The role of convection in the Southern Ocean

ARC-supported researchers at the ARC Centre of Excellence for Climate Extremes (CLEX) have used a supercomputer to accurately simulate convection in the waters of the Southern Ocean, with implications for climate models that may have underestimated the degree to which seawaters are mixed by convection in turbulent ocean regions of the Earth.

CLEX invests in basic science that is fundamental to the long term capability of climate models in predicting how extremes will change in the future. Many extreme climate events are triggered through the influence of the ocean, and new discoveries within the ocean have highlighted the role of ocean heatwaves in impacting ecosystems, heat uptake and currents.

At the heart of how the Earth's oceans work is convection. Convection in the ocean occurs when dense (cold and/or saline) water overlies lighter (warmer or fresher) water. Convective events occur primarily when water is cooled (or becomes more salty) at the ocean surface. During these convection events, cold and salty water sinks to the bottom of the ocean, especially along the Antarctic continent.

Researchers at CLEX say that ocean models used for understanding these processes, and for climate projections such as used by the Intergovernmental Panel on Climate Change, do not represent convection well. Further, the impact of convection on the ocean circulation, transport, and turbulent mixing in the Southern Ocean is unknown.

To resolve some of these uncertainties, researchers at CLEX, including ARC Future Fellow, Dr Bishakhdatta Gayen, examined the role of convection and surface winds in the Southern Ocean using a first-of-its-kind, high-resolution model that conserves mass and energy, and was simulated using Australia’s national peak supercomputing facility at the National Computational Infrastructure at The Australian National University.

A key result of this research is the discovery that both convection and surface winds enhance turbulent mixing. Mixing rates are largest in the convective plumes in cold regions and near the warm surface of the ocean—and the efficiency with which turbulence mixes the ocean is greatest in convecting regions. This has important implications as it is likely that the mixing rate in the Southern Ocean is much higher than previously thought, challenging some well-established assumptions held by the broader oceanographic community. The results further help to estimate the convective mixing in many different regions of the global ocean and are crucial for further improving ocean models.

It is research of this kind that helps improve the basic tools used across weather and climate science, and enable scientists to simulate the processes well, and to obtain good simulations of important phenomena with the underlying physics correctly simulated. This is part of the ARC Centre of Excellence for Climate Extremes’ long term strategy to improve climate models and make them fit for purpose in simulating extremes in the ocean and in the atmosphere.

Image (top): A representation of the model used by CLEX researchers to simulate convective mixing of seawater in the Southern Ocean. Credit: CLEX.

Image (bottom): The Southern Ocean is more turbulent than thought. Credit: Ed Dunens (CC BY 2.0).