mathematical model which predicts that when magnetic fields are strong enough the jets shut down (Constantinou & Parker 2018). This theory offers a partial explanation as to why the jets terminate at about 3,000km, since at around that depth the pressure inside Jupiter becomes so high that the fluid gets ionized and starts being conductive. This paper drew a lot of media attention when it was published in August.

Staff news

Callum Shakespeare began his tenure-track appointment. Bishakhdatta Gayen and Yifei Zhou departed the group. Nicky Wright, Anna Ukkola and Navid Constantinou all joined the group, funded by the ARC Centre of Excellence for Climate Extremes. Yuting Yang joined the group from CSIRO Land and Water for a brief six month period before departing Australia for a tenured appointment in the Department of Hydraulic Engineering at Tsinghua University in Beijing. During his stay, Yuting was funded by the ARC Centre of Excellence for Climate System Science. Andy Hogg was promoted to Professor.

Student news

Jemima Rama joined the group as a PhD student, working on internal waves with Callum Shakespeare.

Emeritus, Honorary staff and Visitors

Emeritus Professor Stewart Turner has renounced his Honorary position as a result of declining health, formally concluding a long association with the group. Prof Ross Griffiths remains as an active Emeritus member of the group. Dr Claire Carouge remains as a long-term visitor from UNSW, heading the Computational Modelling Support team for the ARC Centre of Excellence for Climate Extremes. Dr Annette Hirsch (UNSW) also sits in the group as a long-term visitor, working in the Drought Research Programs of the ARC Centre of Excellence for Climate Extremes.